TM-9-258
TM 9-258
DEPARTMENT OF THE ARMY TECHNICAL MANUAL
ELEMENTARY OPTICS
AND
APPLICATION TO
FIRE CONTROL INSTRUMENTS
HEADQUARTERS, DEPARTMENT OF THE ARMY
DECEMBER 1977
TM 9-258
TECHNICAL MANUAL
HEADQUARTERS
DEPARTMENT OF THE ARMY
WASHINGTON, DC, 5 December 1977
No. 9-258
ELEMENTARY OPTICS
AND
APPLICATION TO FIRE CONTROL INSTRUMENTS
Reporting Errors and Recommending Improvements. You can help improve this manual. If you find any
mistake or if you know of a way to improve the procedures, please let us know. Mail your letter, DA Form
2028 (Recommended Changes to Publication and Blank Forms), or DA Form 2028-2 located in the back
of this manual direct to: Commander, US Army Armament Materiel Readiness Command, ATTN:
DRSAR-MAS, Rock Island, IL 61201. A reply will be furnished
1.
INTRODUCTION ..................................................................................................
Paragraph
1-1
CHAPTER 2.
Section I.
II.
III.
IV.
V.
VI.
VII.
PROPERTIES OF LIGHT
Light ......................................................................................................................
Reflection ..............................................................................................................
Refraction..............................................................................................................
Image formation ....................................................................................................
Color......................................................................................................................
Characteristics of optical systems.........................................................................
Aberrations and other optical defects ...................................................................
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2-9
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CHAPTER 3.
Section I.
II.
III.
IV.
V.
CHAPTER 4.
Section I.
II.
III.
THE HUMAN EYE
General .................................................................................................................
Construction ..........................................................................................................
Binocular (two-eyed) vision and stereovision........................................................
Defects and limitations of vision............................................................................
Optical instruments and the eyes..........................................................................
OPTICAL COMPONENTS, COATED OPTICS, AND CONSTRUCTION FEATURES
Optical components ..............................................................................................
Coated optics ........................................................................................................
General construction features ...............................................................................
3-1
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3-7
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3-2
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4-13
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CHAPTER
5.
I.
II.
III.
IV.
V.
VI.
VII.
ELEMENTARY PRINCIPLES OF TELESCOPES
Introduction ...........................................................................................................
Astronomical telescopes .......................................................................................
Terrestrial telescopes............................................................................................
Functions of collective lenses and eyepieces .......................................................
Adjustments ..........................................................................................................
Functions of telescopic sights ...............................................................................
Optical factors in telescope design .......................................................................
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5-8
5-12
5-14
5-17
5-22
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5-2
5-4
5-8
5-10
5-12
5-13
CHAPTER
6.
PRINCIPLES OF LASERS....................................................................................
6-1
6-1
CHAPTER
7.
INFRARED PRINCIPLES .....................................................................................
7-1
7-1
CHAPTER
*This manual supersedes TM 9258, May 1966.
i
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TM 9-258
Paragraph
Page
CHAPTER 8.
Section I.
II.
III.
IV.
V.
VI.
VII.
VIII.
TYPICAL FIRE CONTROL INSTRUMENTS
Telescopes............................................................................................................
Articulated telescopes ...........................................................................................
Elbow telescopes ..................................................................................................
Panoramic telescopes...........................................................................................
Periscopes ............................................................................................................
Binoculars .............................................................................................................
Range finders........................................................................................................
Specialized pilitary optical instruments..................................................................
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8-5
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CHAPTER
9.
TESTING OPTICAL PROPERTIES OF INSTRUMENTS.....................................
9-1
9-1
CHAPTER 10.
MEASUREMENT SYSTEMS EMPLOYED IN OPTICS........................................
10-1
10-1
APPENDIX A.
REFERENCES......................................................................................................
A-1
B.
GLOSSARY ..........................................................................................................
B-1
INDEX.................................................................................................................................................
I-1
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TM 9-258
LIST OF ILLUSTRATIONS
Figure
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Title
Page
Military applications of Optics ...............................................................................................................................................
Wave carrying energy along rope .........................................................................................................................................
Wave train passing along rope .............................................................................................................................................
Electromagnetic spectrum ....................................................................................................................................................
Light sources send waves in all directions ............................................................................................................................
Light travels in direction of radii of waves..............................................................................................................................
Waves and radii from nearby light source .............................................................................................................................
Waves and radii from distant light source .............................................................................................................................
Speed of light .......................................................................................................................................................................
Wavelength ..........................................................................................................................................................................
Frequency ............................................................................................................................................................................
Symbols and types of illustrations used in this manual .........................................................................................................
Beam of light reflected back .................................................................................................................................................
Terms used with reference to reflected light .........................................................................................................................
Reflection from a plane mirror ..............................................................................................................................................
Regular reflection .................................................................................................................................................................
Irregular of diffuse reflection .................................................................................................................................................
Reflection at optical interfaces ..............................................................................................................................................
Reflection from convex mirror, infinitely distant source .........................................................................................................
Reflection from concave mirror .............................................................................................................................................
Projection by spherical mirror ...............................................................................................................................................
Projection by parabolic mirror ...............................................................................................................................................
Path of beam of light through sheet of glass .........................................................................................................................
Effects of refraction ..............................................................................................................................................................
Terms used with reference to reflected light .........................................................................................................................
Index of refraction.................................................................................................................................................................
Atmospheric refraction bends sun rays.................................................................................................................................
Sun below horizon seen by reflected light.............................................................................................................................
Path of light ray through prism ..............................................................................................................................................
Deviation of rays by two prisms, base to base ......................................................................................................................
Deviation of rays by convergent lens ....................................................................................................................................
Paths of rays of light through convergent lens ......................................................................................................................
Law of refraction applied to lens ...........................................................................................................................................
Deviation of rays by two prisms, apex to apex ......................................................................................................................
Deviation of rays by divergent lens .......................................................................................................................................
Angles of light rays from an underwater source ....................................................................................................................
Total internal reflection in 90 degree prism ...........................................................................................................................
Virtual image reflected by plane mirror .................................................................................................................................
Real image formed on ground glass of camera ....................................................................................................................
Letter F as reflected or refracted by optical elements ...........................................................................................................
Reflection of point of light in mirror .......................................................................................................................................
Reflection of letter F in mirror ...............................................................................................................................................
Image transmission by mirrors..............................................................................................................................................
Image transmission by prism ................................................................................................................................................
Focal lengths of convergent lens ..........................................................................................................................................
Optical center of the lens ......................................................................................................................................................
Focal length of divergent lens ...............................................................................................................................................
Image formed by convergent lens.........................................................................................................................................
Magnification by reading glass-object within focal length ......................................................................................................
Effect of parallel rays on divergent lens ................................................................................................................................
Reduction by divergent (negative) lens.................................................................................................................................
Dispersion ............................................................................................................................................................................
Field of view limited by optical possibilities of eyepiece ........................................................................................................
Comparison of light entering eye with 2-power telescope .....................................................................................................
Exit pupil...............................................................................................................................................................................
Eye distance.........................................................................................................................................................................
Spherical aberration of convergent lens................................................................................................................................
Spherical aberration of divergent lens...................................................................................................................................
Spherical aberration reduced................................................................................................................................................
Effect of compound lens on spherical aberration ..................................................................................................................
Cause of chromatic aberration in lens...................................................................................................................................
Correction of chromatic aberration of lens ............................................................................................................................
Correction of chromatic aberration of prism ..........................................................................................................................
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TM 9-258
LIST OF ILLUSTRATIONS-Continued
Figure
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Title
Proper refraction of light .......................................................................................................................................................
Astigmatic refraction of light .................................................................................................................................................
Formation of coma, greatly magnified...................................................................................................................................
Appearance of coma, greatly magnified................................................................................................................................
Barrel-shaped (A and B) and hourglass (C and D) distortion ................................................................................................
Hourglass distortion in magnification by reading glass..........................................................................................................
Curvature of image ...............................................................................................................................................................
Diffraction pattern, greatly magnified ....................................................................................................................................
Newton’s rings ......................................................................................................................................................................
Parallax ................................................................................................................................................................................
Comparison of eye and camera............................................................................................................................................
Cross section of eyeball........................................................................................................................................................
Action of iris compared with camera diaphragm ...................................................................................................................
Suspension and action of crystalline lens .............................................................................................................................
Cones and rods of retina ......................................................................................................................................................
Field of view of the eyes .......................................................................................................................................................
Cube as seen by left eye, right eye, and both eyes...............................................................................................................
Judging distance...................................................................................................................................................................
Angular discernible difference...............................................................................................................................................
Nearsightedness (myopia)....................................................................................................................................................
Farsightedness (hypermetropia) ...........................................................................................................................................
Visual Limitations ................................................................................................................................................................
Types of simple lenses .........................................................................................................................................................
Comparative focal lengths of elements in compound lens ....................................................................................................
Types of objectives...............................................................................................................................................................
Typical light paths through field lens and eyelens of eyepiece ..............................................................................................
Kellner and ramsden eyepieces............................................................................................................................................
Huygenian and symmetrical eyepieces.................................................................................................................................
Erfle and orthoscopic eyepieces ...........................................................................................................................................
French plossl, and variable magnification eyepieces ............................................................................................................
Porro prism system ..............................................................................................................................................................
Abbe system.........................................................................................................................................................................
Amici or roof-angle prism......................................................................................................................................................
Rhomboidal prism.................................................................................................................................................................
Rotating prisms ....................................................................................................................................................................
Pentaprism (diagrams show effect obtained with mirrors).....................................................................................................
Triple mirror prism ................................................................................................................................................................
Double right-angle Abbe prism .............................................................................................................................................
Rotation of wedge changes direction of path of light.............................................................................................................
Extent of displacement of light may be changed by movement of wedge .............................................................................
Principles of rotating measuring wedges ..............................................................................................................................
Lens erection systems ..........................................................................................................................................................
Erecting system using two double right-angle abbe prisms...................................................................................................
Optical system of panoramic telescope ................................................................................................................................
Image rotation in dove prism (end view) ...............................................................................................................................
Representative types of reticles............................................................................................................................................
Representative types of reticle patterns................................................................................................................................
Diaphragm (stop)location......................................................................................................................................................
Types of ray filter mountings.................................................................................................................................................
Principles of polarization of light ...........................................................................................................................................
Instrument light for panoramic telescope ..............................................................................................................................
Instrument light for BC periscope..........................................................................................................................................
Light ray striking uncoated surface .......................................................................................................................................
Light ray striking coated surface (coating one-half wavelength in thickness).........................................................................
Light ray striking coated surface (coating one-quarter wavelength in thickness) ...................................................................
Low-reflectance coating........................................................................................................................................................
Lens cell, separators, lenses and retaining ring ....................................................................................................................
Eccentric objective mounting ................................................................................................................................................
Sunshade and objective cap.................................................................................................................................................
Diopter scale ........................................................................................................................................................................
Simple magnifier-object at principal focus.............................................................................................................................
Simple magnifier-object with focal length o ...........................................................................................................................
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TM 9-258
Figure
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LIST OF ILLUSTRATIONS-Continued
Title
Simple compound microscope..............................................................................................................................................
Objective lens.......................................................................................................................................................................
Keplerian system ..................................................................................................................................................................
Refracting astronomical telescope ........................................................................................................................................
Concave mirror .....................................................................................................................................................................
Reflecting astronomical telescope ........................................................................................................................................
Terrestrial telescope-simple form..........................................................................................................................................
Symmetrical erectors ............................................................................................................................................................
Simple erector ......................................................................................................................................................................
Asymmetrical erectors ..........................................................................................................................................................
Galilean system ....................................................................................................................................................................
Relation of elements in galilean telescope (zero Diopter setting) ..........................................................................................
Objective diameter limits field in galilean system ..................................................................................................................
Function of collective lens.....................................................................................................................................................
Function of eyepiece ............................................................................................................................................................
Function of field lens in eyepiece..........................................................................................................................................
Zero Diopter setting ..............................................................................................................................................................
Minus setting for shortsighted eye ........................................................................................................................................
Plus setting for farsighted eye ..............................................................................................................................................
Dioptometer..........................................................................................................................................................................
Reticle location .....................................................................................................................................................................
Entrance and exit pupils .......................................................................................................................................................
Exit pupil-virtual image of objective.......................................................................................................................................
Ramsden dynameter ............................................................................................................................................................
Field limit ..............................................................................................................................................................................
Linear true field.....................................................................................................................................................................
Eye-relief-symmetrical erectors with real images at focal points of erectors and image 0 of objective between erectors ......
Eye relief-objective image outside erectors...........................................................................................................................
Eye relief-collective lens in system .......................................................................................................................................
Angular limit of resolution .....................................................................................................................................................
Gas laser ..............................................................................................................................................................................
Solid-state red lasers ............................................................................................................................................................
Photoconductive detector .....................................................................................................................................................
Bolometer circuit...................................................................................................................................................................
Telescope (rifle)--Assembled view, reticle pattern.................................................................................................................
Telescope (sniper’s sighting device)--assembled view..........................................................................................................
Observation telescope--assembled view with tripod..............................................................................................................
Observation telescope-optical elements and optical diagram ...............................................................................................
Articulated telescope-optical system diagram .......................................................................................................................
Articulated telescope (tank)--assembled view.......................................................................................................................
Articulated telescope (self propelled vehicle) --assembled view............................................................................................
Schematic diagram of elbow telescope.................................................................................................................................
Elbow telescope (howitzer) -assembled view, reticle patterns...............................................................................................
Elbow telescope (sight units)--assembled view, reticle pattern .............................................................................................
Elbow telescope (towed howitzer)--assembled view .............................................................................................................
Elbow telescope (self propelled howitzer)--assembled view..................................................................................................
Panoramic telescope (towed howitzer)-assembled view, optical elements, and optical diagram...........................................
Panoramic telescope (self propelled howitzer) --assembled view .........................................................................................
Periscope (tank observation)--assembled view, optical elements and diagram of principle of operation ...............................
Optical diagram and optical elements of periscope...............................................................................................................
Periscope (self propelled vehicle)--assembled view..............................................................................................................
Periscope (tank)--assembled view ........................................................................................................................................
Periscope (tank-diagram of optical system ...........................................................................................................................
Periscope (infrared) -assembled view...................................................................................................................................
Binocular (general use)-assembled view, optical elements, and optical diagram ..................................................................
Battery commander’s periscope-assembled view, reticle pattern..........................................................................................
Optical system schematic .....................................................................................................................................................
Fundamental triangle of range finder ....................................................................................................................................
Range finder, tank M17A1 and M17C (typical coincidence type) ..........................................................................................
Range finder, reticle patterns................................................................................................................................................
Collimator sight-optical system and optical diagram .............................................................................................................
Reflex sight...........................................................................................................................................................................
Aiming circle .........................................................................................................................................................................
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TM 9-258
LIST OF I LLUSTRATIONS-Continued
Figure
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Title
Page
Laser range finder ................................................................................................................................................................
Fire control subsystem .........................................................................................................................................................
Artillery mil-degree conversion chart.....................................................................................................................................
Metric unit-inch conversion table ..........................................................................................................................................
Absorption-selective transmission ........................................................................................................................................
Angle of azimuth...................................................................................................................................................................
Angle of elevation, angle of site, and quadrant angle............................................................................................................
Apertures..............................................................................................................................................................................
Astigmatism..........................................................................................................................................................................
Axis of bore and boresights-boresighting ..............................................................................................................................
Burnishing ............................................................................................................................................................................
Center of curvature...............................................................................................................................................................
Conjugate focal points (conjugate foci) .................................................................................................................................
Converging lenses................................................................................................................................................................
Diverging lenses ...................................................................................................................................................................
Drift.......................................................................................................................................................................................
Lens erecting system............................................................................................................................................................
Eyelens and field of eyepiece ...............................................................................................................................................
Focal length of lens and mirror .............................................................................................................................................
Real image produced by converging lens .............................................................................................................................
Virtual image produced by diverging lens .............................................................................................................................
Index of refraction.................................................................................................................................................................
Optical system of erecting telescope ....................................................................................................................................
Linear field............................................................................................................................................................................
Magnifying power..................................................................................................................................................................
Nearsightedness (myopia)....................................................................................................................................................
Normal and oblique ..............................................................................................................................................................
Porro prism...........................................................................................................................................................................
Prism ....................................................................................................................................................................................
Protractor..............................................................................................................................................................................
Reticle patterns superimposed on image of object ...............................................................................................................
Right angle prism..................................................................................................................................................................
Waves and wave fronts ........................................................................................................................................................
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CHAPTER 1
INTRODUCTION
using personnel and others who must be familiar with the
functioning of fire control instruments (fig 1-1).
1-1.
Purpose. This manual is published primarily for
the information and guidance of Army maintenance and
1-1
TM 9-258
Figure 1-1. Military application of optics.
1-2
TM 9-258
essential, as the use of those subjects in this text is on a
simple level.
b. The glossary contains definitions of specialized
terms and unusual words used in this manual. A list of
current references appears in this appendix.
c. The material presented herein is applicable
without modification, to both nuclear and non-nuclear
warfare.
1-2.
Scope
a. This manual covers the basic principles of
optical theory necessary to understand the operation of
fire control instruments. It contains sufficient descriptive
matter and illustrations to provide a general knowledge
of the principles upon which the design and construction
of military optical instruments are based. A background
in physics and mathematics, though helpful, is not
1-3
TM 9-258
CHAPTER 2
PROPERTIES OF LIGHT
Section I. LIGHT
Fresnel (1788-1827), and others supported the wave
Theories and Known Facts About Light.
theory and the rival corpuscular theory was virtually
a. General. The true nature of light and the
abandoned. These scientists offered their measurement
manner in which it travels have fascinated scientific
of light waves as proof. They accepted the "ether" theory
investigators for centuries. Many theories have been
and assumed light to be waves of energy transmitted by
advanced to explain why light behaves as it does. Later
an elastic medium ether.
discoveries have proved some of these theories
(5) Maxwell, Boltzmann, and Hertz. Light and
unsound. This manual cannot attempt to cover all the
electricity were similar in radiation and speed was proved
aspects of light nor is this knowledge essential to the
the experiments of Maxwell (18311879), Boltzmann
study of the laws of optics as they apply to fire control
(1844-1906), and Hertz (18571894).
From these
equipment. Therefore, only a short summary of the
experiments was developed the Electromagnetic Theory.
various theories of light is given.
These experiments produced alternating electric currents
(1) Ancient Greek theory. The earliest known
of short wavelengths that were unquestionably of
speculations as to the nature of light were those of the
electromagnetic origin and had all the properties of light
ancient Greeks. They believed that light was generated
waves. The Maxwell theory contended that energy was
by streams of particles which were ejected from the
given off continuously by the radiating body.
human eye. This theory could not persist because it
(6) Planck. For a time it was thought that the
could not explain why one could not see as well at night
puzzle of light had been definitely solved but, in 1900,
as by day.
Max Planck refuted the Maxwell theory that the energy
(2) Newton. The first modern theory was that
radiated by an ideal radiating body was given off
of Sir Isaac Newton (1643-1727). This, the corpuscular
continuously. He contended that the radiating body
theory, assumed light to be a flight of material particles
contained a large number of tiny oscillators, possibly due
originating at the light source. Newton believed that light
to the electrical action of the atoms of the body. The
rays moved at tremendous velocity in a state of near
energy radiated could be high frequency (para 2-3b) and
vibration and could pass through space, air, and
with high energy value, all possible frequencies being
transparent objects. This theory agreed with the property
represented. The higher the temperature of the radiating
of light to move only in a straight line, in a medium of
body the shorter is the wave length (para 2-3a) of most
constant optical density, but failed to explain other
energetic radiation. In order to account for the manner in
phases of light behavior. Newton stumbled on light
which the radiation from a warm black body is distributed
interference in his discovery of Newton’s Rings (para 2among the different wave lengths, Planck found an
44)but did not realize their significance.
equation to fit the experimental curves, and then only on
(3) Huygens. In Newton’s time, Christian
the very novel assumption that energy is radiated in very
Huygens (1629-1695) attempted to show that the laws of
small particles which, while invisible, were grains of
reflection and refraction of light could be explained by his
energy just as much as grains of sand. He called these
theory of wave motion of light. While this theory seemed
units quanta and his theory the Quantum Theory. The
the logical explanation for some phases of light behavior,
elementary unit or quanta for any given wave length is
it was not accepted for many years because a means
equal to hn, where n is frequency of the emitted radiation
was lacking by which to transmit the waves. Huygens
and h is a constant known as Planck’s constant. Quanta
proposed that a medium, which he called "ether, " be
set free were assumed to move from the source in
accepted as existing to serve light rays as water does the
waves.
familiar waves of water. He assumed this medium to
(7) Einstein. A few years later, Albert Einstein
occupy all space, even that already occupied by matter.
backed up in Planck and contended that
(4) Young and Fresnel. Experiments made
about 50 years later by Thomas Young (1773-1829),
2-1.
2-1
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the light quanta when emitted retained their identity as
individual packets of energy.
(8) Millikan and Compton. Still later, R.A.
Millikan’s very accurate measurements proved that the
energy due to motion (called "kinetic energy) of
elemental units of light (called "photons") behaved as
assumed by the Quantum Theory. Later proof was given
by A.H. Compton in 1921 in determining the motion of a
single electron and a photon before and after collision of
these bodies. He found that both have kinetic energy
and a momentum and that they behave like material
bodies.
This was somewhat a return to the old
corpuscular theory.
(9) Summary.
The present standpoint of
physicists on the nature of light is that it appears to be
dualistic, both particle and wavelike. It is assumed that
light and electricity have much in common. It is presently
accepted that the energy associated with a light beam is
transmitted as small particle-like packets-photons,
originally called quanta (para 2-1a (6) which may be
described in terms of associated waves, but, which are
best measured relying solely on their particle-like nature.
The simple wave analogy is only a very rough
approximation.
However, the phenomena of light
propagation may be best explained in terms of wave
theory. The wave theory will be used in the remainder of
this manual.
b. Known Facts. All forms of light obey the same
general laws. Light travels in waves in straight lines and
at a definite and constant speed, provided it travels in a
medium or substance of constant optical density. Upon
striking a different medium, light either will bounce back
(be reflected) or it will enter the medium. Upon entering
a transparent medium, its speed will be slowed down if
the medium is more dense or increased if the medium is
less dense. Some substances of medium density have
abnormal optical properties and for this reason "optically
dense" would be a better term. If it strikes the medium at
an angle, its course will be bent (refracted ) upon
entering the medium.
Upon striking all material
mediums, more or less of the light is absorbed.
(1) Transmission of light energy. Visible light
is one of many forms of radiant energy which is
transmitted in waves. The means by which the waves
carry the energy can be illustrated in a simple manner.
Secure one end of a rope to some object. Hold the other
end of the rope, stretch it fairly taut, and shake it. A
wave motion (wave pulse) will pass along the rope from
the end that is held to the end that is secured (fig 2-1). If
the end of the rope is continually shaken, a series of
waves (wave train) will pass along the rope (fig. 2-2). It
will be noted that the different parts of the rope (the
medium) vibrate successively, each bending back and
forth about its own position. The disturbance travels but
the medium does not. Only the energy is carried along
the rope. Now imagine that the light source is a vibrating
ball from which a countless number of threads extend in
all directions. As the ball vibrates, successive waves are
transmitted out along the threads in all directions. In a
similar manner, light radiates from its source.
Figure 2-1. Wave carrying energy along rope.
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Figure 2-2. Wave train passing along rope.
(2) Electromagnetic spectrum. Heat and radio
waves, light waves, ultraviolet and infrared rays, X-rays,
and cosmic rays are forms of radiant energy of different
wavelengths and frequencies. Together, they form what
is known as the electromagnetic spectrum (fig 2-3). The
visible light portion of the electromagnetic spectrum
consists of wavelengths from 0.00038 to 0.00066
millimeter. The different wavelengths represent different
colors of light. Practically all light is made up of many
colors, each color having its distinctive wavelength and
frequency. In as much as light of each color reacts in a
slightly different manner when passed through different
mediums, provision must be made in optical elements to
control the action of light of various colors.
2-3
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Figure 2-3. Electromagnetic spectrum.
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(3) Light rays. In considering visible light,
assume that it starts from a luminous point and travels
outwardly in all directions in waves through a medium of
constant density to form a sphere of which the luminous
point is the center (fig 2-4). The direction in which the
light is traveling is along the radii of the sphere of light
(fig 2-5) at right angles to the fronts of the waves. Light
traveling along these radii is termed light rays.
Figure 2-5. Light travels in direction of radii of waves.
(4) Light rays from distant source. The wave
front radiating from a light source is curved when it is
near the source (fig 2-6) and the radii of the waves
diverge or spread. The wave front becomes less and
less curved as the waves move outwardly, eventually
becoming almost straight (fig 2-7), and the radii of waves
from a distant source are virtually parallel.
Figure 2-4. Light sources send waves in all directions.
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Figure 2-7. Waves and radii from distant light source.
(5) Principle of reversibility of light paths. If a
single narrow beam of parallel light is reflected or
refracted in any combination through a number of optical
elements, it will retrace its path through the elements if
the light were to enter the optical system from the other
end. This is true no matter how many reflections and
refractions the light has undergone.
Most substances reflect light
(6) Color.
selectively. Certain wavelengths (colors) are reflected,
while others are absorbed. The color of an object is the
one which is reflected. A colored transparent medium
(filter) transmits light selectively, absorbing all rays not
transmitted.
(7) Biological effects. Infrared light is not very
effective on living tissues except to produce heat. Visible
light is beneficial in its effects. Shorter waves may
produce serious burns, but possess marked bactericidal
properties and are used for sterilizing milk and other
materials. In general, the shorter the waves the more
violent are they in their effects on living tissues.
2-2. Speed of Light
a. The speed of light (fig 2-8) is an important factor
in the study of the physical nature of light., The lengths of
all waves in the electromagnetic spectrum (fig 2-3) are
logically connected to corresponding frequencies and to
the speed of light (para 2-3). The difference in the speed
of light through air, glass, and other substances accounts
for the bending of light rays or refraction.
Figure 2-6. Waves and radii from nearby light source.
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Figure 2-8. Speed of light
b. Light travels so fast that it was a long time before
its speed could be measured. Galileo Galilei (15641642) tried to measure light velocity by means of blinking
lanterns on widely distant hilltops but, as the speed was
too great to be measured by this method, the velocity of
light was presumed to be infinite.
c. Thirty-four years after Galileo’s death, Olaus
Roemer (1644-1710) determined the approximate speed
of light by astronomical means based on observations
which showed the amount of time required for the light of
the planet Jupiter to cross the orbit of the earth.
d. It was nearly 200 years more before scientists
were able to measure a speed so great as that of light by
laboratory methods.
In 1849, Fizeau (1819-1896)
measured the velocity of light, using intermittent flashes
of light sent out by a source equipped with a swiftly
moving toothed wheel and reflected back to the
observer. Other methods, employing the reflections
from rotating mirrors and prisms, have since been used
to establish the speed of light to an exactness beyond
that required for practical usage.
e. The speed of light is accepted as 186, 330 miles
per second. This is the figure used in computations
involving the speed of light. The speed of light obtained
by the important methods of the past 70 years ranges
from 186, 410 to 186, 726 miles per second.
2-3.
Wavelength and Frequency
a. Wavelength. The wavelength is the distance
from the crest or top of one wave to the crest of the next,
or from any point on one wave to the corresponding
point, in the same phase on the next wave (fig 2-9). The
velocity of all forms of electromagnetic waves in vacuum
is the same, that is, the speed of light is approximately
186, 000 miles per second. The wavelengths, however,
vary greatly
(fig 2-3).
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Figure 2-9. Wavelength
b. Frequency. The frequency is the number of
waves occurring per second. It is determined by dividing
the speed of light by the wavelength. It is the number of
waves passing a given point in 1 second (fig 2-10).
c.
Special Units of Measurement Employed
Wavelengths of
in Measuring Wavelengths.
electromagnetic waves range from many miles long to
those measured in trillionths of an inch.
The
measurements cover such a wide range and the shorter
wavelengths are so minute that special units of
measurements have been provided to avoid long
decimal fractions of millimeters or inches.
(1) The first of the special units of
measurement is the micron, which is abbreviated to µ,
the Greek letter "mu". It represents one one-millionth of
a meter or one one-thousandth of a millimeter. The next
is variously called the milli-micron and micro-millimeter.
It is abbreviated to mµ and represents one onethousandth of a micron. Finally, the micro-micron is
abbreviated to go. It represents one one-millionth of a
micron.
(2) Another important measurement is the
Angstrom unit (AU) which is one-tenth of a milli-micron
or one ten-millionth of a millimeter. Even Angstrom units
are inconveniently long in measuring the shortest
electromagnetic waves so the X-ray unit (XU) is used for
this purpose. It is one one-thousandth of an Angstrom
unit.
2-4.
Light Rays and Other Symbols Used in the
Diagrams.
a. Every point on a luminous body or an illuminated
object sends out a constant succession of wave fronts in
all directions. The action of light in passing through a
lens, for example, might be as shown in A, figure 2-11.
But for simplicity, light will be indicated in this manual by
one, two, or more representative "light rays." These "light
rays" are shown as lines with arrowheads indicating the
direction of travel and are shown as in B, figure 2-11.
Wherever possible light will be indicated as coming from
the left side of an illustration.
Velocity
Frequency =
Wavelength
Figure 2-10. Frequency
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Figure 2-11. Symbols and types of illustrations used in this manual.
b. Single rays of light do not exist. The term "light
ray" is used throughout this manual for the sake of clarity
and convenience to indicate the direction of travel of
light.
c. In studying the diagrams, it must be
remembered that printed illustrations have only height
and width (b, fig 2-11) while everything, including light,
has three dimensions: height, width, and depth (or
length). The reader’s imagination must be employed to
give the illustration depth. Thus, for example, where two
light rays are used to indicate the course of light
from a luminous or illuminated point, these lightrays
indicate a solid cone of light from that point (A, fig 2-11).
In certain of the illustrations, an attempt has been made
to simulate height, width, and depth by preparing the
diagrams to appear as though viewed from an angle (A,
fig 2-11).
d. The letter F is used as the object in a great many
of the diagrams because it shows quickly whether the
image of this letter or object is upright, inverted, or
reverted.
Section II. REFLECTION
directed back along the path of the incoming beam (A,
fig 2-12). If the mirror is shifted to an angle from this
position, the reflected beam will be shifted at an angle
from the incoming beam that is twice as great as the
angle by which the mirror is shifted (B, fig 2-12). For
example, if the mirror is held at angle of 45° to the
incoming beam, the reflected beam will be projected at
an angle of 90° to the incoming beam.
2-5.
Reflection From a Plane Mirror.
a. A plane mirror is a flat polished surface used to
reflect light. If a beam of light is permitted to enter a
darkened room and a plane mirror is held so that the
beam will strike the mirror, the beam will be reflected. By
shifting the mirror, the beam can be reflected to almost
any part of the room.
b. If the mirror is shifted so that it is exactly at right
angles (normal) to the beam, the reflected beam will be
2-9
TM 9-258
Figure 2-12. Beam of light reflecting back.
c. These simple experiments illustrate one of the
dependable forms of action of light. Light can be
reflected precisely to the point where it is required
because any kind of light, on being reflected by a
smooth, polished surface, acts in the same manner.
This action of light is put to use in many types of firecontrol instruments.
d. The ray of light which strikes the surface is
called the incident ray (fig 2-13). The ray which is
reflected is termed the reflected ray. An imaginary line at
right angles to the surface is called the normal or
perpendicular. The angle between the incident ray and
the normal is the angle of incidence, while the angle
between the reflected ray and the normal is the angle of
reflection.
Figure 2-13. Terms used with reference to reflected light.
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e. The best reflectors are very smooth metal
surfaces, some of which may reflect as much as 98
percent of perpendicular light. The glass in a silvered
mirror serves only as a very smooth supporting surface
and protective window for the silver coating on the back.
In front surface aluminized mirrors, the glass serves only
as a smooth supporting surface for the aluminum
reflecting surface. The resulting aluminum oxide (from
the oxidation of the aluminum when exposed to the
oxygen in the air) is both colorless and transparent and
so does not interfere to any appreciable extent with the
passage of light. This type of coating is used on the
reflectors in the huge reflecting astronomical telescopes.
2-6.
Law of Reflection
a. The law of reflection is as follows: The angle of
reflection is equal to the angle of incidence and lies on
the opposite side of the normal; the incident ray,
reflected ray, and normal all lie in the same plane.
b. Diagrams A, B, C, and D in figure 2-14 show
light rays incident upon plane mirrors at successively
smaller angles. By applying the law of reflection, it can
be seen that in all such cases of reflection the angle of
reflection can be plotted as long as the angle of
incidence is known or vice versa.
Figure 2-14. Reflection from a plane mirror.
b. Regular reflection occurs when light strikes a
smooth surface and is reflected in a concentrated
manner (fig 2-15).
2-7.
Types of Reflection.
a. There are two main types of reflection regular
and irregular.
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d. The direction in which any single ray of light will
be reflected can be plotted by erecting a perpendicular or
normal at the point of impact and applying the law of
reflection (fig 2-16). Any rough surface can be regarded
as an almost infinite number of elementary plane
surfaces at each of which the law of reflection holds true.
e. Light is reflected by diffusion or by regular
reflection and diffusion from nearly every object which is
seen. Because the light falling on such an object is, in
part, scattered in all directions, the object is made visible,
generally, when viewed from any direction. Skin, fur, and
every dull surface reflect light in essentially a diffused
manner. The glossy finish of a new automobile or the
bright finish of a polished casting reflect light essentially
by regular reflection and partially by diffusion.
f. Surfaces of transparent mediums such as optical
interfaces also will reflect. The amount of incident light
reflected depends upon the angle of incidence. As the
angle of incidence increases, so does the amount of
reflected light. When light passes through a piece of
glass, reflection occurs at both surfaces as in figure 217. Approximately 4 percent of the incident light is lost or
reflected at the first surface and another 4 percent of the
emergent light is lost by reflection at the second surface.
Thus, both reflection and refraction (the bending of light)
occur at any optical interface. The importance to optical
systems is discussed in detail in paragraphs 4-1 through
4-16).
Figure 2-15. Regular reflection.
c. If a beam of light strikes a rough surface, such
as a sheet of unglazed paper or ground glass, the light is
not reflected regularly but is scattered in all directions (fig
2-16). This is called irregular reflection or diffusion.
Figure 2-17. Reflection at optical interfaces
Figure 2-16. Irregular or diffuse reflection.
2-12
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rays will be deflected in a divergent manner. The reason
for this can be, determined by plotting the angles of
reflection of individual rays in relation to their angles of
incidence and the normals for each ray. In this case, the
normal for each ray is an imaginary line drawn from the
center of curvature of the mirror to the point of incidence
of the ray. The angle of reflection will be equal to the
angle of incidence for each ray.
2-8.
Reflection from Convex Mirror.
a. The law of reflection holds true for all surfaces
whether convex, concave, or plane. The action of
various curved surfaces in directing the course of the
reflected light will vary according to the amount of
curvature of the reflecting surface and the overall effect
will be dependent on the distance of the light source from
the reflecting surface.
b. Assume that light from a practically infinitely
distant source, such as the sun (practically parallel rays),
strikes a convex mirror (fig 2-18). It will be found that the
Figure 2-18. Reflection from convex mirror, infinitely distant source.
c. If the light source is at a finite distance, or close to the mirror, the rays will be reflected in a divergent manner also.
Such rays will be reflected at different angles than would be the case if all of them struck the mirror as parallel rays.
2-9.
Reflection from Concave Mirror.
a. The law of reflection is again applied to locate the path of the reflected rays when plotting the reflection from a
concave mirror. In this instance, the center of curvature of the mirror is in front of the mirror (fig 2-19). Imaginary lines are
run from this center to the points of incidence of the incident rays, to indicate the normals of individual rays. When this has
been done, the reflected rays can be plotted so that each forms an angle of reflection which will be equal to the angle of
incidence of the corresponding ray.
2-13
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Figure 2-19. Reflection from concave mirror.
b. It will be noted that the rays originating from an
infinite (distance) source, such as the sun, are
converging after reflection and that they intersect very
close to a point halfway between the center of curvature
and the surface of the mirror. This point is known as the
principal focal point of the mirror. This point of principal
focus is always one-half of the distance from the center
of curvature to the surface of a concave mirror, provided
the mirror is a true portion of a sphere. Due to "spherical
aberration, " the principal focus is a true point only for a
small central bundle of rays. After passing through the
focal point, the light will diverge.
c. If a very small luminous source is located at the
principal point of focus, the rays will be nearly parallel
after reflection. This is true only if the curvature is very
slight. Actually, the rays will have a slight convergence,
especially those which are reflected from near the edges
of the mirror (fig 2-20). For this reason, if it is desired to
have practically parallel rays after reflection, a parabolic
mirror is used.
Figure 2-20. Projection by spherical mirror.
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d. With a parabolic mirror, light rays emanating
from an infinitely small source located at the focal point
would be parallel after reflection, since this is the
principal focal point of the parabolic mirror (fig 2-21). In
practice, however, the source of light, which may be a
filament or an arc, is located in the principal point of
focus and the rays diverge as there cannot be a true
point source. All rays falling upon the parabolic mirror,
except those which are diffused or scattered, are
reflected nearly parallel to each other. This allows a long
powerful beam of light to be formed which diverges very
slightly.
Figure 2-21. Projection by parabolic mirror.
Section III. REFRACTION
surfaces of this plate are parallel and air contacts both
surfaces. The glass and the air are transparent but the
glass is optically denser than the air. Light travels
approximately one-third slower in glass than in air.
2-10. Refraction Through a Glass Plate.
a. What happens when light is refracted can be
understood by imagining a beam of light, indicated by a
broad "light ray" or "light beam" (fig 2-22), passing in
slow motion" through a sheet of glass. Both plane
Figure 2-22. Path of beam of light through sheet of glass.
2-15
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b. If the beam strikes the glass directly at right
angles (or the normal), the light is not bent or refracted
(A, fig 2-22).
Its speed is slowed down upon
encountering the’ optically denser medium, but as the
entire front strikes the glass at the same instant its path
is not deviated but continues through the medium in a
straight line. Upon reaching the other surface of the
glass, the beam enters the optically lighter medium, air,
and resumes its faster speed in air.
c. If this beam of light strikes the glass at an angle,
one of the edges of the wave front arrives at the surface
an instant before the other edge does and consequently
its path through glass is longer, at this point, while
entering glass. The edge of the front arriving at the glass
first is slowed down upon entering the optically denser
medium (B, fig 2-22). When the other edge enters the
glass, it is slowed down in the same manner but an
instant later, having traveled with greater speed in air,
the effect is to swing the entire beam toward the normal.
This bending of light is termed refraction.
The light follows its new course in a direct line when the
optical density of the medium is constant.
d. Upon leaving the other surface of the glass
plate, the light is again refracted or bent (B, fig 2-22).
The edge which entered the glass first traverses the
glass first. Upon leaving the optically denser medium
and entering the optically lighter medium (air), its path is
deviated again. The effect is to swing the entire beam
away from the normal.
e. Refraction may be visually demonstrated by
placing the straight edge of a sheet of paper at an angle
under the edge of a glass plate held vertically (B, fig 223). The straight edge of the sheet of paper will appear
to have a jog in it directly under the edge of the glass
plate. That portion of the paper on the other side of the
glass will appear to be displaced due to refraction. As
the sheet of paper is moved to change the angle of its
straight edge, the amount of refraction will be increased
or decreased. If the straight edge of the paper is viewed
at right angles to the surface of the glass, there will be no
refraction.
Figure 2-23. Effects of refraction.
toward the normal. In passing from a medium of greater
optical density to a medium of lesser optical density, the
path of light is deviated away from the normal (fig 2-24).
2-11. Law of Refraction.
a. The law of refraction can be stated as follows: In
passing from a medium of lesser optical density to one of
greater optical density, the path of light is deviated
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Figure 2-24. Terms used with reference to refracted light.
b. The greater the angle at which the light strikes
the medium and the greater the difference in optical
density between the two mediums, the greater the
bending. If the faces of the medium are parallel, the
bending at the two faces is always the same so that the
beam which leaves the optically denser medium is
parallel to the incident beam (A, fig 2-23).
c. The ray which strikes the surface is called the
incident ray (fig 2-24). The ray entering the second
medium is the refracted ray. The ray leaving the second
medium is the emergent ray. An imaginary line at right
angles to the surface of the medium at the point where
the ray strikes or leaves the surface is termed the normal
or perpendicular. The angle between the incident ray
and the normal is the angle of incidence. The angle
between the refracted ray and an extension of the
incident ray is termed the angle of deviation. It is the
angle through which the refracted ray is bent from its
original path by the optical density of the refracting
medium. The angle formed by the refracted ray with the
normal is called the angle of refraction. The incident ray,
refracted ray, emergent ray, and the normal all lie in the
same plane (fig 2-24).
d. The angle of refraction depends on the relative
optical density of the two mediums as well as on the
angle of incidence.
e. An equation known as Snell’s law may be used
to determine the angle of refraction (r) if the angle of
incidence (i) and the index of refraction (para 2-12c) of
each medium are known (fig 2-25).
sin i
This equation is
= n where n is the sin r
sin r
quotient of the indices of refraction of the two mediums.
If one of the mediums is air, n is practically the index of
refraction of the second medium.
2-17
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determined from the equation of Snell’s law (para 2-11e)
by actually measuring the angles i and r in a simple
experiment, inasmuch as air can be used instead of
vacuum for all practical purposes.
c. The following listing contains the indices of
refraction of a number of substances.
NOTE
For most computations, the index of air
is considered to be unity (1.000). The
error introduced equals approximately
0.03 percent.
Vacuum ....................................................1.000000
Air .............................................................1.000292
Water ..............................................................1.333
Boro-silicate crown glass................................1.517
Thermosetting cement....................................1.529
Thermoplastic cement
(Canada balsam) .....................................1.530
Gelatin ............................................................1.530
Light flint glass................................................1.588
Medium flint glass...........................................1.617
Dense flint glass .............................................1.649
Densest flint glass ..........................................1.963
Figure 2-25. Index of Refraction.
2-12. Index of Refraction.
a. Light travels through substances of different
optical densities with greatly varying velocities. For
example, the speed of light in air is approximately 186,
000 miles per second; in ordinary glass, it is
approximately 120, 000 miles per second.
b. The ratio between the speed of light in vacuum
and the speed of light in a medium is known as the index
of refraction. The index of refraction is usually indicated
by letter n and is determined by dividing the speed of
light in a vacuum by the speed of light in the particular
medium.
Velocity in Vacuum
Index of Refraction =
Velocity in Medium
The index of refraction n, of a substance, can be
2-13. Atmospheric Refraction
a. At a surface separating two media of different
indices of refraction, the direction of the path of light
changes abruptly when passing through the surface. If
the index of refraction of a single medium changes
gradually as the light proceeds from point to point, the
path of the light will also change gradually and will be
curved rather than in a straight line.
b. Although air at its densest has a refractive index
on only 1.000292, this is sufficient to bend light rays from
the sun toward the earth when these rays strike the
atmosphere at an angle (fig 2-26).
The earth’s
atmosphere is a medium which becomes denser toward
the surface of the earth. The result is that of light
traveling through the atmosphere toward the earth at an
angle does not travel in a straight line but is refracted
and follows a curved path. From points near the horizon,
the bending of light is so great that the setting sun in
seen after it is completely below the horizon (fig 2-27).
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Figure 2-26. Atmospheric refraction bends sun rays.
Figure 2-27. Sun below horizon seen by refracted light.
c. The bending of the paths of rays of light in both
vertical and a horizontal plane is due to variation of the
refractive index of air due to moisture and temperature
variations. This variation is generally so small that it can
be neglected in many types of observation made with
fire-control instruments. In very careful surveys of
large areas, this effect is eliminated by repeating the
observations on different days and under different
conditions. In the aiming of guns over ]wide expanses of
water as must be done by coast artillery, a special
instrument (the depression position finder) takes
atmospheric refraction as well as other conditions into
consideration.
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d. Over large areas of heated sand or over water,
conditions are such as to produce strata or layers of air
differing greatly in temperature and refractive index.
Under such conditions, erect or inverted and sometimes
much distorted images are formed which can be seen
from a great distance. These images are known as
mirages.
e. On a hot day, the columns of heated air rising
from the earth are optically different from the surrounding
air and rays of light are irregularly refracted. The air is
turbulent and conditions under which observations are
made are changing all the time. Consequently, an object
viewed through such layers of air appears to be in motion
about a mean position. In such cases the air is said to
be "boiling" or the image is "dancing" due to "heat
waves." This condition is particularly detrimental when a
high-power telescope is employed.
Under such
conditions it is usually impossible to use an instrument of
more than 20 power.
2-14. Refraction Through Triangular Glass Prism.
a. Unlike a plate of glass, a prism has its faces cut
at an angle. Refraction through a triangular glass prism
differs
from
refraction
through
a
sheet
of glass with parallel surfaces because the light which
penetrates the base or thicker part of the prism must
travel longer and at less speed in an optically denser
medium than the light passing through the apex or
thinner part of the prism. Rays of light emerging from a
plate of glass with parallel surfaces always travel in a
parallel direction with the incident rays; rays emerging
from a prism always travel at different angles from the
incident rays.
b. The laws of refraction apply to prisms just as
they do to plates of glass with parallel surfaces. These
laws may be applied to plot the paths of light through any
prism.
c. When a ray of light strikes the surface of a
prism, the refracted ray (fig 2-28) is bent toward the
normal according to the law of refraction. The refracted
ray is bent away from the normal on leaving the prism.
Thus, the path of the ray is deviated at the first surface of
the prism and then further deviated at the second
surface. In both cases, the ray is bent toward the
thickest part of the prism.
Figure 2-28. Path of light ray through prism.
front surfaces will pass through the prisms. The rays
emerging from one prism will cross the emergent rays of
the other prism.
2-15.
Refraction Through Lenses.
a.
Convergent Lens. If two prisms are
arranged base to base (fig 2-29), rays of light striking the
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Figure 2-29. Deviation of rays by two prisms, base to
base.
Figure 2-30. Deviation of rays by convergent lens
(2) Myriads of rays may be considered to come from
every point of light on an object. Consider the refraction
of three such rays from a point of light passing through a
convergent lens (fig 2-31) to intersect at a point on the
other side of the lens. On the basis that a lens bends the
rays as a prism does, rays passing through the upper
and lower portions of the lens would be bent toward the
thickest part of the lens upon striking the first surface
and bent again toward the thickest part in emerging. As
the result, they would converge on the other side. A
straight line drawn through the center of the two
spherical surfaces is termed the axis of the lens. A
central or axial ray would not be deviated because it
would strike the surfaces of the lens at the normal. Such
a ray would join the outer rays at their convergent point.
(1) If the two prisms are cut in semicircular form,
their surfaces made spherical, and the two bases
cemented together, the result will be an optical element
known as a convergent lens. (All convergent lenses are
thicker at the center than at the edges. ) When parallel
rays of light strike the front surface of a convergent lens
(fig 2-30), all rays pass through the lens and converge to
a single point where they cross (color and other
aberrations are disregarded at this time). Such a lens
may be thought of as consisting of an infinite number of
prisms arranged so that each directs light rays to the
same single point. The lens bends the rays as a prism
does but, unlike a prism, it brings them to a point.
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TM 9-258
Figure 2-31. Paths of rays of light through convergent lens.
(3) The laws of refraction may be applied to
plot the path of any ray through any lens. A ray entering
a lens will bend toward the normal of the lens at that
point (fig 2-32). The normal of an incident ray at any
point on a lens is an imaginary line at right angles to the
surface of the lens at the
point where the ray enters. As the refracted ray leaves
the lens as the emergent ray, it is bent away from its
normal. The normal of any emergent ray is an imaginary
line at right angles to the surface of the lens at the point
where the ray emerges from the lens.
Figure 2-32. Law of refraction applied to lens.
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b. Divergent Lens. A lens of a different type can be
approximated by placing two prisms apex to apex (fig 233). If rays of light strike the front surfaces of the prisms,
the rays will pass through and, in accordance with the
law of refraction, those passing through the upper prism
will travel upward while those passing through the lower
prism will travel downward. Now assume that the front
and rear surfaces of this pair of prisms have been
rendered spherical and that the two prisms have been
converted into a lens (fig 2-34). All rays of light, except
the axial ray, passing through this lens would spread out
or diverge but instead of splitting in two at the axis would
diverge evenly in a spherical manner. This type of lens
is known as a divergent lens. (All divergent lenses are
thicker at the edges than at the center.
2-16. Total Internal Reflection and Critical Angle.
a. When light passing from air into a more optically
dense medium, such as glass, strikes the boundary
surface, it is refracted. This occurs regardless of the
incident angle except when light is on the normal (no
refraction occurs). However, when light attempting to
leave a more optically dense medium for a less optically
dense medium strikes the boundary surface, it may be
reflected rather than refracted even though both
mediums are perfectly transparent. This is known as
total internal reflection and occurs when the incident
angle exceeds a certain critical angle. The critical angle
depends upon the relative indices of refraction of the
media. Critical angles for various substances, when the
external medium is air, are listed in paragraph 2-17.
b. The total internal reflection of light can be
illustrated by following the rays from a light source under
water. Water has a critical angle of 480 36 minutes,
when the external medium is air. Rays of light from such
a source would be incident at various angles on the
surface separating the water and air (fig 2-35). As the
angle of incidence of the light rays increases, the
deviation of the refracted rays becomes proportionately
greater. A point is reached where an incident ray is
deviated to such an extent that it travels along the
surface of the water and does not emerge into the air.
The angle formed by this incident ray and the normal is
the critical angle of the medium.
Figure 2-33. Deviation of rays by two prisms, apex to
apex.
Figure 2-34. Deviation of rays by divergent lens.
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TM 9-258
Figure 2-35. Angles of light rays from an underwater source.
e. Consider a prism with two faces at right angles
and a back or hypotenuse at a 450 angle (A, fig 2-36).
An incident ray, striking one of the right-angle faces in
the direction of the perpendicular or the normal, passes
into the prism without refraction or deviation until it hits
the boundary of the medium (back of the prism). Since
the angle of the back is 450 and the critical angle of the
glass of the prism is 420, the ray cannot pass through
the back surface of the prism. Instead, it is totally
reflected. Inasmuch as the angle of incidence between
the incident ray and the normal (at the back surface) is
450, the angle of reflection would be the same because
these angles are always equal. The result is that the ray
is reflected a total of 900, strikes the surface of the other
right-angle face of the prism in the direction of the
normal, and passes out of the prism without further
deviation.
c. All rays traveling in an optically dense medium
and striking the surface with an angle of incidence that is
less than the critical angle of the mediums are refracted
and pass into the optically lighter medium in accordance
with the laws of refraction. All such rays striking at an
angle of incidence greater than the critical angle of
mediums are reflected inwards in accordance with the
laws of reflection.
d. The critical angle has a practical application in
the design of prisms used in fire-control instruments. In
such instruments, it is quite often necessary to change
the course of the path of light. This deviation could be
accomplished by mirrors but the prisms used perform the
task more satisfactorily. When prisms are employed at
angles greater than the critical angle of the substance of
which they are made, their reflecting surfaces do not
require silvering, yet these surfaces appear to be silvered
when one looks into such a prism.
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TM 9-258
Figure 2-36. Total internal reflection in 90-degree prism.
f. At any boundary between different media there
is both reflection and refraction of an incident ray,
excepting total reflection (para 2-7f). Figures 2-35 and 236, therefore, are only approximations.
g. The critical angle of any substance can be easily
calculated from the equation of Snell’s law
(para 2-11. e), n =sin n
sin r
It follows that:
sin r = sin 90° = 1
n
n
water,n=1. 333 (para 2-12c), sin
1= 0. 750187, and the critical angle
1. 333
r = 48° 36 min (para 2-17a).
For
r=
2-17. Critical Angles of Various Substances.
a. The critical angles for various substances, when
the external medium is air, are as follows:
Water ...................................................... 48° 36 min
Crown glass ............................................ 41° 18 min
Quartz ..................................................... 40° 22 min
Flint glass ................................................ 37° 34 min
Diamond.................................................. 24° 26 min
2-25
TM 9-258
b. The small critical angle of diamond accounts for
the brilliance of a well-cut diamond. It is due to a total
internal reflection of light which occurs for a greater
variety of angles than in any other
substance. The light entering the diamond is totally
reflected back and forth a great number of times before
emerging, producing bright multiple reflections.
Section IV. IMAGE FORMATION
(2) Real images. A real image actually exists.
It can be thrown upon a screen. The lenses of the
human eye and the camera form real images. The real
image formed by a camera can be seen on the ground
glass (fig 2-38).
2-18. General. In considering the principles of the
formation of images by lenses, for simplicity of
explanation it will be assumed that perfect single lenses
are used.
2-19. Classification of Images.
a. Virtual and Real Images. Two types of images
are produced by optical elements, virtual images and real
images.
(1) Virtual Images. A virtual image is so called
because it has no real existence. It exists only in the
mind and cannot be thrown upon a screen because it is
apparent only to the eyes of the observer. A familiar
example is the virtual image formed by a mirror. The
image of the person looking into the mirror appears to be
on the other side of the mirror (fig 2-37). No trace of the
image can be seen on the back of the mirror. A plane
mirror produces a virtual image resulting in an optical
illusion in which the object appears to be behind the
mirror.
Figure 2-38. Real image formed on ground glass of
camera.
b. Erect, Inverted, Normal, and Reverted Images.
When an image is reflected or refracted by optical
elements, the parts of the object may be seen as being
transposed horizontally, vertically, or both. This is
illustrated by what happens to the letter F shown in figure
2-39 and by B, figure 2-36.
Figure 2-37. Virtual image reflected by plane mirror.
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TM 9-258
Figure 2-39. Letter F as reflected by optical elements
(1) An image, regardless of size, that shows
the face of an object unchanged is said to be normal and
erect (A, fig 2-39). When the object is seen as
horizontally transposed through 1800, it is termed
reverted and erect (B, fig 2-49). Reversion is the effect
seen in a vertical mirror where the image is reversed so
that the right side of the object becomes the left side of
the image (fig 2-37).
(2) The image of an object with its face
unchanged but upside down in termed normal and
inverted (C, fig 2-39). Inversion is the effect seen in a
horizontal mirror where the image is inverted so that the
top of the object becomes the bottom of the image. If it
appears turned from left to right as well as upside down,
it is said to be reverted and inverted (D, 2-39). This is
the effect seen on the ground glass of a camera
(fig 2-38).
mirror, it appears to be located at a distance behind the
reflecting surface equal to the distance of the object from
the front of the mirror. The image is erect and reverted
as are all single reflections from vertical plan mirrors.
b. In a dark room, the image of a tiny point of light
as viewed in the mirror appears to the observer to be
located behind the mirror and on the other side of the
room where it actually is. The observer sees along the
path of the reflected ray to the point where the ir dent ray
is reflected by the mirror. His line of sir :is extended in
his mind in a direct line through and beyond the mirror.
The apparent position of the point of light in the mirror is
located directly across the room from the light source
and at the same distance behind the mirror as the light
source is in front of the mirror (EYE "A", fig 2-40).
2-20. Image Reflection by Plane Mirror.
a. When the image of an object is seen in a
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TM 9-258
Figure 2-40. Reflection of point of light in mirror.
c. Regardless of where the observer stands, if he
can see the reflection of the point of light in the mirror, its
apparent position is unchanged. This holds true whether
he walks forward along his line of sight without affecting
the angles of incidence and reflection of the ray or steps
to either side changing his line of sight and the angles of
incidence and reflection (EYE"B", fig 2-40). It is the
object (source of light) that is reflected and the apparent
position of its reflection is changed only when the
position of the object or of the mirror is changed.
d. Now, assume that the light source is replaced
by a cutout letter F covered with luminous paint (fig 2-41).
Light from every point on the letter sends out
incident rays which are reflected by the mirror. Each
incident ray and reflected ray obeys the laws of reflection
and their paths can be plotted accordingly. As the result,
the entire image formed by the combination of this
infinite number of images of individual points of light is
reflected to the eye of the observer. The observer,
looking along the paths of the reflected rays, sees the
image formed by the points of light. The image he sees
appears to be in back of the mirror as an erect, reverted
image (fig 2-41).
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TM 9-258
Figure 2-41. Reflection of letter F in mirror.
(1) Consider two mirrors placed at 900 angles
to one another. The light coming from the letter F falls
onto the reflecting surface of one of the mirrors
(fig 2-42). Light rays from every point on the letter F are
reflected by the first mirror, according to the laws of
reflection, to the second mirror, and reflected again, this
time parallel to the original paths of the light rays. The
image will have been reflected a total of 180ƒ.
2-21. Image Transmission by Mirror and Prism.
a. Image Transmission by Mirror.
Image
transmission by mirror is image reflection put to practical
use. The mirror is mounted so that it will transmit the
light that falls upon it to whatever point is desired. If this
cannot be accomplished with a single mirror, a second
mirror is placed to catch the reflected light from the first
mirror and transmit it again.
2-29
TM 9-258
Figure 2-42. Image transmission by mirrors.
Figure 2-43. Image transmission by prism.
c. Comparison. When mirrors are used, the image
is reflected through air; when a prism is used, the image
is reflected through glass. While being transmitted
through glass, the paths of the rays of light are not
deviated because they travel in a medium of constant
optical density. These paths are not deviated upon
entering or leaving the glass of the prism provided the
light rays strike the surface of the glass in the direction of
the normal, at right angles to the surface.
(2) The combination of two mirrors at 90° angle
does not invert or revert the image when reflection takes
place in the horizontal plane (the figure shows the rear of
the object and the front of the image). If reflection takes
place in the vertical plane an inverted, reverted image is
obtained.
b. Image Transmission by Prism.
Image
transmission by prism can be brought about in practically
the same manner as an image is transmitted by one
plane mirror (B, fig 2-36) or two plane mirrors (fig 2-43).
Actually, the reflecting surfaces of a prism are plane
mirrors. These surfaces are silvered when the angle at
which the light strikes is less than the critical angle (para
2-16) of the material from which the prism is made; the
surfaces require no reflecting agent when this angle is
greater than the critical angle.
2-22. Focal Length and Focal Plane.
a. Focal Point of Convergent Lens. When light
strikes and passes through a convergent lens, the rays
from a point of light intersect at a point on the opposite
side of the lens. The point at which the rays intersect is
termed the focal point.
b. Focal Point-Location. The focal point of a lens
will vary in relation to the distance of the
2-30
TM 9-258
object. If a sheet of ground glass or thin paper is placed
across the back of a camera or held in back of a lens at
a distance that will permit the lens to focus the images of
objects on the glass or paper, images of all distant
objects may be brought into sharp focus at one time
regardless of their distance providing the distance is
great -and rays of light are almost parallel. An object
very close to the lens will be out of focus. The ground
glass or paper will have to be moved backward to bring
such an object into sharp focus; then the more distant
objects will be out of focus.
c. Point of Principal Focus. The focal point where
almost parallel rays from a very distant object intersect is
called the point of principal
focus (fig 2-44). The point of principal focus of a lens
does not vary, it always remains at the same distance
from the lens. The optical center of a lens is a point at
the center of the thickest part of a converging lens or the
thinnest part of a divergent lens. It lies on a straight line
connecting all the principal focal points of the lens (fig 245). Rays of light pass through the optical center without
deviation. The distance from the point of principal focus
to the optical center of a convergent lens is the focal
length of the lens. This is a fundamental concept which
must be kept in mind throughout to understand optical
diagrams. The thicker the lens in relation to its size or
the steeper the curves of the lens, the shorter its focal
length will be.
Figure 2-44. Focal lengths of convergent lens.
d. Focal Plane. The focal plane of the lens (fig 244) is a thin flat area at right angles to the central axis of
the lens in all directions from the focal point. It is the
area that is occupied by the ground glass or paper which
might be used to make the image visible. In a camera,
the film is placed in the focal plane of the lens. The focal
point (fig 2-46) is the image of a single point of light on
the object; the focal plane is the combined image of all
the points of light on the object. The principal focal plane
of the lens is the focal plane passing through the point of
principal focus (fig 2-44). In a camera, when taking a
photograph of very distant objects, the film is placed in
the principal focal plane of the lens.
e. To Determine Focal Length of Convergent
Figure 2-45. Optical center of lens.
2-31
TM 9-258
Lens. A rough determination of the focal length of a
convergent lens can be secured by holding the lens to
focus the image of some distant object on a sheet of
paper or ground glass. When the image is clear and
sharp, this will indicate that the point of principal focus
has been reached. Measure the distance from the
image to the optical center of the lens.
f. Focal Length of Divergent Lens. The point of
principal focus and any focal points and focal
planes resulting from the nearness of the object to the
lens are located on the side of the divergent lens toward
the object or light source. The point of principal focus
and other focal points are located where the emergent
rays would intersect on the axis if they were extended
backward as imaginary lines toward the side of the lens
on which the light strikes (fig 2-46). The distance from
the point of principal focus to the optical center of the
lens is the focal length.
Figure 2-46. Focal length of divergent lens.
telescope will produce an image, the apparent size of
which is eight times as high and eight times as wide as
the image when viewed by the unaided eye.
d. Military targets can often be seen better with 10power magnification than with 20-power, due to the
change in the field of vision which accompanies a
change in power and due to varying conditions of light.
For this reason, some fire-control instruments are fitted
with means for changing the power of magnification.
Certain instruments provide no magnification; their
primary purpose is to place a reticle in the field of vision.
2-23. Magnification and Reduction.
a. Magnification is the apparent enlargement of the
image of the object to the eye of the observer. Common
examples are the reading glass and microscope. Inafirecontrolinstrument, magnification, by marking targets
appear larger, makes them appear closer.
b. Optical reduction is the apparent reduction in the
size of the image of the object to the observer. A
negative or divergent lens placed in the floor of an
airplane to permit observation functions on this principle.
It makes objects appear smaller than when viewed with
the unaided eye but has the advantage of increasing the
field of view. A much larger area may be scanned
through the relatively small opening than would be
possible if the lens were not used.
c. The degree of magnification of an optical
instrument is expressed as the power of the instrument,
for example, 8-power. An 8 power (8X)
2-24. Image Formation by Convergent Lens.
a. Types of Images. Images which are formed by
convergent lenses may be either real or virtual images.
They may be larger or smaller than the object, depending
b.
upon the distance of the object from the lens.
Perfect Lens Assumed. Multiple or com2-32
TM 9-258
pound lenses (para 4-2) may be used to correct for color
and other faults. A theoretically perfect single lens is
assumed in this paragraph.
c. Plotting the Image. To plot the formation of an
image by a convergent lens, consider that every
point of an object is emitting cones of rays of light which
are incident upon the front face of the lens. Two rays
from a point will be sufficient to indicate the action of the
rays and to located the image of this point (fig 2-47).
Figure 2-47. Image formed by convergent lens.
(1) Assume that one ray from a point at the top
of the object will pass through the center of the lens.
Since this ray will strike the lens at the center, its path is
not deviated. A second ray from this point of the object
strikes the lens above the axis, is refracted upon entering
and upon leaving the optically denser medium, and
intersects the first ray, For this second ray, we chose a
ray parallel to the axis. After passing through the lens, it
will converge to the point of principal focus and thus can
be
easily
located.
The
point
where
the two rays intersect becomes the corresponding point
for the image.
(2) Two similar rays from a point at the bottom
of the object form a point of the image corresponding to
the bottom of the object. Every point on the object forms
its point of light on the image in the same way. Light
rays from the upper part of the object form points of light
on the lower part of the image and vice versa. The
image is transposed diametrically and symmetrically
across the optical axis from the object resulting in in-
2-33
TM 9-258
version and reversion as in figure 2-47. An image
formed in this way is real.
d. Object Location and Image Size. The image of
an object located at any distance beyond two focal
lengths of the lens is smaller than the object. If the
object is at two focal lengths, the image is the same size.
If the object is between one and two focal lengths away,
the image is magnified. All of these images are inverted,
reverted, and real. If the object is at one focal length, the
rays are parallel after refraction; the image is said to be
at infinity.
(2) Magnification by reading glass. If the object
is inside one focal length of a convergent lens, the image
will be on the same side of the lens as the object
magnified, normal, and erect. The image will no longer
be real but will be virtual. This is a condition which can
be illustrated easily by the use of a reading glass (figs 248 and 2-69). Eyepieces of all optical instruments
provide the eye with a virtual image. If the observer
looks at the object, for example, a portion of a printed
page, through a reading glass held close to the type, the
printed letters appear larger than they actually are. As
he moves the glass away from the type, each letter
appears larger and remains clear provided he moves his
eye backward as he moves the glass away from the
printed page. At a certain point the printed letters appear
at their largest and clearest. Beyond this point they
become blurred and then inverted. The time when the
largest and clearest view of the letters is obtained is just
before the focal point of the lens has been reached. The
rays from the object can be plotted for any point in the
preceding demonstration (fig 2-48). A ray of light
through the center of the lens passes through in a
straight line unchanged. A ray which is originally parallel
to the axis is directed towards the point of principal focus
(para 2-22). The reason for the magnification of the
image is clear considering that the virtual (not real)
image is located where the emergent rays would
intersect if they were extended backward as imaginary
lines. These rays form a magnified image of the object
which appears to be in back of the object.
e. Magnification by Convergent Lens.
(1) Conjugate foci. These are two points so
related that an object at either point will be imaged by a
lens at the other point and will be a real image. The
mathematical relationship is as follows
1
Object Distance
or
1
0d
plus
1
Id
1
1
Image Distance Focal Length
1
f
(fig 2-47)
Magnification = Size of Image = Image Distance
Size of Object Object Distance
If any three of the above quantities are known, it is
possible to determine the fourth by the application of
simple algebra. To state magnification another way, it is
simply comparing the size of the image with the size of
the object or:
Magnification= Image Size
Object Size
Y
M = Y1=f
Y Od-f
f=x
x
Y1
Y
= Id
Od
or
1
f
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TM 9-258
Figure 2-48. Magnification by reading glass-object within focal length.
2-25.
distant object strike the face of a divergent lens in almost
parallel rays and, after refraction, are diverted by the lens
away from the axis (fig 2-49). In looking along the paths
of the rays, the observer sees an apparent intersection of
the rays at a point on the same side of the lens as the
object. The point where the rays appear to intersect is
the focal point. It is backward continuation of the
emergent rays as imaginary lines. As the observer looks
along the paths of these rays, he sees the virtual (not
real)--image of the object located inside the intersection
of these imaginary continuation lines (fig 2-50).
Image Formation by Divergent Lens.
a. Type of Image. Divergent lenses can form only
virtual images which are erect and smaller than the
object. This holds true regardless of the distance of the
object from the lens. Divergent lenses are termed
negative because they will produce only an image
smaller than the object, they produce only virtual images,
and they deviate the light outward. The virtual images
formed by them are always on the same side of
b. Plotting the Image. Light rays from a very
2-35
TM 9-258
Figure 2-49. Effect of parallel rays on divergent lens.
Figure 2-50. Reduction by divergent (negative) lens.
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TM 9-258
c. Image Size.
An observer receives the
impression that the image seen on the other side of the
lens is smaller than the object. This effect is known as
reduction as contrasted with
magnification produced by a simple magnifier. It is
useful in some camera viewfinders, but since it does not
magnify or produce a real image its usefulness is limited
in military instruments.
Section V. COLOR
2-26.
through a triangular cross-section glass prism, the
emergent light is split into a number of refracted rays. If
these refracted rays are made to fall upon a screen, they
produce a band of colored light called the solar
spectrum. The colors of the spectrum are, on order; red,
orange, yellow, green, blue, indigo, and violet. About
130 distinct hues have been observed.
b. Each color of the spectrum represents light
vibrating at a different frequency or wavelength. The
shorter the wavelength, the more the rays are bent upon
being refracted. Red light has the longest wavelength
(A, fig 2-51); violet, the shortest. Red rays are bent the
least; violet, the most; the rays of other colors are
interspaced accordingly
General.
a. Ordinary sunlight or any other "white" light is a
mixture of visible light of all wavelengths. "White" light,
is, therefore, a mixture of all colors.
b. The amount of deviation of light from its path
depends upon the color of the light as well as upon the
shape and optical density of the optical element. Each
color of light has its own distinctive wavelength and there
is a different index of refraction for each color. The
unequal deviation of rays of light of various colors,
dispersion as it is called, has an important effect on the
formation of images, and, therefore, upon the design of
fire-control instruments.
2-27. Separation of "White" Light.
a. If sunlight is passed through a slit and then
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TM 9-258
Figure 2-51. Dispersion.
c. Glass of various optical densities disperses light
to different degress. A prism of flint glass disperses the
rays of different color more than the less optically dense
crown glass. A lens would also separate light because a
lens may be considered as an infinite number of prisms.
The properties of different kinds of glass are employed to
neutralize the dispersion of the rays of colored light in
lenses and prisms, and to neutralize their tendencies to
form different focal points when refracted through lenses
(para 2-39).
d. The principle of dispersion is put to practical use
in a scientific instrument known as the spectroscope.
This instrument is especially designed to disperse light
coming from an incandescent source.
Since any
element upon being
heated to a point of incandescence emits light of a
frequency unique to that element, the spectrum of an
incandescent solid can be used to show of what that
solid is composed. By the use of the spectroscope,
helium was discovered in the atmosphere of the sun
before it was discovered on earth.
e. An instrument similar to the spectroscope but
fitted with a camera permits the photographing of
spectra.
The photographic records made by this
instrument are called spectrographs. This instrument is
also used to measure refractive indices and is extremely
accurate.
f. The rainbow is a natural phenomenon resulting
from refraction and total internal reflection of sunlight in
raindrops (B, fig 2-51).
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TM 9-258
reflection.
b. An object appears to be of a color because it
absorbs light of certain wavelengths and reflects others
striking it. In sunlight, it may appear to be of one color
because it reflects the light of that color and absorbs all
others. Under a single color or monochromatic light, it
might appear to be of another color or almost black,
depending upon what colors and how much light it
reflected.
2-28. Selective Reflection and Absorption.
a. Objects derive their color from the selective
reflection and selective absorption of light. If an object
reflects practically all of the light striking it, the object
appears white. If it absorbs the greatest percentage of
light, it appears black. The surfaces of most objects
absorb light of certain frequencies and reflect light of
other frequencies. The colors of these surfaces are
determined by the frequencies they reflect. This is
known as selective
Section VI. CHARACTERISTICS OF OPTICAL SYSTEMS
eyepiece magnifies the image formed by the objective, it
cannot supply sharpness or detail lacking in the image.
The shorter the focus of the eyepiece, the greater is the
magnifying power.
2-29. General.
Military fire-control instruments
employing optical systems are required for a great
variety of purposes. To cite a few, these instruments are
used to see at great distances, to magnify small objects,
to determine positions, to gage distance, and to aim
weapons.
2-32. Field of View.
a. The true field of view of any optical instrument is
the extent (the width and height) of what can be seen at
one time by looking through the instrument. The field of
view is cone-shaped; it becomes wider and higher as the
distance is increased from the objective. In the majority
of fire-control instruments the angle of the true field of
view is relatively small (para 5-25).
b. The true field is limited by the optical possibilities
of the eyepiece(fig. 2-52). The maximum angle which
might be imaged by the eyepiece, if it were not for a
number of limiting factors, is termed the apparent field of
view (para 5-26)
2-30. Military Telescopes. Necessary characteristics
of military optical instruments are usually the following a
minimum of weight and bulk, the largest possible field of
view and brightness of image consistent with the
necessary magnifying power; freedom from distortion;
and a combination of ruggedness with simplicity. The
instrument must form a normal erect image of the object.
In the majority of instruments, the optical system must
include a reticle pattern.
2-31. Magnification. Magnification of a telescope
optical system depends both upon the objective and the
eyelens or eyepiece. Although the
Figure 2-52. Field of view limited by optical possibilities of eyepiece.
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TM 9-258
c. An apparent field of view of approximately 450
may be considered as a practical maximum for a highly
corrected eyepiece. Thirty--five or 400 is a more
common value and in the more simply constructed
eyepieces the field may be only 250 or 300. This
apparent field of view, divided by the power of the
eyepiece, determines the maximum value of the true
field for any given magnification. For example, if an
eyepiece corrected for an apparent field of 400 is used
and a telescope of 10-power is required, the maximum
true field of the instrument would be only 4°.
d. Other factors can further limit this maximum true
field. The components of the erecting system of the
instrument might not be sufficiently large to transmit the
full useful area of the image and would decrease the
field. Some aperture in the instrument might be so small
as to restrict the field.
2-33. Brightness of Image.
a. Brightness of Image and Exit Pupil. The more
light that can be brought from each point of the object to
the eye, the brighter will be the image that is formed.
This characteristic is known as brightness of image. The
point where the image is brought to the eye is called the
exit pupil of the instrument.
b. Magnification and Brightness. When one looks
at very distant objects with the naked eye, almost parallel
rays of light enter the pupil of the eye and form an image
of a certain size on the retina. When one uses a
telescope of 2-power magnification, the light forms an
image on the retina which is twice as high and twice as
wide or an image that covers four times the area of the
image that would be formed by the naked eye of the
same parts of the objects (fig 2-53).
Figure 2-53. Comparison of light entering eye with 2-power telescope.
c. Loss of Light. There is always a considerable
loss of light by absorption and by reflections at the
surfaces of the lenses. While this loss may be as great
as 75 percent, every effort is made to reduce the loss to
a minimum (para 4-13).
d. Objective Aperture. A contributing factor in
brightness of image is an objective lens aperture (usable
portion) large enough to permit the eyelens to produce
an emergent beam that will fill the pupil of the eye. On
the other hand, enlarging the objective beyond this point
affords no greater brilliance because additional light is
prevented by the iris from entering the eye.
e. Exit Pupil Aperture. During the day, the pupil of
the eye is from 1/10 to 2/10 inch in diameter. At night,
the pupil may dilate until its
diameter is from 1/4 to 3/10 inch. A telescope or other
instrument for use at night should have an exit pupil
aperture (fig. 2-54) of this size. The aperture of the
objective must then be sufficiently large (1/4 to 3/10 inch
times the magnification of the instrument) to insure that
the pencil of light will fill the pupil of the eye. During the
daytime, the useful aperture of the objective of such an
instrument is correspondingly smaller.
f. Sacrificing Brightness of Image. Some of the
telescopes used for military fire control purposes are
made with objective smaller than that indicated by/this
formula. Brightness of image has been sacrificed to
obtain a more compact instrument.
g. Aperture Ratio of Camera Lens. This is
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TM 9-258
the ratio of focal length to lens diameter of aperture. It is
also known as the speed of a camera lens and is written
as f/16 (for example) or f/3. 5. At f/16 the lens is being
used at an aperture (opening) of 1/16 of its focal length
and at f/3. 5 the lens is being used at an aperture of 1/3.
5 of its focal length. As the denominator increases, the
focus becomes sharper (lessened aberration) but the
light and brightness of image decrease as the aperture is
smaller. The time of exposure, therefore, must be
lengthened proportionally.
As the denominator
decreases, the lens is said to become faster (wider
aperture or lens opening) and it can be used at faster
shutter speeds (shorter exposure time). If it is desired to
photograph action, a fast lens must be used; otherwise,
the picture will be blurred from the motion.
2-34. Eye Distance or Eye Relief.
a. The exit pupil (fig. 2-54) is the term applied to
the rays of light which emerge from any optical
instrument with a collective or convergent eyepiece and
form the image that is seen by the eye. The exit pupil of
such an instrument can be seen as a brightly illuminated
disk floating in space if the instrument is directed toward
an illuminated area and its eyepiece is held about 15
inches from the eye. The position of the exit pupil can be
fixed on a screen of translucent paper or on a plate of
ground glass. In this manner, the distance of the exit
pupil from the last glass surface of the eyepiece can be
measured.
Figure 2-54. Exit pupil.
termed the eye distance or eye relief (fig 2-55). The eye
must be placed at this distance from a collective
eyepiece in order to see most effectively through the
instrument.
Figure 2-55. Eye distance.
c. If the eye distance of an instrument is so limited
that the exit pupil cannot enter the eye pupil or if the
eyelashes touch the eyelens, observation through the
instrument will become tiresome and the instrument
cannot be used most effectively. When the instrument is
to be mounted on a weapon, if the eye distance is not
considerable, recoil, of the gun will make observation
dangerous.
d. In the design of the instrument, the proper
location of the exit pupil must be given most careful
consideration. If it is too far back, the eyepiece will
become too cumbersome in design; if it is placed too
close, the user of the instrument will suffer discomfort
and will be unable to use the instrument to the best
advantage.
Proper eye distance depends on the
purpose for which the instrument is to be used
(para 5-27).
2-35. Interpupillary Distance.
a. The spacing between the pupils of the eyes
make stereovision possible because each eye views an
object from a slightly different viewpoint. From the fused
image gained from the two viewpoints, the observer
receives the impression of depth. This spacing of the
eye pupils varies to a considerable degree in different
individuals and is known as Interpupillary distance. This
distance is measured in millimeters by the
interpupillometer M1.
b. In the designing of instruments to be used by
b. The full field of view of a telescope can be seen
only when the pupil of the eye of the observer is at the
same position as the exit pupil. The distance from the
eyelens to the exit pupil is
2-41
TM 9-258
both eyes of an individual, provision must be made to
adjust the spacing between the eyepieces of the
instrument to conform to the interpupillary distance of
different observers. The interpupillary distance of such
instruments generally is adjustable from 58 to 72
millimeters because the eye spacing of nearly everyone
is within this range. If the interpupillary distance of an
individual is greater than 72 millimeters or less than 58
millimeters, he is incapable of using instruments of this
type to the best advantage.
2-36.
movement. The two lenses of the eyepiece are mounted
in a simple tube and its distance from the reticle or focal
point of the objective or erecting system can be adjusted
by rack and pinion, by a simple draw tube or by rotating
the entire eyepiece causing it to screw in or out.
d. On fire-control instruments a graduated scale
generally is provided around the eyepiece. This scale is
calibrated in diopters.
The diopter is the unit of
measurement of the converging power of lenses (para
10-2). For a normal eye the scale on the eyepiece is set
at zero.
If a positive 2-diopter spectacle lens is
commonly worn, the eyepiece should be set at +2
provided the spectacles are removed as they should be.
After carefully focusing the instrument, the reading of the
eyepiece should be memorized and used for future
focusing.
e. Low-power telescopes are frequently made
without any means for focusing. Such an instrument is
termed a fixed-focus telescope. When assembled, such
an instrument is often so adjusted that light from a
distant source emerging from the instrument, instead of
being essentially parallel, appears to diverge from a point
40 to 80 inches in front of the eyelens. A telescope so
focused (approximately minus 3/4 to minus 1 diopter) is
more readily adaptable to the eye of the average
observer than one focused so that the rays of light are
essentially parallel. Because a fixed-focus instrument is
simple, it can be made entirely waterproof. However, if
the power of the telescope is greater than 3. 5 or 4, the
accommodation of the average eye is not sufficient to
permit its use.
Focusing.
a. For perfect focusing of a monocular instrument
under all conditions, two adjustments are necessary. If
the instrument has a reticle, means must be provided for
adjusting the distance between the reticle and the
objective so that there will be no parallax (para 2-46).
Also, the distance between the eyepiece and the reticle
must be adjusted for the eye of the observer. In a
binocular instrument, it also is necessary to adjust the
instrument to conform to the interpupillary distance of the
eyes of the observer.
b. If the focal length of the objective is short and
the targets in general are at a great distance, the
objective may be adjusted for a target at an infinite
distance when the instrument is assembled in the factory
and no field adjustment is provided. This is the case with
some fire-control instruments and low-power telescopes.
c. Focusing eyepieces are commonly found on firecontrol instruments. This adjustment is primarily
designed for focusing the instrument for different eyes
and is referred to as the diopter
Section VII. ABERRATIONS AND OTHER OPTICAL DEFECTS
2-37.
coma, curvature of image, and distortion. Other factors
which may affect the operation of the optical system of
an instrument are resolving power, Newton’s rings, light
loss, and parallax.
General.
a. Aberration is a lens or prism imperfection
resulting in an image that is not a true reproduction of the
object.
b. In designing an instrument, correction is usually
made for optical defects with special attention being
given to the use to which the instrument is to be put.
Correction is achieved by using lenses or prisms made
of two or more kinds of glass (called compound lenses or
compound prisms), and by eliminating rays which would
be refracted through the outer edges of lenses (called
marginal rays) by equipping the instrument with
diaphragms (field stops) (para 2-38 e), and a suitable
eyepiece.
c. There are six general types of aberrations:
spherical and chromatic aberrations, astigmatism,
2-38.
Spherical Aberration.
a. Light rays refracted through a lens with spherical
surfaces near its center and those refracted through the
outer portion or margin do not intersect the axis at a
single point. The outer rays of a convergent lens
intersect the axis closer to the lens than the more central
ones (fig 2-56) and the opposite is true of a convergent
lens, considering the imaginary extension of the
refracted rays (fig 2-57). The result is a blurred image.
This fault is common to all single lenses with spherical
surfaces and is termed spherical aberration.
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TM 9-258
b. The thickness of a lens and its focal length
influence the amount of spherical aberration. Spherical
aberration is least in thin lenses of long focal length.
c. If the convergent lens were ground with
constantly flattening curves to the edge instead of being
ground with each refracting surface as a true portion of a
sphere, the rays would be caused to cross the axis at
nearly a single point and a sharp image would result.
Such lenses, however, would be too difficult and costly to
manufacture and would be correct for only one distance
from the object. Instead, two other methods are utilized
either singly or together which satisfactorily eliminate
spherical aberration (f and g below).
d. Hourglass distortion, barrel distortion, and
curvature of image all result from spherical aberration
(para 2-42). Replacing lenses so they face the wrong
way ’usually will introduce distortion into the system.
This is of interest, therefore, to the instrument repairman.
e. In a lens system such as a complex camera
objective or in a single lens used at a wide angle,
spherical aberration can be reduced at the expense of
light intensity by using an aperture stop, field stop, or
diaphragm. The central portion of a lens is most free of
spherical aberration. Tests of a lens will show how much
of the area around the axis may be safely used to form a
sharp image. The practice is to mask out all rays
passing through the lens beyond this circle (A, fig 2-57).
The mask used for this purpose is led an aperture stop.
It is a flat ring or diaphragm of metal or other opaque
material covering the outer portion of the lens. It stops
the rays from entering the margin of the lens but, as it
cuts down the effective size of the lens, it limits the
amount of light passing through the lens. Obviously, in
general, it would be cheaper to manufacture a 4maller
lens rather than to make a large lens and a stop for
masking out marginal rays. Diaphragms or stops have
other uses. See diaphragms or stops in paragraph 4-10.
Figure 2-56. Spherical aberration of convergent lens.
Figure 2-57. Spherical aberration of divergent lens.
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TM 9-258
Figure 2-59. Effect of compound lens on spherical
aberration .
g. Spherical aberration is minimized by "bending"
the lens. Bending is accomplished by increasing the
curvature of one surface and decreasing that of the
other. This tends to eliminate spherical aberration while
retaining the same focal length. In telescope design, it is
common practice to minimize spherical aberration in yet
another way by placing the greater curvature of each
lens toward the parallel rays so the deviation at each
surface is nearly equal as in B, figure 2-58. The angles
of incidence and emergence must be equal for minimum
spherical aberration. In accord with this rule, telescope
objectives always are assembled with greater curvature
(the crown side) facing forward.
2-39.
Chromatic (Color) Aberration.
a. White light consists of all colors. Upon being
refracted through a prism, white light is dispersed into
rays of different wavelengths forming a spectrum of
various colors (para 2-27). The rays of different colors
are refracted to different extents; red undergoing the
least refraction and violet the most. Inasmuch as a lens
may be thought of as being composed of an infinite
number of prisms, this dispersion also exists where light
is refracted through a lens. This produces an optical
defect, present in every uncorrected single lens, known
as chromatic aberration or chromatism. The violet rays
focus nearer the lens than the red rays (fig 2-60) and the
rays of the other colors focus at intermediate points.
Thus, such a lens would have a different focal length for
each color and the image would be fringed with
Figure 2-58. Spherical aberration reduced .
f. In fire-control instruments, spherical aberration is
commonly eliminated by the use of a convergent and a
divergent lens cemented together to form a single
element known as a compound lens (para 4-2). The
compound lens approximately corrects spherical
aberration because the concave curves of the divergent
lens neutralize the positive aberration of the convex
curves. The refractive power of the combination is
retained by the proper choice of indices of refraction for
each of the two lenses. A lens in which spherical and
other aberrations have been minimized or eliminated is
said to be aplanatic (fig 2-59).
2-44
TM 9-258
color. Similarly, spherical aberration is greater for blue
rays than for red rays because the focal length. of a lens
is shorter for blue than for red rays. This is due to red
rays being refracted the lesser of the two. The marginal
parts of a blue real image
formed by a positive lens will show compression, while
the marginal parts of a blue virtual image viewed through
a positive lens will be more distended than the red rays.
Figure 2-60. Chromatic Aberration
b. The aforementioned chromatic aberration, to
prevent trouble in optical instruments, must be corrected
by spacing between lenses and by adjusting curvatures.
A, figure 2-61, illustrates the diminishing to such effects
by equalizing the deviation at the two surfaces. Like
spherical aberration, chromatic aberration is corrected by
making a compound lens of two separate lenses; one
positive (convergent) and one negative (divergent). The
positive lens is made of crown glass while the negative
lens is made of flint glass (B, fig 2-61). Crown glass is
more strongly convergent for blue rays than for red, while
flint glass is more strongly divergent for blue rays than for
red. The high color dispersion of the flint glass negative
lens is sufficient to compensate for the lower color
dispersion of the crown glass positive lens without
entirely neutralizing its refractive power. A compound
lens so designed is said to be achromatic. Construction
of a lens of this type is described in paragraph 4-2b.
Figure 2-61. Correction of chromatic aberration of lens.
2-45
TM 9-258
c. Spherical and chromatic aberrations usually are
corrected by the same two elements of a compound
lens.
In some lenses, the negative element is
planoconcave (only one surface ground spherically). In
this case, the adjacent surfaces are ground to
compensate for these two defects. In others, the inner
curves of the lenses are calculated to eliminate
chromatic aberration and the outer curves to remove
spherical and other types of aberration.
d. The majority of prisms employed in fire-control
equipment are used to reflect light; some are used to
refract light. When light is reflected by a prism, there is
no chromatic aberration because the light rays are not
divided up or dispersed. Only when light is refracted by a
prism, as through a measuring wedge of a range
finder, is the light affected by chromatic aberration.
e. Chromatic aberration in a refracting prism is
corrected in much the same manner that it is in a lens.
Two prisms made of different kinds of glass are
cemented together (fig 2-62). The prism having the
larger refracting angle and which is to be the means of
refracting light from its normal path is made of crown
glass. The prism having the smaller refracting angle is
made of heavy flint glass which has a large dispersion of
colors. The flint prism, by reason of its large dispersion,
neutralizes the dispersion caused by the crown prism
without entirely neutralizing the deviation of the path of
light. This compound prism is known as an achromatic
prism.
Figure 2-62. Correction of chromatic aberration of prisms.
b. A perfect lens refracts rays from a point of light
to a sharply defined point of light on the image. The
useful rays which form the image are refracted as a cone
(fig 2-63). Each cross section of one of these cones is
circular in form; each successive circle becoming smaller
until the focal point is reached
.
2-40. Astigmatism in Lenses.
a. Astigmatism is a lens aberration which makes it
impossible to get images of lines equally sharp when
these lines run at angles to one another. This optical
defect is found in practically all but relatively complex
lenses known as anastigmats which are designed to
practically eliminate this condition.
2-46
TM 9-258
Figure 2-63. Proper refraction of light .
c. A lens with spherical or plane faces properly
ground will not show astigmatism for points near the axis
but will show astigmatism for points laying at a
considerable distance from the axis. The face of the
lens is then presented obliquely to the incoming light
rays. Cross sections of the cone of light refracted by the
lens become successively narrow ovals until they
become a line in the
vertical focal plane; then they become broader ovals until
they are circular; and then they again become a line in
the horizontal focal plane at right angles to the first line
(fig 2-64). Between the two focal planes is an area
known as the circle of least confusion. It is in this plane
that the most satisfactory image is secured.
2-47
TM 9-258
Figure 2-64. Astigmatic refraction of light .
d. Astigmatism is reduced to acceptable quality by
the use of several lenses in the same manner as
spherical and other aberrations. Lenses are made of
optical glasses possessing different degrees of
refraction, ground to different curvatures, so that the
aberrations of all types cancel each other.
2-41.
relatively wide portion of a lens. The lens may be
considered to be divided into concentric circular zones or
rings of varying thickness. To form a sharply defined
point of light, the rays from each zone must come to a
focus at exactly the same place in the focal plane.
c. In a lens producing coma, rays of light originating
at a point located off the axis and refracted through the
inner zone form a well defined image of the point. Rays
refracted through the next zone form a larger, lessdefined image of the point which is offset slightly from
the first. The image formed by each successive zone is
larger, less defined, and farther removed from the initial
point of light (fig 2-65). The displacement of the
successive images is in a direction toward or away from
the center of the field of the lens.
Coma.
a. Coma is due to unequal refracting power of the
various zones or concentric ring surfaces of a lens for
rays of light coming from a point which lies a distance off
the axis. It is caused by the rays from the various zones
coming to a focus at slightly different points so that they
are not exactly superimposed. It appears as blurring of
the images for points off the axis.
b. The image of a point of light is formed by a cone
of useful light rays refracted through a
2-48
TM 9-258
Figure 2-65. Coma
d. The total image of the point offset from the axis
may be any of a wide variety of patterns, an egg-1, pear-,
coma-, or comet-shaped blur (fig 2-66). Its general
appearance of a comet gives it the name of coma.
When viewed under a microscope, a point of light
influenced by coma may have a very fantastic shape due
to its being affected at one time by all types of
aberrations. Inasmuch as coma causes portions of
points of light to overlap others, the result is blurred
images of objects in the portion of the field affected by
coma.
e. In producting compound lenses the several
types of glass and the curves of the various faces are
carefully chosen to give approximate correction for coma
as well as for other aberrations. Aplantics are lenses
corrected for spherical, coma and chromatic aberration.
2-42. Curvature of Image and Distortion.
a. Definition of Distortion. Distortion is a form of
spherical aberration in which the relative location of the
images of different points of the object is incorrect.
When imaged by a lens having this defect, a straight line
extending across the field is curved. This is a serious
defect in instruments of high magnifying power because
the amount of distortion present is increased in
proportion to the power of the instrument.
b. Causes of Distortion. Distortion (fig. 2-67) is
particularly harmful when the object is held in close
proximity to lens and is caused by rays of light from
different points of the object being refracted by dissimilar
portions of a lens. When a line extends across the field,
is located close to lens, and is off clear, rays from the
middle of the line strike the lens nearer its center. These
rays are refracted at a different angle from that of rays
striking near the margin of the lens, giving a curved
instead of straight appearance to the image.
AR910072
Figure 2-66. Appearance of coma greatly magnified
2-49
TM 9-258
Figure 2-67. Barrel-shaped (A and B and hourglass (C and D) distortion .
c. Forms of Distortion. If the curvature of the lines
of the image is away from the center of the lens, the
distortion is termed barrel-shaped (A and B, fig 2-67). If
the lines curve toward the center, the distortion is called
hourglass or pincushion (C and D, fig 2-67).
d. Curvature of the Image. The field of a lens is the
area in which the image produced by the lens
is formed. This area should be flat in order to form an
undistorted image. When it is not flat, but is concave or
saucer-shaped, the condition is termed curvature of the
image or curvature of the field. An ordinary reading
glass plainly shows curvature of the image (fig 2-68). In
fire-control equipment this defect causes distortion in
portions of the field located a distance off the axis.
2-50
TM 9-258
Figure 2-68. Hourglass distortion in magnification by reading glass .
e. Formation of Image Curvature. Curvature of the
image is readily visualized by assuming that a cross is
imaged by a lens with this defect (A, fig 2-69). Rays
from points of light at the ends of the arms of the cross
are brought into focus nearer the lens than rays from the
center of the cross. If the
lens is focused on the enter of the cross, the ends of the
arms are not _ sharp focus and vice versa. This is
particularly disadvantageous in a photographic lens
where a flat plate or film is used. If the image is to be in
focus over the entire plate, the image also should be flat.
2-51
TM 9-258
Figure 2-69. Curvature of image .
f. Rays and Pencils. A definition of eccentric and
paraxial rays and of centric, eccentric, and paraxial
pencils will clarify curvature of image in spherical
aberration. Eccentric rays are rim rays which pass
through a lens remote from its center. Paraxial rays are
those along the axis of the lens Oblique centric pencils
are cones of light which pass through the center of a lens
at a considerable angle to the principal axis. Paraxial
pencils are
those along the axis. Eccentric pencils are those which
pass through the lens near the rim, remote from its
center.
g. Hourglass Distortion. Paraxial magnification of a
convergent lens (magnification through the lens near the
optical axis) is actually less than the true magnification of
the lens. Spherical aberration thus appears as hourglass
or pincushion distortion when a vertual image is viewed
through an un
2-52
TM 9-258
corrected lens as in C and D, figure 2-67.
The
extremities of such a virtual image will be curved away
from the lens so that we call this phenomenon curvature
of image as in B, figure 2-69.
h. Barrel Distortion. Eccentric rays refracted more
by a convergent lens and cross the axis closer to the
lens than paraxial rays. Oblique centric pencils also
focus closer to the lens than paraxial pencils. As the real
image is formed where the eccentric pencils and the
centric pencils come to a focus, it is obvious that such an
image formed by an uncorrected convergent lens will be
curved with the extremities of the image close to the lens
as in A, figure 2-69. This curvature of image is
especially troublesome in wide-angle instruments. This
image on a screen will present barrel distortion (A and B,
fig 2-67) and it will be impossible to focus all the image
clearly at one time on the screen. On a screen of the
same curvature as that of the image, no distortion or outof-focus effect would appear. The human eye used such
a screen as in C, figure 2-69.
i. Correction for Distortion and Curvature of Image.
Distortion or curvature of image is partially corrected by
employing a compound lens, consisting of one
convergent and one divergent lens, the two having
different types of distortion. In certain cases, field stops
or diaphragms may also be used to restrict the passage
of undesired marginal rays. A highly corrected distortionfree eyepiece or lens system is called orthoscopic; it
gives an image in correct normal proportions and
gives a flat field of view.
2-43. Resolving Power of Lens.
a. When two points viewed through a lens are so
close together that they cannot be distinguished as two
distinct points, these points are not resolved. To make
separation of such points possible, a lens of greater
resolving power is required.
The measure of the
resolving power of a lens is the limiting angle of
resolution which is the angle subtended at the optical
center of the lens by two points which are close together
and can be just barely distinguished as two distinct
separate points.
b. It might be supposed that a point of light on the
object on being refracted through a high quality
compound lens would form a single point of light on the
image. However, a factor termed diffraction tends to
enlarge the point of light so that it merges with another
point close to it.
(1) Diffraction causes the image of a point of
light to become a tiny disk of light surrounded by a series
of concentric rings of light which rapidly fall off in
intensity, a minute bull’s-eye target of light. This can be
demonstrated to the best advantage by sharply focusing
the lens on a very small single point of light. The image,
if it were greatly magnified, would resemble a series of
concentric rings (fig 2-70). The images of more detailed
objects are made up of these tiny bullseyes.
(2) Diffraction sets the final limit to the
sharpness of the image formed by a lens.
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Figure 2-70. Diffraction pattern, greatly magnified .
c. In practice, the resolving power of a lens
selected for a specific purpose is usually so great that
any lack of sharpness of detail is too slight to be
observed and, therefore, too slight to be objectionable in
the performance of the service for which the lens is
intended.
2-44. Newton’s Rings.
a. When position (convergent) and negative
(divergent) lenses of slightly unequal curvature are
pressed one against the other, irregular bands or
patches of color appear between the surfaces. This
pattern is called Newton’s rings (fig 2-71) after Sir Issac
Newton who directed attention to it. This condition is a
defect if it occurs in a compound lens. It may be utilized
to advantage as a means for testing the accuracy of
grinding and polishing lenses.
Figure 2-71. Newton’s rings .
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b. When testing lenses for accuracy of grinding and
polishing, a test lens having the desired curvature is
used. The test lens is placed in contact with the lens to
be tested, care being taken that both surfaces are
perfectly clean and dry. When the lenses have been
squeezed together, if the lens being tested is perfect, the
air film between the two lenses will be of uniform
thickness and the color will be uniform all over the
surface. Irregular bands or patches of color show that
the surface of the tested lens is not perfect and point out
clearly the parts of the lens needing attention.
elements have been coated produces brighter, effective
images.
2-46. Parallax.
a. Parallax is apparent displacement of one object
with respect to another due to the observer’s change of
position (A, fig 2-72).
b. Parallax in a telescope sight is observed as a
relative displacement (apparent movement) between the
reticle and the field of view. Parallax is present when the
reticle is displaced from the image plane; in other words,
it is present when the image is not formed at the reticle
but is located either in front of or behind the reticle as in
B, figure 2-72. Parallax can be eliminated either by
moving the objective lens, thus shifting the image, or by
moving the reticle to coincide with the image. In a highpower instrument (above 4X), it is necessary to provide
means of moving the objective to eliminate parallax if the
instrument is to be used at all ranges, for example, from
50 feet to infinity. The engineer’s transit has such an
objective-focusing device as do all target-type telescopic
sights used on rifles.
2-45. Light Loss.
a. Whenever light rays strike the surface of any
lens or prism, a certain amount of light is lost by
absorption and reflection. Light is absorbed by every
element it strikes or travels through. The more elements
in the optical system, the more light is lost by absorption
and reflection.
b. Light loss by reflection from surfaces has been
greatly reduced by coating the surfaces of elements
which are intended only to refract light (paras 4-13 thru
4-16). The additional light transmitted by instruments in
which various
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Figure 2-72. Parallax
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CHAPTER 3
THE HUMAN EYE
Section I. GENERAL
light, and response of the whole patchwork makes a
rough image but is coarse and inefficient in making
visible any details of the outline in pattern resolution.
3-1. Introduction.
All creatures endowed with sight
have eyes of some sort; some are simple and others
complex, some adapted to long range vision and others
to short distances. Animals and birds of the mountains
and plains, where vision is unobstructed for great
distances, have visual ability which enables them to pick
out objects too small for human perception. This ability
appears to be associated with small nerve, endings in
the retina, faster retinal response to observed motion,
and better interpretation by the brain. The other extreme
is represented by the short range compound eye as
found in insects and crabs. This arrangement consists
of a patchwork of individual "eyes," each of which
records a spot of shade or
3-2. Comparison of Eye and Camera. The human
eye, which is rather a sturdy organ of wonderful design,
may be called a living automatic camera. A high-grade
camera closely resembles the eye in basic essentials.
Each has a compound lens to refract light rays and to
project them to definite points by focusing. Each has a
diaphragm to regulate the entering light, a sensitized
surface to receive and record optical images, and a
lightproof chamber to shut out extrenous light and to
protect the receiving and recording mediums (fig 3-1).
Figure 3-1. Comparison of eye and camera .
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Section II. CONSTRUCTION
protection, supply it with nutrition, shut out extraneous
light, and transmit visual impressions to the brain (fig 32). It has a refracting mechanism consisting of the
crystalline lens, cornea, aqueous humor, and vitreous
humor; together, these form a compound lens. The
shape of the lens is changed by the ciliary muscles which
serve to change the focus of the lens. The iris serves as
a diaphragm to regulate the amount of incoming light
3-3. Structure of the Eye.
a. The eye is an organ nearly spherical in shape,
approximately an inch in diameter which rotates through
quite a wide range in a socket in the bone structure of
the head. Muscles control the eyes in associated
movements to a considerable extent and in individual
movement to a lesser degree. Protection and shade are
furnished by the eyelids, lashes, brows, and by the
configuration of bones.
b. The eye has three coats which afford it
Figure 3-2. Cross section of eyeball .
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c. The clearest and most distinct vision of the
response mechanism of the eye is centered in a small
area called the macula or yellow spot on the inner coat or
retina and in a still more sensitive smaller spot in the
center of the macula called the fovea or fovea centralis.
Sensations of light are received by tiny cones and rods
attached to nerves and distributed over the fovea,
macula, and retina. These sensations are transmitted to
the brain by the optic nerve at the lower rear of the
eyeball.
thin jelly-like substance, the vitreous humor, which is
transparent.
d. Light entering the eye passes successively
through the cornea, the aqueous-humor, the crystalline
lens, the vitreous humor, and falls on the retina.
3-5. Refracting Mechanism.
a. The chief refracting surfaces of the eye are
those of the cornea and the crystalline lens. The cornea
provides the constant portion of the refraction. The
crystalline lens supplies a variable degree of refraction
which permits the eye to focus on near or distant objects.
This gives to the eyes the ability to see far or near
objects distinctly and is termed the power of
accommodation.
b. The iris is a diaphragm which contracts and
dilates to regulate the amount of light entering the eye
(fig 3-3). The iris is the variously colored portion of the
eye around the pupil. The pupil is the circular opening in
the center of the iris. Its size is limited by the contraction
or dilation of the iris. It appears black because of the
comparative darkness of the inside of the eyeball. The
pupil varies in size from 2 to 8 millimeters, depending
upon the illumination. In intense illumination, the iris
steps the pupil down to about 2 millimeters. In moderate
(daytime) illumination, the opening is about 4 or 5
millimeters. This is considered to be the opening for
maximal acuity or best resolution. In very faint (night)
illumination, the diameter of the pupil is approximately 8
millimeters
3-4. Three Coats or Tunics.
a. The outer of the three coats or tunics of the eye
is the sclera (fig 3-2) to which are attached the six
muscles that hold the eye in place. It is tough, white, and
flexible and is the white portion of the eye normally seen.
The slightly protruding tough transparent portion at the
center front of the eye, the cornea, is part of this coat.
The cornea and the transparent liquid or aqueous humor
behind it are part of the refracting mechanism of the eye.
The aqueous humor is essentially a weak salt solution.
b. The middle coat, the choroid, is a deep purple
layer made up of veins and blood vessels which supplies
nourishment to the eye tissues. The coloring of the
choroid forms a dark housing and prevents external light
from diffusing into the eye through the walls of the
eyeball.
c. The inmost coat of the eye is the retina, a highly
sensitive layer of nerve fibres by means of which visual
impressions are transmitted to the brain. The retina is a
part of the response mechanism. The interior of the eye
is filled with a
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Figure 3-3. Action of iris compared with camera diaphragm .
c. The crystalline lens is transparent and is
suspended by the ligaments and muscles of the ciliary
body which encircles it (A, fig 3-4). The front of the lens
rests against the aqueous humor, the rear against the
vitreous humor. The suspensory ligaments and muscles
of the ciliary body draw upon or release the outer edges
of the lens to change its refracting power by altering its
shape.
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Figure 3-4. Suspension and action of crystalline lens .
d. The crystalline lens is double-convex and the
front surface is flatter than the rear (fig 3-2). When the
eye is relaxed (B, fig 3-4) the lens of a normal eye will
focus upon distant objects. To increase the refractive
power of the lens, for focusing the eye upon a close
object, the surfaces are made more strongly curved,
more convex by the compressive action of the tense
muscles of the ciliary body (C, fig 3-4). This process is
known as accommodation, permitting the normal adult
eye to view near objects as close as 25 centimeters
(about 10 inches). An object viewed at this distance is
said to be at the near point of the eye. The ability to
accommodate usually decreases as a person gets older
making the wearing of bifocals
necessary in most cases. Bifocals contain lenses having
a small section ground to a higher power through which
the user looks at near objects.
e. The image formed on the retina is inverted, but
we do not see the image inverted, as there is simply a
correspondence between the retina and external
directions.
3-6. Response Mechanism.
a. The response mechanism of the eye, the area
on which images are formed and examined consists of
the retina (fig 3-5) or thin inner coat of the eyeball which
is sensitive to light. The cause of light sensitiveness of
this area is an almost infinite number of visual cells.
They are connected by nerve fibers to the optic nerve
.
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Figure 3-5. Cones and rods of retina .
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b. The light sensitive elements or visual cells are of
two distinct types called rods and cones (fig 3-5). The
rods are cylindrical-shaped and longer than the cones;
the shorter cones are bulb-shaped. All are so small that
they must be magnified to 300 times their actual size to
be visible. While the mechanism of the eye has not been
fully explained it appears that the cones provide daylight
vision, where light intensities are high; the rods are more
sensitive to light, and give twilight vision, where light
intensities are low. The cones give clear sharp vision for
seeing small details and the distinguishing of colors; the
rods detect motion.
c. A slight depression in the retina on the visual
axis of the eye is called the macula or yellow spot. It is
about 2 millimeters in diameter and contains principally
cones and only a few rods. At the center of the macula
is the fovea or fovea centralis, where the retina is much
more highly developed than elsewhere, s small area
about 0.25 millimeter in diameter containing cones alone.
This area of the retina, therefore, gives the sharpest
detail and the best appreciation of color. No fibers of the
optic nerve overlie it.
d. The rods are stimulated by a substance known
as visual purple to increase their sensitiveness of
subdued illumination. This substance builds up and
accumulates during the time an eye is becoming
accustomed to darkness. An eye affected in this manner
is termed a dark-adapted eye. The visual purple is
bleached out by light. A different substance serves the
cones in the same way to produce what is known as a
light-adapted eye.
e. One small portion of the retina is insensitive to
all light. This is an area known as the blind spot or optic
disk. It is where the optic nerve enters the eye and is
located slightly to one side of the macula. In most
instances, this blind spot has no effect on vision because
the mind has learned to ignore it.
f. The optic nerve carries visual sensations to the
brain. This nerve is really a cable of nerves through
which the rods and cones activated by light images send
messages to the brain which result in what is termed
sight.
g. Foveal vision is much more acute than
extrafoveal vision and the muscles controlling the eye
always involuntarily rotate the eyeball until the image of
the object toward which the attention is directed falls on
the fovea.
h. The outer portions of the retina serve to give a
general view of the scene and to warn of an object
approaching within the field, while the fovea enables the
object of chief interest to be examined minutely. If the
eye is directed at a particular point on a printed page only
the words close to that point are seen distinctly.
i. An impression registered upon the retina and the
optic nerve persists, at least as a mental effect, for an
appreciable time (about 1/15 sec) after the stimulus has
been removed. Thus, the glowing end of a whirling stick
gives the impression of a streak of light. This is known
as persistence of vision. It is this effect which is the chief
aid in securing the illusion of smooth motion in moving
pictures so that we are not aware of the gaps between
successive pictures.
Section III. BINOCULAR (TWO-EYED) VISION STEROVISION
between the pupils of the eyes which enables the eyes to
see objects from slightly different angles. Eyes normally
are about 62 millimeters apart. This distance between
the eyes is called the interpupillary distance and is
abbreviated "ipd".
c. The ability to record with each eye a slightly
different picture or image of the same object often
enables the observer to see more of one side of a given
object with one eye than with the other. Together, the
two eyes see farther around the object than one eye can
see. They get a better impression of the shape of the
object, its depth, and its position with relation to other
objects. This ability is increased through the use of
certain firecontrol instruments which increase the virtual
distance between the pupils of the eye of the observer by
optical means.
3-7. General.
a. Binocular vision is coordinated two-eyed vision.
The two eyes of a person are normally identical and they
work together as a team.
The muscles used in
adaptation and accommodation of the eyes are
sympathetic in action; that is, they tend to dilate,
contract, and focus together. Both eyes usually meet
with the same light conditions and are turned to converge
on the same object or field of view. The two images
received by the eyes are not seen separately but are
fused by the brain into a single image.
b. Stereoscopic vision, stereovision, or stereopsis,
as it is called, is inseparably associated with binocular
vision because it is the result of seeing with both eyes.
Stereovision is the power of depth perception; the ability
to see in three dimensions. This power is due to the
spacing
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of interest is located, the eye turns in its socket until the
image falls upon the central portion, the fovea and
macula, and distinct vision is secured.
c. The field of vision of both eyes includes portions
which are viewed by the right eye, the left eye, and both
eyes (fig 3-6). The binocular field exists only in that
portion of the field of view where the field of the separate
eyes overlap. This explains why stereovision or depth
perception is appreciated only in the center of the field of
view, it is not apparent, except by association, in the
outward portions of the field. The extreme outer areas of
the field of view serve in the perception of motion, light,
and shadow and form in two dimensions.
3-8.Field of View.
a. What may be seen by the eyes rotated in their
sockets without movement of the head is termed the field
of view; what may be seen without movement of the eye
is termed the field of fixation. The field of view of the
eyes is a horizontal extent of about 1600 and a vertical
extent of about 70°.
In comparison, the average
telescope has a field of view of about 10°.
b. The field of distant vision of immobile eyes is
extremely limited, distinct vision being limited to only a
very small central portion of the retina. Portions of the
field focused on the remainder of the retina are seen
indistinctly and serve as a finder to locate objects of
interest. When an object
Figure 3-6. Field of view of the eyes .
(haze), light, and shadow; the difference in focus
(accommodation of the eye) required for more than one
object in the field of view; and the relative apparent
movement of distant objects as compared with that of
near ones when the head is moved from left to right, all
influence the estimate of distance.
c. None of these means of depth perception are of
value if its contributing factors are lacking. This is
difficult to realize completely in any practical case,
however, because one or more of these factors are
present in almost all commonplace observations and are
subconsciously used by persons with two-eyed vision
whenever possible to augment their stereovision.
Binocular vision can be investigated by placing two very
small objects on a large table and then scanning them
with the eyes
3-9. Principles of Stereovision.
a. In a military sense, stereovision implies the
ability to recognize the existence of differences in range
to objects by visual means only. It is an ability that can
be developed by training and practice. Certain fire
control instruments are designed to greatly increase the
normal stereovision possessed by the individual.
b. There are various means requiring the use of but
one eye which can be used to determine which of two
objects is the more distant.
(1) When one object is known to be larger than
another and both appear to be the same size, the larger
object is known to be more distant.
(2) If an object partly conceals another, the
second is recognized as being the farther away.
(3) Differences in the amount of atmosphere
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placed at the edge of the table. To gage their relative
distances, the use of both eyes and the employment of
stereovision will be required.
d. A simple demonstration of stereoscopic vision
can be made by looking at a near object such as a small
cube. The observer will see a different view with each
eye. The left eye will see the front and left side of the
cube (A, fig 3-7). The
right eye will see the front and right side (B3, fig 3-7).
The brain fuses these two separate pictures into a single
image giving the impression of depth. The observer
sees the front, both sides, and forms a conception of the
depth of the cube, and in this way pictures the cube in
three dimensions (C, fig 3-7).
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Figure 3-7. Cube is seen by left eye, right eye, and both eyes.
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e. In much the same manner when two objects are
observed simultaneously, stereovision enables the
observer to judge the relative distance of one object from
the other, in the direction away from the observer, or
depth. This can be done with considerable accuracy
without utilizing other characteristics which suggest
nearness of distance.
f. The ability to distinguish the relative positions of
two objects stereoscopically depends upon the
interpupillary distance of the observer’s eyes, the
distance of the objects from the observer, and their
distance from each other (A, fig 3-8). Other factors of
depth perception being equal, the wider the spacing
between the pupils of the eyes, the better appreciation of
depth, one should be able to secure by stereovision.
The farther one object is from the observer, the farther
away the second object must be from the first, if the
observer is to distinguish their relative positions
stereoscopically
.
3-11
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Figure 3-8. Judging distance .
3-12
g.
When a person looks at two objects and
attempts to determine which is farther away, the lines of
sight from both his eyes converge or come together to
form angles on both of these objects. The angles
formed by the converging lines of sight are the angles of
convergence (B, fig 3-8). If the angles of convergence to
both objects appear to be identical, there is no
impression of one being farther away than the other. If
there is a difference
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in the angles of convergence to the two objects, one of
the objects will appear more distant.
h. The
difference
between
angles
of
convergence may be slight but the brain possesses the
faculty of distinguishing between them. The ability to see
stereoscopically is dependent upon the ability to discern
the difference between angles of convergence
(A, fig 3-9).
Figure 3-9. Angular discernible difference.
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be developed by practice and training which aids
recognition.
The smallest difference between
convergence anges that an observer is able to
distinguish is known as his minimum discernible
difference of convergence angles.
i. Angles of convergence become smaller and
the difference between them becomes more difficult to
discern as the objects are farther removed from an
observer or as their distance from one another is
decreased. This difference is known as the discernible
difference of convergence angles (B, fig 3-9) and is
measured in fractions of minutes and seconds of arc.
This provides a means of gaging the ability of observers
to see stereoscopically.
j. The sense of depth perception or
stereoscopic vision for the unaided eye is effective, at
most to 500 yards. In using a binocular or range finder,
the lines of sight for the two eyes are further separated
thus increasing stereoscopic vision.
l. The minimum difference that can be
discerned between two angles of convergence depends
upon the vision of the individual observer, his state of
training, and other general conditions such as visibility. A
well-trained observer should consistently discern an
average difference of 12 seconds of arc and at times,
under excellent observing conditions, this difference may
be reduced to as low as 4 seconds of arc for a series of
observations.
k. Stereoacuity (or stereopsis) is sharpness of
sight in three dimensions. It is the ability to gage
distance by perception of the smallest discernible
difference of convergence angles. This ability may
The average untrained observer should be able to
distinguish a minimum difference of 30 seconds of arc
between two angles of convergence under normal
conditions of visibility.
Section IV. DEFECTS AND LIMITATIONS OF VISION
Old-sightedness (presbyopia)
Astigmatism
Colorblindness (dichromatism)
a. Classification of Eye Defects. Common eye
Night blindness
defects fall into two classifications: those pertaining to
Muscular imbalance (heterophoria)
sight itself and those which are the result of muscular
Double vision (diplopia)
disorder. Names ending in "ic" signify the condition while
Squint, cross-eye, wall eye (strabismus)
"ia" signifies the name of the disorder. Likewise, the
c. Nearsightedness and Farsightedness. The
"tropias" derived from the Greek "op" meaning sight,
size of the eyeball, the curvature of the cornea, and the
signify optical or sight abnormalities while the "phorias,"
focal length of the compound elements are not always
from the Greek "phora" meaning movement, signify
matched. If the lens is too convex or the retina too far
muscular disorders. The normal eye is said to be
back, the eye is nearsighted or myopic. This condition is
emmetropic; the person enjoys emmetropia.
The
corrected by negative or digerging spectacles (fig 3-10).
abnormal eye is ametropic; the person suffers from
If the lens focal length is too long or the retina too close,
ametropia.
the eye is farsighted or hypermetropic. This condition is
b. Common Eye Defects. The common eye
corrected by positive of converging spectacles (fig 3-11).
defects are listed below. Explanation may be found in
the glossary.
Nearsightedness (myopia) Farsightedness
(hypermetropia
or
hyperopia)
3-10.
General Eye Defects.
Figure 3-10. Nearsightedness (myopia)
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Figure 3-11. Farsightedness (hypermetropia)
observation. This is not to be confused with actual size.
d. Astigmatism of the Eyes. This condition
A plane flying away from one will shrink in apparent size
occurs when at least one refracting surface is not
until it is only a speck in the sky. Obviously, it does not
spherical but is somewhat cylindrical (curvature not
become any smaller but, as it recedes, light rays from
symmetrical). In this case the image of a point source is
the wingtips make a progressively smaller angle until the
not a joint image but a short line image as in A, figure Beye no longer can separate them and the entire plane
5. Such line images will form from sources on the optical
appears as a single point. Apparent size then is
axis. Thus, visual astigmatism is different from that
inversely proportional to distance and is expressed
encountered in optical instruments (where astigmatism
mathematically as follows:
of a properly centered spherical lens system is zero on
Apparent Size =Size of Object
the axis). This condition is corrected by cylindrical or
Distance to Object
toroidal (toric) spectacles.
When two adjacent objects become so small in apparent
size that any further reduction in size results in failure of
3-11. Visual Limitations.
the eye to separate them, the angle of resolution of the
eye has been reached.
a. Apparent Size (A, fig 3-12). This is the basis
of all magnification (para 2-4 e (1)). It is measured by
the angle the object subtends at the point of
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Figure 3-12. Visual Limitations
b. Resolving Power of the Eye. The retina is a
mass of nerve endings. Their individual diameters
(between 0.0015 millimeter and 0.0054 millimeter)
determines the resolving power of the particular eye.
The size of these nerve endings vary in different
individuals. These nerve endings may be compared to.
the sensitive particles in a photographic film. Although a
perfect image may be formed in the retina, it will not be
perceived as such, for the distance between two
adjacent image points must be greater than the distance
between rods or cones (nerve ends) for perception.
c. Acute Vision. Acute vision is limited to the
area of the fovea centralis, a spot (about 0.25 millimeter
in diameter) on the macula which lies on the retina at the
visual axis. Here are cone nerve endings alone instead
of the longer rods with a diameter of approximately 0.002
millimeter. This area of acute vision covers a true field of
less than 1° of arc (ordinarily) or a circle approximately
3.5 millimeters in diameter at 25 centimeters distance
from the eye. Least angular separation between any two
discernible points in this field of acute vision is normally
one minute of arc (B, fig 3-12). Coincident readings, as
on a vernier or micrometer, can be read closer as
angular
displacement
of
two
lines can be distinguished by adjacent cone endings
closer than displacement of points. Some persons can
read accurately angular measurement (between two
displaced lines) as small as 10 seconds of arc.
d. Measure of Visual Acuity. Visual acuity is
keeness of sight. It is the ability to see near and distant
objects clearly in order to compare minute details and to
discriminate between them. Visual acuity commonly is
measured by viewing a standard letter of one-minute
details, such as the following example: A person able to
view the letter E composed of lines one minute of angle
of width at the standard distance of 20 feet is said to
possess 20/20 vision. In case a letter is used at 20 feet
which should normally be identified at 40 feet, vision is
said to be 20/40. Some eyes, on the other hand, have
better than average resolving power and can perceive
letters having less than one-minute details. An example
of this is 20/10 vision, which it the ability to discern at 20
feet what the normal eye can appreciate at 10 feet. A
whole series of letters have been designed (subtending
one-minute details at their respective distances) for use
at the standard distance, so that the testing can be done
at a fixed distance rather than at various distances.
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Section V. OPTICAL INSTRUMENTS AND THE EYES
siderations, when the eyes are seriously affected by
3-12. General. Optical fire-control instruments may be
astigmatism, the observer should not remove his
placed in two general classifications: monocular, for the
spectacles as no compensating adjusting can be
use of one eye and binocular, for the use of both eyes.
secured by focusing the eyepiece of the instrument.
Both types require certain adjustments to accommodate
d. Eyeshields. The proper fit and use of the
them to the eye as the use of any optical instruments
eyeshields should be such that it excludes stray light
affects the functioning of the eye itself.
from the eyes. This is particularly important in night
observation and under conditions of poor illumination so
3-13. Accomodation of Optical Instruments to the
that the pupil may dilate as much as possible. If a
Eyes.
monocular instrument is used, the eye not is use should
a. Conditions for Clear Vision. In order that the
also be shielded from light; if it receives too much light,
eye may function properly in the use of a monocular
the pupil of the eye in use will contract sympathetically.
instrument, the instrument must be focused to permit
Rubber shields at the eyepiece make the most effective
light entering the instrument from an object to form a
seal. Any light used to illuminate the reticle must be held
distinct image on the retina without undue effort on the
to a bare minimum.
part of the eyecontrolling muscles of the observer. The
e. Interpupillary Adjustment.
A binocular
exit pupil (rear opening of the eyepiece) must be
instrument must be adjusted to the eye spacing or
sufficiently large to let the maximum amount of light
interpupillary distance of the observer. If this is not done,
enter the pupil of the eye. Stray light must be kept out of
the lines of sight of both eyes cannot traverse the most
the eye. In addition, the use of a binocular instrument
effective optical paths of the instrument; the observer will
demands that the two optical systems of the unit be
not have full binocular vision, nor view the most distinct
properly alined with each other and conform to the
images, nor be able to seal out unwanted light from the
interpupillary distance of the individual.
eyepieces. Practically all instruments of this type are
b. Focusing.
Precise focusing of the
equipped with means for interpupillary adjustment. They
instrument permits light from the object to form a distinct
are marked with calibrations for such adjustment. The
image on the retina. Focusing changes the position of
observer should determine his interpupillary distance
the eyepiece with relation to the focal plane of the
(para 2-35), memorize it, and adjust any such instrument
objective and the angles at which the light rays are
he may use to conform to the spacing between the pupils
brought to a focus. The eyepieces of focusing-type
of his own eyes.
telescopes generally are designed to accommodate the
refracting qualities of the eyes of individual observers. If
3-14. Eye Tension or Fatigue
the instrument is used consistently by an individual, the
a. Blinking of the eyes is an automatic process
eyepiece setting can be memorized to expedite the
resulting from eye tension or fatigue. It will occur in the
focusing of the instrument. Low-power telescopes have
use of optical instruments because the eyes cannot
a wider range of accommodation without adjustments
focus steadily very long without relaxing. It is muscular
than high-power telescopes. Telescopes in which the
rather than retinal, and is least apparent when the eye is
magnifying power is 4X or less have a sufficiently wide
relaxed as when accommodated for distant objects. For
range of accommodation so that a single focus setting
this reason, in focusing an instrument, it should be
will be satisfactory inasmuch as the eye correction is not
focused for distance and the first distinct focus setting
extremely large. They are generally constructed with a
should be used.
fixed-focus eyepiece which cannot be adjusted during
b. Fatigue of the eye muscles will be
operation. They are called fixed-focus telescopes, and
experienced after comparatively short periods of
usually have a minus 3/4 to minus 1 diopter setting (para
continuous observation. This fatigue usually is greater
10-2).
with low illumination. Inasmuch as the eyes quickly
c. Use of Spectacles. If the proper spectacles
recuperate, frequent rest periods are advisable.
are worn, the corrected eyes will focus at the normal
c. A particular type of fatigue results from the
setting of the instrument.
However, the use of
use of binocular instruments if not set at proper
spectacles prevents the eyes from coming up properly to
interpupillary distance. This is due to the fact that both
the eyeshield of the instrument and the eye may be
eyes involuntarily adjust themselves so that a
placed so far from the eyepiece as to restrict the field of
view. In spite of these con3-17
TM 9-258
the eyes are placed in a condition. of forced equilibrium
by the expenditure of an amount of nervous and
muscular force resulting in rapid fatigue.
single image is formed when the image is focused on the
macula of each eye where best vision is obtained. Under
ordinary conditions this is done when viewing a distant
object. In using an instrument, images may be at
different points and
3-18
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CHAPTER 4
OPTICAL COMPONENTS, COATED OPTICS, AND
CONSTRUCTION FEATURES
Section I. OPTICAL COMPONENTS
(6) Chemical stability and mechanical hardness
Optical Glass.
determine the resistance of the finished optical element
a. Types. Two of the most important types of
to handling and contact with atmospheric moisture.
optical glass are known as crown glass and flint glass.
(7) Freedom from internal strain depends upon
Crown is an alkali-lime glass. Boro-silicate crown glass
annealing or very slow cooling of the glass in
contains salts derived from boric and silicic acids. Flint
manufacture. Any large amount of internal strain may
glass contains lead. The refractive indices of these
cause the glass to break during grinding or if subjected
types of glass are listed in paragraphs 2-12c and 5-31a.
to shock at any time after grinding.
b. Characteristics.
The
characteristics
c. Manufacture. Although an understanding of
affecting the values of all types of optical glass may be
the manufacture of optical glass is not essential to a
divided into the purely optical properties, which directly
knowledge of elementary optics, some appreciation of
influence the light in passing throught the glass, and the
the many processes involved will show why fine optical
general physical qualities. The purely optical properties
glass is considered a critical material. The finely divided
are constant density (homogeneity), transparency
materials are very thoroughly mixed, usually by hand.
refraction, dispersion, and freedom from color and
This mixture is melted in small charges, small quantities
defects. The general features of optical glass are
being fed to the melting pot at intervals regulated by the
chemical stability, mechanical hardness, and freedom
melting time of the charge previously introduced. Each
from internal strain.
charge is brought to a liquid state before the next charge
(1) Constant density or homogeneity is the most
is placed in the pot. The next operation, the fining
important property. The uniformity of the index of
process, consists of holding the molten glass mixture at
refraction of a given sample of glass is dependent upon
a high temperature, sometimes as long as 30 hours, to
its constant density.
drive off the bubbles contained in the liquid. To secure
(2) The greater the transparency, the less light
homogeneity and prevent striae, the molten glass is then
is absorbed and the more light will pass through.
stirred during the gradual cooling of the mass until it is so
(3) The refractive and dispersive qualities are
stiff that the stirrer cannot be moved. The mass is stirred
dependent upon the type of glass. Crown glass has
from 4 to 20 hours, depending on the glass and the size
approximately half of the dispersive power (ability to
of the pot. As soon as the stirring ceases, the pot is
spread the colors of so-called white light) of flint glass.
withdrawn from the furnace and allowed to cool to about
(4) Freedom from color tint, while preferable, is
the annealing temperature. It is then placed in an
not as essential as the other requirements. It is obtained
annealing kiln at a temperature of from 4000 to 5000 C.
by careful selection of raw materials which must be of
and slowly cooled to ordinary temperature. Annealing
the greatest possible purity.
takes from 1 to 2 weeks, according to the size of the
(5) Types of defects are known as stones,
batch. When cooled, the pot is withdrawn from the
bubbles, seeds, and striae (layers). Stones are due to
annealing kiln and broken away from the glass.
solid material that accidentally may get into the molten
glass at the time of manufacture. Bubbles are air
4-2.
Lenses.
pockets in the glass. Seeds are very minute bubbles.
a. Single Lenses. Lenses are optical elements
Striae are wavy bands which appear to be of different
with polished faces, one plane and one spherical or
density and color than the surrounding medium.
4-1.
4-1
TM 9-258
both spherical. Lenses deceive the eyes by bending rays
of light so that, depending on the type of lens, they
appear to come from closer and larger objects or from
farther and smaller objects. Lenses are divided into two
general classes (fig 4-1). One
class forms real images and lenses in this class are
termed convex, convergent, positive, or collective lenses.
The other class can form by itself only virtual images and
lenses in this class are termed concave, divergent,
negative, or dispersive lenses.
Figure 4-1. Types of simple lenses.
(1) Convex lenses. All convex or converging,
spectacles because they permit undistorted vision
positive lenses are thicker in the center than at the edges
through their margins ’as the eyes are rotated in their
and will converge light from sources and objects. Both
sockers.
faces of a convex lens may be convex, one surface may
(2) Concave Lenses. Concave or diverging,
be convex while the other is flat (termed plano or plane),
negative lenses (fig 4-1) are thinner in the center than at
or one face may convex while the other is concave.
the edges and will diverge light from sources and
(a) A double-convex lens (fig 4-1) is one in
objects. Both surfaces of a divergent lens may have a
which both surfaces have convex curvature. Both
concave curvature (doubleconcave); one surface may be
surfaces contribute to the converging power of the lens.
concave and the other plane (planoconcave); or one face
The greater the convexity of the surfaces, the shorter the
may be concave while the other is convex (concavofocal distance.
convex or divergent meniscus), with both centers of
(b) A planoconvex lens (fig 4-1) has a plane
curvature on the same side of lens.
(3) Cylindrical lenses. Cylindrical lenses are
surface and a convex surface. The plane surface does
ground with a cylindrical surface instead of a spherical
not contribute to the converging power of the lens.
(c) The convexo-concave
or, meniscus
one; they are either positive or converging, or else they
are negative or diverging. Their use is very limited. They
converging lens (fig 4-1) possesses both a convex and a
are used in some of the coincidence range finders.
concave surface with both centers of curvature on the
b. Compound Lenses. As an optically perfect
same side of lens. The more powerful convex curve
single lens cannot be produced, two, three, or more
makes this a positive lens despite the fact that the
lenses ground from different types of optical glass are
concave surface tends to diverge the light, thus
frequently
combined
as
a
unit
to
cancel
subtracting from the converging power of the lens.
Meniscus
lenses
are
mainly
used
for
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aberrations or defects which are present in the single
lens (paras 2-37 thru 2-42).
(1) The refractive power of a compound lens is
less than that of the convex lens alone. For example, if a
double-convex lens of crown glass is combined with a
planoconcave lens of flint glass, the latter would have
little more than half the
refraction power of the former but sufficient dispersive
power to neutralize the dispersion. The result would be
that light passing through this compound lens would be
brought to a practical focus at a point that would be
about double the distance of the point of principal focus
of the crown lens alone (fig 4-2).
Figure 4-2. Comparative focal lengths of elements in compound lens.
(2) The elements are frequently cemented
at the two surfaces in contact, if the cement used, such
together with their optical axes in alinement. Two lenses
as thermosetting cement (para 2-12c), is a substance
may be cemented together as a doublet or three may be
having approximately the same index of refraction as
glass.
cemented together as a triplet, or each lens of the unit
may be mounted separately as a dialyte (fig 4-3). In the
dialyte compound lens, the inner surfaces of the
4-3.
Objectives.
a. General. The lens nearest the object in any
dissimilar faces of the two lenses cannot be cemented
together inasmuch as they are ground to different
optical system of the refracting type is called the
curvatures in order to correct for aberrations; the two
objective. Its function is to gather as much light as
lenses are separated by a thin ring spacer and are
possible from the object and form a real image of that
secured in a threaded cell or a tube with burnished
object. Objectives lenses in most optical instruments
edges. The cementing of the contact surfaces, ground
form real images (except Galilean telescope).
to the same curvature, generally is considered desirable
b. Construction. The majority of objectives are
because it helps to maintain the two elements in
constructed of two elements, a double-convex
alinement under sharp blows, it aids cleanliness, and it
converging lens of crown glass and a planoconcave flint
decreases the loss of light through reflection
lens (A, fig 4-3).
4-3
TM –9-258
Figure 4-3. Types of objectives.
(1) When the elements of the objective are of
may be magnified is, therefore, governed to a great
large diameter or when the faces of the elements are of
extent by the size of the objective. It is likewise
different curvature, the elements are not cemented but
governed by the limits set by distortion due to diffraction
are held in their correct relative positions in a cell by
(para 2-43b).
separators and a retaining ring (B, fig 4-3 and 4-35).
(2) Increasing the size of the objective beyond a
This construction permits giving the inner surfaces of the
certain point does not improve the brightness of the
two elements different values and allows greater
image appreciably because of the restriction imposed by
freedom in the correction of aberrations. An objective of
the size of the pupil of the eye.
this type is often called a dialyte or Gauss objective.
(3) Some fire-control instruments, notably tank
(2) Certain objectives are composed of three
telescopes, have objectives which are smaller than the
elements securely cemented together (C, fig 4-3) or with
eyelenses of the instruments. Most instruments of this
two of the elements cemented and one mounted
nature, however, have low magnification.
Others
separately, or with all three elements mounted
sacrifice a certain amount of the light-gathering qualities
separately. Such objectives afford a total of six surfaces
of the larger objective for compactness of instrument.
upon which the designer can secure the best possible
correction for aberrations.
4-4.
Eyepieces.
c. Relation of Objective to Optical System.
a. General. The eyepiece may be considered
(1) The size of the objective affects the amount
a magnifying glass which enlarges the image produced
of magnification that can be used in the instrument. Only
by the objective (fig 4-4). The eyepieces of modern fireso much light can pass through a given objective. If the
control instruments generally consist of two or three
magnification were increased, that amount of light would
lenses; one, two, or all three of which may be compound
be distributed over a larger image area and a dimmer
lenses.
image would result. The effective size to which an image
4-4
TM 9-258
Figure 4-4. Typical light paths through field lens and eyelens of eyepieces.
e. Types of Oculars or Eyepieces. Types of
b. Construction. The lens of the eyepiece
nearest the eye is called the eyelens. The eyelens does
eyepieces used in optical fire-control instruments are
the actual magnifying of the image and can affect the
discussed in the following subparagraphs.
They
quality of the image as seen by the eye. The lens in the
represent general types rather than the specific
eyepiece nearest the objective is called the field lens.
eyepieces of instruments inasmuch as a specific type of
The field lens gathers the light from the objective and
eyepiece generally is modified to make it best suited for
converges it into the eyelens. If it were not for the field
a particular instrument.
f. Kellner. The Kellner eyepiece (achromatized
lens, much of the marginal light gathered by the objective
would not be brought into the field of the eyelens (A and
Ramsden, g below) consists of two lenses, field lens and
B, fig 4-4).
eyelens; the eyelens is corrected for color and is a
c. Relation of Eyepiece to Optical System. The
doublet or compound lens with the crown forward and
objective takes nearly parallel rays of light from a distant
the flat flint facing the eye. For full chromatic correction,
object and brings them to a focus turning them into
image lies in plane surface of field lens, but to use with
angular rays. The eyepiece takes these rays, which are
reticle is positioned forward of field len as illustrated in A,
diverging after having been brought to a focus, and
figure 4-5. In this case, some color correction is
directs them into the eye.
sacrificed. To reduce aberration a dense barium crown
d. Magnification. Magnification by the eyelens
and light flint are used in the eyelens. This ocular gives
an achromatic and orthoscopic field that in some models
or eyelenses may be fixed of variable, or there may be
is as large as 500. Its most serious disadvantage is the
no magnification. Some instruments are provided with
pronounced ghost resulting from light reflected
changeable eyepieces to secure different degrees of
successively from the inner and outer surfaces of the
magnification. In other instruments, the relative positions
field lens. This system is common in prism binoculars.
of lenses not in the eyepiece are changed to effect
changes in magnification.
4-5
TM 9-258
Figure 4-5. Kellner and Ramsden eyepieces.
4-6
TM 9-258
h. Huygenian.
This eyepiece employs a
collective or field lens moved away from the eyelens so
that the real image lies between the two lenses. The
Huygenian eyepiece (A, fig 4-6) is not suitable for use
with reticles in telescopes as distortion of the reticle
would result since the individual components are not
corrected for aberration. It is sometimes used however
with a small reticle in the center of the field in some
microscopes and it is useful in observation instruments.
Eyepiece focal length usually used is over 1 inch for
sufficient eye relief. Both elements are normally of the
same type of crown glass with convex surfaces facing
forward. This type of ocular is of negative type in
contradistinction to the Ramsden or positive type (g
above).
g. Ramsden. The Ramsden eyepiece consists
of two planoconvex lenses made of ordinary crown glass
of equal focal lengths separated by a distance equal to
two-thirds of the numerical value of the focal length of
either. The reticle is located in front of the field lens at
one-quarter of its focal length as illustrated in B, figure 45. The field lens thus contributes to the magnification
and quality of the image. Dirt on its principal plane is not
in focus and, therefore, not visible. The convex surface
of each lens faces the other. This eyepiece has the
disadvantage of considerable lateral color which can be
defined as a difference in image size for each color. It is
widely used with reticles, however. This type of ocular
can be used as a magnifier and it is therefore called a
positive ocular.
4-7
TM 9-258
Figure 4-6. Huygenian and symmetrical eyepieces
4-8
TM 9-258
portion of the field providing a more uniformly.
illuminated image. The third is a double convex doublet
eyelens with a slight curvature facing the eye and the
greater curvature facing the collective lens and the
convex surface of the field lens. The periscope using
this eyepiece is the battery commander’s periscope.
k. Orthoscopic. This eyepiece is illustrated in
B, figure 4-7. It employs a planoconvex triplet field lens
and a single planoconvex eyelens with the curved
surface of the field lens facing the curved surface of the
eyelens. It is free of distortion and is useful in highpower telescopes because it gives a wide field and high
magnification with sufficient eye relief. It is useful in
range finders to permit use of any part of the field. It is
so named because of its freedom from distortion.
i. Symmetrical. This eyepiece employs two
doublet (compound) double convex lenses, with the
greater curvature (crown side) of each lens facing that of
the other. The symmetrical eyepiece illustrated in B,
figure 4-5, provides long eye relief (related to low power
and large exit pupil). It has a larger aperture than a
Kellner of the same focal length providing a rather wide
field. It is used commonly in rifle scopes and gunsights
requiring long eye relief between the eye of the gunner
and the eyepiece when the weapon recoils, and low
power with a large field.
j. Erfle. The Erfle eyepiece illustrated in A,
figure 4-7 employs three elements. One is a concavoconvex doublet field lens. Another is a symmetrical
collective center lens which adds to the power of the
eyepiece by shortening the focal length and improves the
illumination of the outer
4-9
TM 9-258
Figure 4-7. Erfle and orthoscopic eyepieces.
4-10
TM 9-258
l. French. This eyepiece is illustrated in A,
figure 4-8. It employs elements substantially resembling
those of the orthoscopic eyepiece in the reverse order. It
was generally indicated on French drawings furnished.
the United States of America during World War I.
eye, the second eyelens (C, fig 4-8).
Variable
magnification is obtained by changing the distances
between elements of the eyepiece and the erecting
system of the telescope. A collective lens in the body of
the instrument is often one of the elements of the optical
system of such an instrument.
4-5.
a.
Prisms.
General.
(1) A prism, unlike a lens, is bounded by plane
surfaces and can be designed to deviate, displace, and
reflect light in numerous ways. The introductions of
prisms into optical instruments permits design variations
otherwise impossible.
(2) In fire-control instruments, it frequently is
desirable to bend the rays of light through an angle in
order to make a shorter, more compact instrument, to
bring the eyepiece into a more convenient position or to
erect an image. Plane mirrors are sometimes used to
change the angles of the light rays but the silvered
surfaces tend to tarnish and cause a loss of light which
grows more serious as the instrument becomes older. A
prism used for the same purpose can be mounted in a
simpler and more permanent mount, the angles of its
surfaces cannot be disturbed, and it can produce more
numerous reflection paths than would be practical with
mirrors.
(3) Optical prisms are blocks of glass of many
shapes especially designed and ground to permit them
to perform the various functions. They are used singly or
in pairs to change the direction of light from a few
seconds of arc (measuring wedges) to as much as 3600
(Porro prism system).
(4) When the incident rays strike the reflecting
surface of a prism at an angle greater than the critical
angle (para 2-16) of the substance of which the prism is
composed, the reflecting surface does not need to be
silvered. This surface must be silvered, to insure total
reflection, if the incident rays strike it at an angle less
than the critical angle.
b. Right-Angle Reflecting Prism. Depending
upon which reflecting surface is struck by the incident
ray, the right-angle reflecting prism will bend the light
rays through an angle of 900 or 1800. The manner in
which this prism reflects rays through an angle of 900 is
illustrated and described in paragraph 2-16e (A and B, fig
2-36) and through an angle of 1800 in paragraph 221a(1) (fig 2-43). When used to reflect rays through an
angle of 900, the rays are reflected only once with the
result that the image is reverted with reflection in a
horizontal plane or inverted with the reflection in a
vertical plane. When used to reflect rays through an
angle of 180°, the rays are reflected twice and a
Figure 4-8. French, Plossl, and variable magnification
eyepieces.
This eyepiece employing an
m. Plossl.
achromatic doublet for each lens is illustrated in B, figure
4-8. The converging components of the doublets face
each other.
n. Eyepiece With Variable Magnification.
These eyepieces generally employ two eyelenses in
addition to the field lens. The forward eyelens is called
the first eyelens and the eyelens nearest the
4-11
TM 9-258
normal erect image is produced with reflection in a
horizontal plane while inverted, reverted image is
produced with reflection in a vertical plane.
c. Porro Prism System.
(1) Each element of the Porro prism system is a
right-angle reflecting prism placed to bend rays of light
through an angle of 1800. One element is arranged to
bend the rays horizontally and the other to bend the rays
veertically; one element reflects the rays into the other
(fig 4-9). With one prism in horizontal position and the
other vertical, the Porro prism produces an inverted,
reverted image. The Porro prism system is used as the
erecting system in many fire-control instruments; it is
commonly used in binoculars. The sharp corners of
these prisms are removed making the ends round. This
reduces the weight and lessens the chance of breakage.
(2) A second type of Porro system is known as
the Abbe system (fig. 4-10). It consists of a single
system which is three prisms in one or it can be made in
the form of two prisms (fig 4-21). It is substantially the
same in principle as the first Porro type but the
reflections occur in different order. It likewise produces
an inverted, reverted image.
Figure 4-10. Abbe system.
d.
Amici or Roof-Angle Prism.
(1) This is a one-piece prism which may be
considered as made up of a right-angle reflecting prism
(fig. 2-43) with the hypotenuse face or base replaced by
two faces inclined toward each other at an angle of 900
(fig 4-11). The latter two faces form the "roof" and give
this prism its name. When inserted in the optical system
of a telescope, it bends the light rays within the
instrument through an angle of 900 while, at the same
time, inverting and reverting the image.
Figure 4-9. Porro prism system.
4-12
TM 9-258
Figure 4-11. Amici or roof-angle prism.
(2) When employed as the erecting prism of an
elbow telescope (fig 8-11), one of its uses, the light ray
enters One short face (fig 4-11) where it strikes the
reflecting surface of one side of the roof. It is reflected to
the other side of the roof where it is reflected outwardly
at right angles to the direction in which it entered the
prism.
Usually, the parts of the prism not used by light rays are
removed affecting the appearance of the prism but not
its fundamental shape or function.
(3) The Amici prism is compact and it transmits
a great amount of light but is one of the most difficult of
all prisms to manufacture because the angle between
the two inclined faces of the roof cannot differ from 90°
by more than 2 seconds if the prism is to function
satisfactorily.
e. Rhomboidal Prism. The Rhomboidal or
Rhomboid prism (fig 4-12) may be considered as made
up of two right-angle reflecting prisms built in one piece,
or as a block of glass with the upper and lower or two
other opposite faces cut at an angle of 450 and parallel
to each other. This prism has two parallel reflecting
surfaces providing two reflections in the same plane and
transmits the image unchanged. It does not invert or
revert the image nor change the direction of the light rays
but displaces the light rays parallel to themselves.
Figure 4-12. Rhomboidal prism.
This is due to double reflection without reversal of the
direction of light. This result is obtained from mirrors in
certain periscopes (fig 8-19). Rhomboidal prisms are
employed to shift the lines of sight to secure
interpupillary distance adjustment. It is used in the range
finder.
f. Rotating Dove Prism. The rotating Dove
prism (A, fig 4-13) resembles the rhomboidal prism
except that the angles of its ends are opposite to one
another
and
this
prism
serves
an
4-13
TM 9-258
in panoramic telescopes by a second inversion in the
900 objective prism. When this prism is rotated about its
longitudinal axis, the image rotates in the same direction
with double angular speed (para 4-7d(4)).
entirely different purpose. Light rays entering the upper
face of the prism are reflected once against the longest
face of the prism after which they continue along their
original path out of the lower face. The image is inverted
only by the single reflection. This inversion of the image
is corrected
4-14
TM 9-258
Figure 4-13. Rotating prisms.4-15
TM 9-258
horizontal or vertical plane (the figure shows the front of
the object and the rear of the image). This prism is
useful in range finders where the angles of the ray must
remain constant, it is necessary to reflect light rays
through an angle of 900 in a horizontal plane, and the
image should neither be reverted nor inverted. The
angles measured by the range finder are so small that; if
a greater number of prisms were used, any deviation in
the angles brought about by slight bending of the tube
and consequent rotation of the prisms would be sufficient
to impair the accuracy of the instrument. It is necessary
to silver the two reflecting surfaces of the pentaprism as
the rays strike the reflecting surfaces at angles less than
the critical angle and total reflection otherwise would not
result. It also is used in elbow telescopes to erect the
image and produce the 90ƒ deviation.
g. Pechan or Z Rotating Prism. The rotating
Pechan or Z prism (B, fig 4-13) consists of two prisms
separated at an angle of 45°. Light rays entering this
prism assembly are reflected five times before emerging;
twice in one element and three times in the other
providing reversion because there are five reflections in
one plane. The incident rays and the emergent rays are
in alinement and travel in the same directions. Two of
the external reflecting surfaces must be silvered. When
this prism is rotated about its longitudinal axis, the image
rotates in the same directions with double angular speed.
h. Pentaprism or Five-Sided Prism.
(1) The pentaprism (A and B, fig 4-14) reflects
light rays through an angle of 90° by two reflections from
two silvered surfaces at a 450 angle to one another. It
does not invert or revert -he image if reflection takes
place in either the
4-16
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Figure 4-14. Pentaprism (diagrams show same effect obtained with mirrors).
4-17
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(2) In the obsolete long base height finders, the
pentaprism that would be required would be so large that
it would be difficult to find blocks of glass sufficiently
large and homogeneous for the construction of such
prisms. Two mirrors were used to replace the two
reflecting surfaces of a pentaprism (C, fig 4-14). These
mirrors were fastened in a rigid mount at precisely the
same angles as the reflecting surfaces of the pentaprism
of the same size and produced the same effect. The
effects of temperature changes were eliminated insofar
as possible to hold the reflecting surfaces in the proper
relation to each other.
(3) This prism is known as a constant deviating
prism because it deviates light constantly 90ƒ regardless
of slight deviation of incident rays from the perpendicular.
i. Triple Mirror Prism. The triple mirror prism
(fig 4-15) has four triangular faces, any three of which
can be reflecting surfaces. It may be considered as
consisting of two triangular roofs, one above and one
below, with the crests of the roofs at right angles to one
another. This prism has the unique property of deviation,
through an angle of 180ƒ, any ray of light entering it. If
the angles of this prism are accurately made, incident
light will be returned back along a course parallel to its
original path. It produces an inversion of the image.
Figure 4-15. Triple mirror prism.
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such slight deviations by means of reflection surfaces.
Wedges are used in the measurement systems of range
finders and to correct or adjust the alinement of paths of
light. They are of two general shapes--rectangular and
round.
b. The angle at which a wedge will deviate the
path of light is dependent upon the relative slant of its
two faces which are inclined toward each other like the
surfaces of a common house shingle. Some wedges
used in fire-control instruments appear to be plates or
disks of glass with parallel surfaces, because the angle
between the surfaces is so slight it cannot be detected
except by careful examination or by actual
measurement.
c. Because deviation of rays of light by wedges
is produced by refraction instead of reflection, a certain
amount of dispersion or separation of colors results from
the use of wedges. For this reason achromatic wedges,
composed of two different kinds of glass to neutralize the
dispersion, must be used where the angles through
which the rays are bent are relatively large or where the
work performed by the wedges is of the most exacting
nature.
d. All wedges cause a certain deviation of the
path of light.
Instruments employing wedges are
sometimes designed so that the path of light entering the
wedge has a certain amount of initial deviation which is
termed the constant deviation. The constant deviation
makes it possible for the wedge to neutralize the
deviation in the path of light or divert the light at a minus
angle.
e. A wedge may be caused to change the path
of light in various directions by rotation of the wedge (fig
4-17). The extent to which a wedge may be made to
divert the path of light may be varied, likewise, by
changing the position of the wedge with relation to the
other elements of the optical system (fig 4-18).
j. Double Right-Angle Abbe Prism.
The
double right-angle Abbe prism consists of two right-angle
prisms joined as shown in figure 4-16. This prism
reflects light twice at right angles to the direction of the
incident ray. The image is rotated 90ƒ two times.
Figure 4-16. Double right-angle Abbe prism.
4-6.
Wedges.
a. The optical wedges used in fire-control
instruments are prisms with two plane surfaces at slight
angles which divert the paths of light through small
angles by refraction instead of by reflection. They are
used where the angle of deviation required may be a
matter of fractions of seconds and it would not be
practical to produce
Figure 4-17. Rotation of wedge changes direction of
path of light.
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f. Another means of varying the path of light is
through the use of pairs of wedges which are geared to
rotate in opposite directions. The wedges of such a
system are referred to as rotating wedges or rotating
compensating wedges. When the thicker edges of two
of these wedges are together (A, fig 4-19), they produce
a deviation twice that of one wedge and in the direction
of the thicker edges of the wedges. When rotated until
the thick edge of one wedge is toward the thin edge of
the other (B, fig 4-19), the wedges neutralize one another
and the wedges produce no deviation. Intermediate
positions cause a deviation which increases as the thick
edge of one wedge rotates farther from the thin edge of
the other. When both wedges are rotated through 180ƒ
(C, fig 4-19), the thick edges again are together and the
wedges function as in A, figure 4-19, except in the
opposite direction.
Figure 4-18. Extent of displacement of light may be
changed by movement of wedge.
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Figure 4-19. Principles of rotating measuring wedges.
length in the optical system is a distinct advantage as in
periscopes. This system also is employed in instruments
with variable magnification because different degrees of
magnification can be obtained by changing the relative
positions of the lenses.
(2) Where compactness of instrument is a
desirable feature, a prism erecting system is used. Such
a system may consist of a single element or several.
The great reduction of length resulting from the use of a
prism system is due to the doubling back of the path of
light. The prism erecting systems in most general use
employ Porro and Abbe prisms, the Amici or roof prism,
and single and double right-angle prisms.
c. Lense Erecting Systems.
(1) The simplest form of lens erecting system
4-7. Erecting Systems.
a. General.
(1) The image generally is reverted and
inverted by the objective.
(2) The function of the erecting system is to
bring the image upright and correct reversion.
b. Types. Erecting systems are of two general
type, lens and prism. While most lens erecting systems
are of the same general design, differing only in the
types of lenses used and the spacing of the elements,
the prism erecting systems employ elements of a wide
variety of shapes causing the rays of light to be diverted
in different paths in the different systems.
(1) A lens erecting system greatly increases
the length of an instrument. It is often used where
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TM 9-258
places a second convex of converging lens in the proper
position to pick up the inverted image framed by the
objective and forms a second erect image which is
magnified by the eyepiece (upper, fig 4-20). In most
fire-control instruments, a lens erecting system of two
and sometimes three lenses is substituted for the single
erecting lens (lower, fig 4-20). The entire lens erecting
system functions as a single lens.
Figure 4-20. Lens erecting systems.
(2) In a two-element lens erecting system, both
lenses usually are of compound (achromatic) type. In a
three-element system, the third lens is of planoconvex or
double-convex type and serves as a collective lens. It
converges the light rays of the image formed by the
objective so they will pass-through the erecting lenses. It
increases the field of view and brightness of image for a
given diameter of the lenses of the erecting’ system.
Inasmuch as the collective lens is sometimes placed at
the point where the image of the objective lens is formed,
and a reticle must be placed at this same point, the
reticle markings are simply engraved on the flat face of
the planoconvex lens used as the collective lens.
(3) According to the placement of the erecting
lenses, a lens erecting system may increase or decrease
the magnifying power of the instrument or it may not
affect magnification. Moving the erecting lenses closer
to the focal point of the objective lens and farther from
the eyepiece increases the magnification but cuts down
the field of view. When the erecting lenses are located
at equal distances from the focal points of the objective
and the eyepiece there is not additional magnification
due to erecting lenses. This method of changing the
degree of magnification is employed in instruments of
variable power.
d. Prism Erecting Systems.
Prism erecting
systems function by internal reflection.
With the
exception of rotating prisms, if the path of light is to
continue in an unchanged direction, a total of four
reflections is required.
(1) Porro erecting system. This system (fig 4-9) is
employed in prism binoculars and in a number of
telescopes. It tends to decrease the curvature of the
field and its use is particularly advantageous when a
compact instrument having a large field of view is
desired as is the case in observation telescopes. This
system is not well suited to telescopes which are to be
mounted gun carriages as the prisms are of awkward
shape and it is difficult to clamp them so tightly that they
will not shift their positions when the gun is fired. In
some periscopes and in the battery commander’s
periscope, the 900 prisms in the heads together with the
lower prisms constitute a Ponrro prism erecting system.
(2) Abbe erecting system.
This erecting
system (fig 4-21) employs two double right-angle prisms
(fig 4-16). The two prisms are assembled in the
instrument to give the same optical effect as in the three
piece Abbe prism (fig 4-10). The two prisms are not
joined, a slight amount of space being left between them.
This system, which inverts and reverts, is used in some
telescopes, including those incorporated in periscopes,
as an erecting system.
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Figure 4-21. Erecting system using two double right-angle Abbe prisms.
(3) Amici or roof-angle prism. When used in
an elbow telescope, this single prism (fig 4-11) serves as
an erecting system as well as a right angle prism. By its
use the light rays are diverted through the right angle of
the telescope and the erection of the image is
secured by reflection on two of its surfaces. It can be
mounted securely and thus is suitable for telescopes are
mounted on gun carriages. Combined with a rotating
prism and a right-angle prism it is used in the erecting
system of panoramic telescopes (fig 4-22).
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Figure 4-22. Optical system of panoramic telescope.
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TM 9-258
(4) Dove rotating prism.
(a) The Dove rotating prism (A, fig 4-13) is the
means by which the image in some panoramic
telescopes is caused to remain erect as the head of the
instrument is rotated. It usually is placed either outside
the telescope system proper or between symmetrical
erectors. The line of sight of the rotating head can be
revolved in azimuth 360ƒ (a complete circle) while the
eyepiece is fixed so that the observer does not move.
Some panoramic telescopes (fig 8-16) employ the
objective 90ƒ prism in the head, and the Dove (rotating)
prism is caused to revolve in the same direction and
through one-half the arc of the 90ƒ prism in the head.
The field, inverted by the objective 90ƒ prism, and again
inverted by the Dove prism, also is rotated through twice
the arc of travel of the Dove prism itself as illustrated in
figure 4-23.
aiming stake viewed through a horizontal Dove prism
positioned with the reflecting face at 45ƒ angle to the
horizontal (fig 4-23). Since the image is reverted in the
plane of reflection, it can be considered to revolve about
the axis AB, which is parallel to the reflecting surface. In
this case, angle "a" is 45ƒ; therefore, angle "b" is 45° and
their sum is 90ƒ or twice the angular rotation of the prism
itself. When the 90ƒ prism (in the panoramic head) is
turned 180ƒ, it will transmit an upright but horizontallyreverted image to the Dove prism. Because the field is
behind the observer, the effect is the same as in viewing
the user in a mirror where the user's image appears
reverted horizontally. In order to provide a normal
image, the Dove prism is so positioned by the 90ƒ
rotation of the plane of reflection that horizontal reversion
occurs.
(d) As both incident and emergent rays are
refracted by a Dove prism, it must be fed by parallel light,
otherwise an aberration is introduced. For this reason it
is usually placed outside (in front of) the telescope
system proper. If it is desired to place a rotating prism
within a telescope system where rays are not parallel, the
Pechan prism is used as its incident and emergent faces
are perpendicular to its surfaces and it is, therefore, not a
refracting prism and will not cause aberrations. If it is
desired to introduce a Dove prism within a telescopic
system, it must be placed between erectors so
positioned that principal rays between erectors are
parallel.
(5) Pechan or Z rotating prism. The Pechan or
Z rotating prism (B, fig 4-13) can be used, similar to a
Dove rotating prism, as a rotating prism in panoramic
telescopes to maintain an erect field by neutralizing the
reverting effect of the objective prism and when the
rotating head is turned ( (4) above). When this prism is
rotated about its axis, the image rotates in the same
directions with double angular speed. By rotating the
prism, erection of the image is accomplished.
4-8. Mirrors. The mirrors used to reflect the paths of
light in fire-control instruments are not the common
household type made of plate glass. The reflecting
surfaces on household mirrors produce a faint reflection
from the front glass surface in addition to that produced
by the silvered back. To eliminate this "ghost image," the
reflecting surfaces of optical mirrors are placed on their
front faces. Mirrors of this type are termed front surface
mirrors. The reflecting surface is a thin film of metal
chosen for its resistance to tarnish. Such mirrors must
be handled carefully as they can easily be scratched and
become useless until they can be resilvered or
realuminized.
Figure 4-23. Image rotation in Dove prism (end view).
(b) An object such as a vertical aiming stake
will appear upside down when viewed through a Dove
prism positioned with its long face horizontal. When
looking forward through the panoramic telescope this
inversion corrects that initially introduced by the 90ƒ
prism in the head. As the panoramic head prism is
revolved from this forward looking position, it introduces
angular tilt equal to its angular rotation so that when it is
turned to look to the user’s left (for example) a vertical
aiming stake will become horizontal in the image. Since
the rotation, or tilt, of the field image produced by rotation
of the Dove prism is twice the rotation of the plane of the
reflecting surface (in accord with the law of reflection),
this 90ƒ tilt will be eliminated by a 45ƒ rotation of the
Dove prism.
(c) As an example, consider a horizontal
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TM 9-258
plate as in a reflex system.
(1) Crosswires (A, fig 4-24), commonly used in
rifle scopes, give increased light transmission by
eliminating one piece of glass. With a wire reticle, there
is no glass surface present to become dirty and require
cleaning. Crosswires may be cleaned with little effort by
using a camels-hair brush dipped in carbon tetrachloride.
A dirty glass surface in the image or reticle plane is in
sharp focus and under magnification. Such a glass
reticle surface, therefore, must be clean.
4-9. Reticles.
a. The majority of reticles (para 5-19) are glass
disks with plane parallel surfaces. Appropriate markings
are engraved or etched on one of the surfaces. In some
cases, a planoconvex lens is required at the point where
the reticle would be mounted. In such cases, the
markings are engraved on the plane surface of the lens.
b. Military reticles are of several types: wire (or
some filament material), a post (picket), an etching on a
plate glass or lens surface, or a punched metal
Figure 4-24. Representative types of reticles
(2) Reticles on glass commonly are used in
most military sights. The reticle pattern is etched and
filled on a planoparallel plate so that it is seen as a black
silhouette. If illuminated, it will glow with reflected light.
Glass provides a surface for any desired reticle design or
pattern as illustrated in B, C, and D, figure 4-24. This
glass surface must be perfectly clean as it is in a focal
place and all dirt is in perfect focus and magnified. A
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TM 9-258
lensatic retical (reticle-lens) is one etched on the plane
surface of a collective or field lens.
c. There are many different types of reticle patterns
(fig 4-25) designed for different purposes. The reticle of
an instrument designed for use on a particular weapon,
when
used
with
specific
ammunition,
is
identified by a group of small figures and letters which
appear near the bottom or top of the field of view (C and
E, fig 4-25). These figures and letters may be firing table
numbers or F.T. numbers to indicate the firing table used
in designing the reticle or may be other characteristic
identification information.
Figure 4-25. Representative types of reticle patterns.
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or ghost images caused by internal reflections from
curved lens surfaces.
b. Antiglare Stops.
(1) Antiglare stops improve contrast by
preventing rays exterior to the field from bouncing off the
interior of the instrument and fogging the field or causing
glare ray (2, fig 4-26). Nonreflecting paint and baffle
finish on inner wall of the tube likewise eliminate most of
this undesired light. Light from a billiant source may still
be reflected off a dark wall and cause serious trouble
especially with a glass reticle. This is particularly true in
wide-angle telescopes and antiaircraft instruments which
may be pointed toward the sun.
4-10. Diaphragms or Stops.
a. General. Diaphragms are rings of opaque
material placed in an optical system so that light passes
through their centers. As their apertures limit the field of
view they are sometimes called field stops. When
placed around the edges of lenses, they prevent rays
from passing through the margins of the lenses and
causing aberrations.
When placed between the
objective and the erecting system or between the
erecting system and the eyepiece, or in both or these
positions or between parts of the erecting system,
they eliminate marginal rays which would otherwise
cause glare and haze by reflecting from the inside walls,
Figure 4-26. Diaphragm (stop) location.
(2) In straight-tube telescopes, the stops are
merely washers or disks with a hole in the center. The
prism shelf in a binocular is designed to act as a stop.
The groove in the face, the rounded corners, and
flattened apex of Porro prisms are designed to function
as stops.
(3) Antiglare stops usually are located between
the objective and the erectors or wherever an image of
the aperture stop is formed (fig 4-26). They are known
as erector stops if they are located between erectors.
Such stops provide balance illumination by limiting the
rays from the center of the field to the same area as
those from the edge of the field.
c. Field Stops. The field stop (fig 4-26) limits the
field to that area which is fully illuminated and sharply
focused by eliminating the peripheral region of poor
imagery (caused by aberrations) and also prevents the
observer from viewing the inside of the instrument. It is
located in an image plane providing a sharply defined
limit to the field. It is designed to admit all the light or
limiting rays needed by the next element whether
erectors or eyepiece. The field stop also may function as
an antiglare stop preventing rays exterior to the field from
being reflected off inner surfaces back into the field (ray
1, fig 4-26). If a field stop is used in each image plane,
the second such stop is slightly larger than the
image of the first at that point so that slight inaccuracy in
size or positioning will not conflict with the sharply
defined image of the first.
d. Aperture Stops. In telescopes, this stop (fig 4-26)
usually is the objective lens cell or retaining ring as there
is no reason for reducing the size of the aperture of the
single compound objective lens usually used in such an
instrument (as contrasted with the need for such stops in
wide-angle camera lenses to reduce aberrations). A
stop in close proximity to such a single compound lens
objective would only reduce the illumination and exit pupil
size (the same as using a smaller lens) without reduction
in aberrations from the lens area used. In a complex
objective (two or more separate lenses), however, a stop
located between the lenses may reduce aberrations.
Also, in a complex eyepiece of several elements, a stop
between the elements may serve to reduce aberrations.
e. Field Effects. All stops, excepting an objective
aperture stop, can be considered to limit the field since
they may prevent rays exterior to the field from reaching
the eye. In designing a system, however, this field limit
is determined from the apparent field of the eyepiece so
that in the final analysis the true field of the instrument
must be governed by the eyepiece.
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TM 9-258
intensities. They are provided as separate elements
which may be attached and detached (A, fig 4-27) or are
mounted so they may be placed in or out of position as
desired (B and C, fig 4-27).
4-11. Filters.
a. General. Filters or ray filters are colored glass
disks with plane parallel surfaces. They are placed in the
path of light through the optical system of a fire-control
instrument
to
reduce
glare
and
light
Figure 4-27. Types of filter mountings.
b. Colored Filters. Filters of various colors are
used to improve visibility under different atmospheric and
light conditions. Among the colors of filters used are:
smoke, yellow, amber, blue, red and greenish-yellow.
(1) The smoke (neutral) filter reduces the
intensity of light and is effective when observing against
or in close vicinity of sun or a searchlight; usually it is too
dark for other purposes.
(2) The yellow and amber are used to protect
the eyes from the reflection of sunlight on water and
other general conditions of glare.
(3) The amber and red filters are usually
employed under various conditions of fog and ground
haze. Red filters are also used in observing tracer fire.
(4) The blue filter is helpful in detecting the
outlines of camouflaged objects.
(5) The greenish-yellow filter is intended to
serve the purpose of both smoke and amber.
c. Polarizing filters.
(1) Polarizing filters do not change the color of
objects but merely decrease light intensity and are used
to eliminate glare. When mounted in pairs, they can be
used to provide continuous control of light intensity.
(2) Light polarizing substances can be
considered as being made up of very minute parallel
bars or grain (A and C, fig 4-28). Light vibrates in waves
with the crests of the waves in all directions. A polarizing
substance, placed in the path of the light, permits waves
vibrating parallel to the direction of the grain to pass
through; waves vibrating at right angles to the bars or
grain are stopped (B, fig 4-28). By eliminating a portion
of the light, the intensity of the light is decreased. When
two polarizing substances are held with the grain of one
at right angles to that of the other, no light passes
through. By shifting the substance to intermediate
angles, light at different degrees of intensity is permitted
to pass through.
4-29
TM 9-258
Figure 4-28. Principles of polarization of light.
(3) Glare is due to light which is reflected from
smooth or wet horizontal surfaces. It can be very
annoying and detrimental to clear vision. Such light as
being reflected at a glancing angle from a smooth
horizontal surface is found to be also polarized in the
horizontal plane, the vertical waves being substantially
eliminated. A polarizing filter consisting of vertical grain
or grid will destroy the remaining horizontal component of
that portion of the light which causes glare while
permitting normal vision to continue practically
unobstructed. The principal function of polarizing filters
is to eliminate glare in the field of vision.
4-12. Lighting.
a. The reticles of fire-control instruments require
illumination for night operation and under certain
conditions for day operation in order that their markings
may be seen clearly.
b. Reticles are edge-lighted. The markings are
etched or engraved into the surface of the glass. When
light is introduced at the edge of the reticle, it travels
through the glass and is diffused at these etched or
engraved marks illuminating them. Light is introduced at
the edges of the reticles through small glass windows set
in openings in the body of the instrument. At night, tiny
shielded electric lamps are placed over the windows.
c. Lucite is a transparent plastic having the
property of transmitting light which enters one end of a
rod of this material. The light travels from end to end of
the rod with little loss through the outside surface,
despite the fact that the rod may be bent. Lucite is
employed in the lighting systems of a number of
instruments (fig 4-29).
Figure 4-29. Instrument light for panoramic telescope.
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TM 9-258
d. Instrument lights for the majority of fire-control
instruments are detachable. They consist of a container
for one or more dry-cell batteries, a light switch which is
sometimes combined with a rheostat, a clamping
arrangement, and a miniature electric bulb with facilities
for directing the illumination to one or more points on the
instrument (fig 4-29). Some systems have an additional
"finger light" (fig 4-30) to permit directing light to any part
of the instrument.
Figure 4-30. Instrument light for BC periscope.
e. Some instrument lights are not supplied with drycell batteries, but use the vehicle power source.
Section II. COATED OPTICS
light which enters the telescope is transmitted to the eye;
the remaining 40 percent is lost by reflection and
absorption at the surfaces of the various elements. The
result is that the image becomes dimmer, the stray
reflections produce glare which reduces contrast, and
"ghost images" are produced when viewing bright objects
at night due to reflection from the curved lens surfaces.
c. A process has been developed for coating the
air-to-glass surfaces of optical elements with a very thin
film of transparent substance. The coating reduces the
loss of light by reflection from each optical interface
surface to less than 1 percent (about 1/2 percent),
resulting in a corresponding increase in the amount of
4ight transmitted through the element. Optical elements
so treated are termed coated optics.
4-13. Introduction.
a. Light loss by absorption and scattered dispersive
reflection on a silvered surface is about 7 percent.
Whenever light strikes an ordinary glass surface, most of
the light is transmitted through the glass and undergoes
refraction. However, even under the most favorable
conditions generally about 4 to 6 percent of the light is
lost by reflection from the surface. This loss takes place
when light passes from glass to air as well as when light
passes from air to glass.
b. This condition occurs at each optical surface of
the elements in a fire-control instrument. For example,
in a straight telescope with 6 optical elements there are
12 surfaces at which there is reflection and loss of light.
Consequently, if each surface transmits 96 percent of the
light which it receives, only about 60 percent of the
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TM 9-258
d. A number of types of coating have been tested
and applied to optical elements. Some have proved too
soft to stand up under handling in normal use and
service. The present transparent coating consists of
magnesium fluoride applied by an evaporation method of
sublimation under heat in vacuum. The coating is
durable enough to permit handling during assembly and
disassembly and to withstand careful cleaning.
e. The main advantages of coating are as follows:
(1) Increased light transmission. When all elements
of a straight telescope are coated, except the reticle and
filters, there is approximately 90 percent transmission of
light instead of 60 percent, or half again as much light as
would pass through the instrument were its optics
uncoated. This improvement will result in a brighter
image at times when light is scarce. It will be most
obvious at dawn, dusk, or at night; in other words, at
times of poor visibility. Targets are visible from 1/4 to 1/2
hour longer at dusk and at dawn. The range of vision at
night is increased approximately 20 percent for standard
binoculars. Therefore, coating is essential in night
glasses or in instruments to be used under adverse
conditions and now is used in all military optical systems.
(2) Reduction of haze. Light reflected from
the surfaces of optical components in an instrument
tends to throw a veil of stray light over the field of view.
Coating will reduce these internal reflections. Better
contrast and sharper definition of the image will be the
result even under intense illumination.
(3) Reduction of ghost images.
Internal
reflections from the curved lens surfaces sometimes
form an image or series of images which are slightly out
of focus and not quite as bright as the image of focus.
These ghost images are very distractive for proper
aiming. Coating will greatly reduce the formation of
ghost images.
(4) Reduction of front surface reflections.
Reflections from the front surface of observation
instruments are very noticeable at night and might give
the observer’s position away. By reducing reflections by
means of coating, this danger is reduced.
4-14. Theory.
a. Consider a tiny portion of a light ray so highly
magnified as to show its wave structure. A ray of light
striking an uncoated glass surface could then be pictured
as a wavy line as shown in figure 4-31. The reflected
ray, which can represent 4 percent of the total light, is
represented by the wavy dotted line and will shift in
phase 180° (reverse direction of vibration).
Figure 4-31. Light ray striking uncoated surface.
b. If a thin film of transparent substance were
placed on the glass, there would be two surfaces from
which the wave would be reflected and similarly shifted in
phase 180ƒ. If this film were one-half as thick as a
wavelength of light, the reflected waves from the two
surfaces would follow the same path (fig 4-32). In doing
so, they would reinforce each other resulting in the
double amount of light being reflected and the minimum
amount of light being transmitted through the surfaces.
This condition would be desirable in a reflector but
undesirable in a lens.
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TM 9-258
Figure 4-32. Light ray striking coated surface (coating one-half wave length in thickness).
c. If, however, the film were only one-quarter of a
wavelength in thickness, the reflected waves would
follow paths which would cause them to interfere with
each other and the reflected waves would cancel
each other (fig 4-33). The radiant energy, which is light,
is not lost when the reflected waves are canceled;
instead, this energy is contributed to the refracted light.
Figure 4-33. Light ray striking coated surface (coating one-quarter wavelength in thickness).
d. Another way to understand the action of coated
optics is to consider two incident light waves: ray 1 and
next oncoming wave, ray 2. With a film of one-quarter
wavelength in thickness, the time required for a wave in
ray 1 (fig 4-34) to travel from the top of the film to the
bottom and return results in a one-half wavelength lag
relative to the reflected wave from the next oncoming
wave, or to the coincident reflected wave in ray 2 at the
top surface so that the two are 180ƒ out of phase. The
vibrations of these two coincident reflected waves then
are in opposite directions and cancel each other, the
same as two matched teams in a tug of war, pulling with
equal force, remain stationary. Because of the 180°
4-33
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phase shift previously mentioned however, the oncoming
wave in ray 2 coincident (at the air-to-film interface) with
the wave from ray 1 (reflected from film-to-glass
interface) i will be in phase. Reinforced transmission of
ray 2 will result.
the reflections from the eyepiece and objective end
surfaces. If the optical elements in the instruments are
coated, the reflection of light will have a distinctive
purplish tinge. Under certain conditions, the faces of
lenses may appear to have a dull film or patina.
b. An alternative method is to compare the
illumination of the field of view of an instrument with the
same model number of series known to have’ optical
elements that are uncoated. The instrument with coated
optics will have a much brighter field.
c. All fire-control instruments with coated optics
bear a decalcomania transfer with the following wording:
"This Instrument Has ’COATED OPTICS’. Clean Lenses
Carefully."
d. The magnesium fluoride coating is referred to as
hard coating. There are a few instruments in the field
with elements coated with what is referred to as soft
coating. The coating on these elements is not as
durable as the hard coating. A simple test will distinguish
a hard coating from a soft coating. With a soft rubber
eraser, rub the coated element near the edge so as not
to impair the usefulness of the element. About 20
strokes will remove a soft coating, but hard coating will
not be affected.
4-16. Serviceability.
a. Hard coating films on coated optics are tough
and durable. They are insoluble in water, are not
affected by oil and alcohol, and salt water will not harm
them if cleaned off promptly. The coat will withstand
temperature from -60 to +200ƒ F. They are specified to
withstand a rubbing of a 3/8-inch pad of dry cotton,
exerting a force of 1 pound rubbed in any direction 50
times. However, due to the critical significance of the
thickness of the coating the effectiveness of the coating
may be completely destroyed by carelessness,
ignorance, or rough treatment.
b. Scratches tend to lower the quality of the lens.
Wearing down of the film due to repeated cleaning is not
harmful to the element itself, but a reduction in thickness
of the film much below one quarter of a wavelength of
green light reduces its efficiency as a coated optical
element. Partial or complete removal of the coating does
not make the optical element useless but merely partly
wholly takes away the benefit of the coating, leaving the
element as if it had not been coated in the first place. It
is important that remnants of a partly deteriorated
coating should not be removed, since even a partially
coated lens will be more effective than an uncoated one.
c. Coated optics slowly deteriorate under fingerprints, under prolonged action of atmospheric dust and
moisture, and salt water. Instruments containing coated
optics should be well sealed. Exceptional precautions
must be taken to prevent sealing and cementing com-
Figure 4-34. Low-reflectance coating.
e. Since the wavelength is different for each
different color of light, one color must be selected to
determine how thick the coating shall be. Green light,
with a wavelength of 20 millionths of an inch, is the
logical color to use since it is in the middle of the visible
spectrum. This color is in the brightest region and
contributes most to vision. The thickness of the coating,
therefore, should be 1/4 of 20 millionths of an inch, or
equal to 5 millionths of an inch and result in increased
transmission of green light and adjacent colors
(diminishing toward outer colors in spectrum). Reflection
of the colors at the extreme ends of the spectrum (red
and violet) is not eliminated completely. Therefore, light
reflected from a coated optical element appears purplish,
similar to a mixture of red and violet regardless of
coating material used. Thus presence of coating is
apparent to visual inspection. The common soap bubble
or oil slick on water reflects iridescent colors because of
the varying thickness of the film.
f. Another useful application of magnesium fluoride
coatings is on front surface aluminized mirrors used in
optical instruments. In this case, a film thickness of onehalf wavelength causes reflected waves to reinforce
each other (fig 4-32).
Such coatings also may be applied to front surface
silvered mirrors. An aluminum or silver surface coated in
this manner is no longer soft and easily scratched, but
becomes harder and more durable and may be cleaned
like other optical elements.
4-15. Identification
a. Whether or not an instrument is fitted with
coated optics can be detected readily by holding the
instrument at an angle to a source of light and observing
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pounds from getting on the coated surfaces of lenses.
The removal of such compounds undoubtedly would
result in injury to the coated surfaces.
d. Every precaution should be taken to prevent
inexperienced personnel from misunderstanding the
function of the coating and attempting to remove it.
Section III. GENERAL CONSTRUCTION FEATURES
thin edge of the cell (burnishing). A setscrew locks the
retaining ring in position in the lens cell. When two or
more lenses are mounted in a cell, the different elements
are maintaintained at the proper distance from one
another by spacers of separators. The cell mounting
permits the entire assembly to be handled and mounted
as a unit.
4-17.
Lens Cells, Separators, Lens Retaining
Rings, and Adapters.
a. A lens cell (fig 4-35) is a tubular mounting or
frame made of metal, plactic, or hard rubber which holds
a lens or a number of lenses in the proper position within
an optical instrument. The lens usually is secured into
the cell by a lens retaining ring or by turning over a
Figure 4-35. Lens cell, separators, lenses, and retaining ring.
b. Separators or spacers are smooth or threaded
tubular sections which separate or space the elements of
a lens system in the proper relation with one another (fig
4-35). Adjoining faces are usually beveled to fit the faces
of lenses snugly.
c. Lens retaining rings are generally threaded
about their outer diameters and are screwed in over
lenses to secure the lenses to their cells.
d. Adapters may be used to mount an element or
part of smaller diameter into a part of the instrument
body which is of larger diameter.
4-18. Centering Devices.
a. Light always bends toward the thickest part of a
lens or prism. When the center of a lens is moved it can
be made to cause the light to be bent in a different
direction. In cases where extreme exactness is required,
proper adjustment of the center of the objective is
accomplishing by mounting it in a pair of eccentric rings
(fig 4-36). By rotating the inner ring about in the outer
ring, or by rotating the outer ring in the lens cell, the axis
of the objective may be moved to any point in a relatively
large area. When properly positioned, the rings may be
locked in position. Skill is required to set the rings in the
proper manner.
4-35
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caused by sunlight striking the outer face of the
objective, but they protect from extreme heat the
thermosetting cement used to cement the elements of
the objective.
Figure 4-36. Eccentric objective mounting.
b. Prisms were formerly often placed in mountings
designed to hold them firmly and yet afford movement for
exact adjustment. Screws bear upon two faces of the
prism to lock it in place in such mountings. By letting out
on one screw and taking up on the other, the prism may
be positioned exactly. Extremely fine adjustments can
be made. Care must be exercised in tightening screws
to insure that prisms will not chip or crack from excessive
pressure.
c. In modern instruments, mirrors and prisms are
bonded to metal brackets or holders. Adjustment of the
various components is accomplished by the screws
bearing against the metal holder.
4-19. Eyeshields. Eyeshields or eyeguards are fitted
to fire-control instruments to maintain proper eye
distance and to protect the eyes of the observer from
stray light, wind, and injury due to the shock of gun fire or
similar disturbances. Eyeshields are made of rubber,
plastic, and metal but only those made of soft rubber can
most effectively meet all of these requirements.
4-20. Sunshades and Objective Caps.
a. Sunshades are tubular sections of metal, usually
with the lower portion cut away, which are fitted into slots
around the objective cells of many instruments to protect
against rain and the direct rays of the sun (fig 4-37).
They not only reduce glare which would be
Figure 4-37. Sunshade and objective cap.
b. Objective caps (fig 4-38) are leather covers
which are fitted over the sunshade or objective end of
the instrument to protect the objective when the
instrument is not in use. The caps are usually attached
to the instrument by strips of leather to prevent loss.
4-21. Focusing Devices. The distance between the
reticle and the eyepiece must be adjusted to the
observer’s eye so that the reticle and image of the object
will be sharply defined and to eliminate eye fatigue. To
provide this adjustment, the two or more lenses of the
eyepiece are mounted in a single tube or lens cell and its
distance from the reticle (and the focal plane of the
objective) can be adjusted by a rack and pinion, by a
simple draw tube, or by rotating the entire eyepiece when
adjusting the diopter scale (fig 4-38) causing it to screw
in or out. This is referred to as the diopter movement
(para 5-14). The knurled ring of the "screw out"
adjustment type is provided with a scale reading in
diopters by which this adjustment can be made directly if
the correction required by the eye is known.
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4-22. Optical Bars.
a. In a range finder, the angles of the lines of sight
from both ends of the instrument are employed to
determine the range. Heat and cold cause unequal
expansion and contraction of metal parts disturbing the
angular alinement of elements mounted on or in them.
The angles determined by the range finders are so small
that an excessive error could be introduced by the
deviation caused by any misalignment if special
measures were not taken to keep the optical systems
stable and in perfect alinement.
b. One of these measures consist of mounting the
most sensitive parts of the instrument in a metal tube
known as the optical bar. The optical bar is made of
specially composed metal with a low coefficient of
expansion. It is perfectly balanced and is supported by
the inner or main tube of the instrument in such a
manner that the outer tube may expand or contract
without affecting the alinement of the inner tube. In
addition, the instrument is insulated to reduce
temperature changes to a minimum.
Figure 4-38. Diopter scale.
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CHAPTER 5
ELEMENTARY PRINCIPLES OF TELESCOPES
Section I. INTRODUCTION
the eye. It should be remembered that rays from a
distant point are, for all practical purposes, parallel and
that the eye can focus or form an image with these
parallel rays without accommodations. If the object is at
the focus (focal point) of the lens, it is seen by the eye
without accommodation as the emergent rays are then
parallel (as if originating at a distant object) as illustrated
in figure 5-1. If the object is within the focal length of the
lens, accommodation by the eye is necessary as the
emergent rays no longer are parallel but are diverging,
as if from a near object, as in figure 5-2.
5-1. History of Optical Instruments. The earliest
record of the use of a lens as a magnifier is estimated to
have been in the eleventh century and attributed to an
Arabian philosopher names Alhazen. In the thirteenth
century, spectacles were devised based on work done
both by Roger Bacon and by a Florentine monk named di
Spina. But as the seventeenth century opened, nobody
had as yet devised a telescope. Quite by accident a
Dutch optician, Jan Lippershey, made the discovery in
1608. His telescope used a 1 1/2-inch diameter positive
(convergent) objective lens of 18-inch focal length and a
negative (divergent) eyelens of 2-inch focal length.
Lippershey was refused a patent by the Dutch
government because of confusing counterclaims from
supposed competitors but in the spring of 1609 the
Italian scientist Galileo carried on the work.
He
duplicated the original using an instrument of 8 power.
Not satisfied, he made another instrument 37 inches long
of 8 power and still another 49 inches long with a very
short focus eyepiece giving 33 power. His famous
astronomical discoveries were made with the latter
instrument and the optical system he used is still known
as the Galilean telescope. A refracting astronomical
telescope of modern design, using a positive objective
lens, and a positive eyelens, was designed by the
German astronomer Kepler in 1610 and some 30 years
later a Scottish mathematician named Gregory designed
the first reflecting telescope which is the basis of many
modern astronomical instruments. In the latter part of
the nineteenth century, buffalo hunters on our western
plains used telescopic sights on their rifles.
The
telescopic sight, the most accurate of all sighting
devices, is too easily damaged to be an item of issue
with the infantry rifle except those weapons used by
snipers. It is used, however, on all types of field pieces
and Naval rifles. The remainder of this text is devoted
mainly to a study of the principles involved in such
applications of telescope as telescopic sights and
observation instruments.
5-2. Simple Magnifier. The simple magnifier consists
of a positive (converging) lens at the first focal plane of
the eye, although this positioning is not too important. If
the object being viewed is at or within the focal point of
the lens, a virtual, erect, and enlarged image is seen by
Figure 5-1. Simple magnified-object at principal focus
Figure 5-2. Simple magnifier-object within focal length
5-3. Compound Microscope. The real purpose of the
microscope is to aid the human eye, essentially a longrange high-acuity instrument, to be useful at short dis-
5-1
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tances. If it is recalled that apparent size equals actual
size divided by distance to object, it should be apparent
that the closer an object is to the eye the larger will be its
image. The microscope uses an objective lens of
extremely short focal length (fo) which forms a real,
enlarged, and inverted image of an object placed just
behind its principal focus. To view this image, a positive
eyepiece of short focal length (fe) is used as a magnifier.
If the real image is at the first principal focus
of the eyelens, the image is seen at infinity and no
accommodation is required. Final image may be formed
at any distance from the eyelens exceeding the shortest
distance of distinct vision (about 10 inches). In this case,
the image formed by the objective lens must be within
the principal focus of the eyepiece as in figure 5-3.
Magnification in a microscope depends on the focal
lengths of the objective and the eyepiece and the
distance between them.
Figure 5-3. Simple compound microscope
called either the objective or objective glass (if a mirror)
which usually forms a real image of the field or area
which the telescope can "see," and an eyelens or
eyepiece used to view this image.
This general
description is exactly the same for the microscope,
except that in the microscope the first image is actually
larger than the object while in the telescope the first
image is smaller than the object.
5-4. Telescopes. Most persons think of telescopes as
magnifiers but they may be designed to provide the eye
with an image the same apparent size as the object or
they may provide the ’eye with a smaller image which is
an example of reduction. The primary purpose of the
telescope is to improve vision of distant objects. In its
simplest form, it consists of two parts; a lens or mirror
Section II. ASTRONOMICAL TELESCOPES
cannot be brought to focus by the eye (fig 5-4). The eye
must be fed either by parallel rays or rays only slightly
diverging as if from an object no closer than the near
point of the
5-5. Refracting Telescopes. Refracting astronomical
telescopes use lenses to form images through refraction.
A positive lens alone will form only real images of distant
objects.
Such
real
images
in
space
5-2
TM 9-258
eye. However, if another positive lens is placed between
such an image and the eye, and the real image lies at
the first focal point of this eyelens, the eye can see
without accommodation of a virtual image of the object
seen by the objective lens. This is the Keplerian system
(fig 5-5). Such an arrangement is a telescope of the
simplest form and is illustrated in figure 5-6 with the
virtual image moved in to the near point of the eye. This
system generally is limited to astronomical observation
becasue the image is inverted. The Army uses this
system only to adjust other types of telescopes and when
it is so used, calls it a collimating telescope.
Figure 5-4. Objective lens.
Figure 5-5. Keplerian system
Figure 5-7. Concave mirror
Figure 5-6. Refracting astronomical telescope
(a) An object beyond center of
curvature forms a real image between center of
curvature and focal point. This image is smaller than the
object and inverted and reverted.
(b) When the object is between the
center of curvature and the focal point, the image is
formed beyond the center of curvature and is real,
enlarged, and inverted and reverted.
b. Reflecting Telescope.
This telescope,
diagramed in figure 5-8, utilizes a concave mirror of long
focal length (instead of an objective lens) to form the real
image. This is viewed as a virtual image through an
eyepiece which magnified it or is photographed by a
camera attachment. The Mt. Palomar telescope, for
example, has a 200-inch diameter mirror of long focal
length, great light-gathering ability, and great resolving
power. (For resolving power see para 5-30).
5-6.
Reflecting Telescopes.
a. Types of Spherical Mirrors.
(1) Convex. In this case, the mirror surface is
on the outside of a spherical surface, the center of
curvature of which lies on the side opposite the incident
light. This type of spherical mirror produces small virtual
image only. It is frequently used on trucks as a rearview
mirror.
(2) Concave. In this type, the light is incident
on the same side as the center of curvature (C) of the
sphere. The focal point (f) is located halfway between
center of curvature and reflecting surface (fig 5-7). The
concave mirror forms virtual and enlarged images of
objects within its focal length which is one-half the radius
of curvature. Some of the uses to which it is put are
headlamp reflectors, searchlight reflectors, and objective
glasses in astronomical telescopes.
5-3
TM 9-258
5-7.
Magnification of Telescopes.
a. Computation of power in an astronomical
telescope is done by dividing the focal length of the
objective by the focal length of the eyepiece. This is true
only when the virtual image is at infinity or when
emergent rays from point object are parallel. If the
image is moved to the near point of the eye (10 inches),
it increases slightly in size.
b. This equation cannot be applied to all
terrestrial systems using lens erecting systems since
such erecting systems can, and usually do, contribute to
the power. It can be applied to any terrestrial system
using a prism erecting system.
Figure 5-8. Reflecting astronomical telescope.
Section III. TERRESTRIAL TELESCOPES
the eyepiece to erect the image as in figure 5-9. A prism
erecting system is placed between the objective and its
focal point, but a lens erecting system requires
repositioning of objective and eyepiece so that erectors
are between objective focal point and first principal focus
of eyepiece.
5-8.
General. This instrument gets its name from the
Latin term "terra" which means earth and is basically
useful for looking at objects as they actually appear to us
on earth. Any astronomical telescope can be converted
into a terrestrial telescope by inserting an erecting
system. either lens or prism, between the objective and
Figure 5-9. Terrestrial telescope—simple form.
erecting system. The first lens may be placed one focal
length behind the image formed by the objective and the
second spaced a distance equal to the focal length of
either to minimize spherical aberrations (fig 5-10).
5-9.
Lens Erecting Systems.
a. Location. A lens erecting system is placed
between the focal plane of the objective and the front
focal plane of the eyepiece. In practice, to minimize
aberrations, two or more lenses usually comprises a lens
5-4
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Figure 5-10. Symmetrical erectors.
b. Symmetrical. A true symmetrical erecting
system is composed of symmetrical lenses so positioned
that the object (real image formed by the objective) and
image distances are symmetrical or equal (fig 5-10). In
other words, the image formed by the objective is at the
focal point of the front erector and the rays between the
two erectors are parallel. Therefore, spacing between
erectors is not critical and does not affect magnification
as the magnification in such a system (always-1)
indicates an inverted image of same size as object
regardless of lens separation.
c. Variable
Magnification.
Variable
magnification erecting systems used in variable power
telescopes may provide (in the high-power position) from
two to three times the power produced in the low-power
position depending on the design. In the simplest lens
erecting system (a single lens), magnification can be
shown to be a function of object distance and image
distance (A and B, fig 5-11, and para 2-24e). The same
is true if a combination of lenses (usually two) is used.
separation between the elements in the erecting system
(A, B, and C fig 5-12).
Figure 5-12. Asymmetrical erectors.
(2) When magnification is increased by
shifting erectors forward, the image position shifts toward
the eyepiece as in B, fig 5-12. A change in distance
then, between the asymmetrical elements in this forward
position, further affects magnification and image
distance. A decrease in the distance between the
elements, i.e., moving the second lens nearer the first,
lessens the increase in magnification slightly but more
important decreases the shift in the resulting image
position as in C, figure 5-12. By design, the image
position can be the same for any of the possible
magnifications. In this case, the separation between
erector elements is always such that the image position
remains fixed for any magnification and the eyepiece can
be fixed (C, fig 5-12).
(3) If magnification is increased, either by
shifting the erector system forward as a complete
Figure 5-11. Simple erector.
(1) Magnification of an erecting system
composed of a combination of asymmetrical lenses
(lenses of different focal lengths) can be varied either by
varying the distance of the erecting system from its
object (image formed by objective) or by varying the
5-5
TM 9-258
unit or by moving only the rear erector element forward,
the image formed by the erectors will move back. In this
case, it is necessary to move the eyepiece back a
distance equal to the image shift to keep the image in
focus (B, fig 5-12).
5-10. Prism Erecting Systems. A prism erecting
system (para 4-7d) may be used in a terrestrial telescope
instead of a lens erecting system. In prism, offset, and
elbow telescopes, it is placed between the objective and
its focal plane. Either all or part of the prism erecting
system, however, may be outside (in front of) the
telescope system. In the BC periscope, the prism in the
head is outside the periscope system but still a part of
the prism erecting system. The prism in the head,
together with the prism system in the elbow below,
comprises a Porro prism erecting system. All observing
and sighting telescopes used by the Army employ an
erecting system.
5-11. Galilean Telescope.
a. Basic Principles. The system employed in
the Galilean telescope is based on two major principles.
(1) It makes use of a negative eyelens
positioned a distance equal to its focal length (fe) in front
of the objective focal point (figs 5-13 and 5-14).
This negative eyelens is placed so that rays from the
objective meet it before forming the real image, which
does not exist, and is called the virtual object. Rays
approaching this virtual object will be bent outward and
made to diverge as if from the enlarged virtual image (fig
5-13). This renders converging rays from the objective
parallel before they have converged to form a real
image; therefore, no real image exists in this system.
The real image, which would be formed by the objective
if the eyelens weren’t there, becomes the virtual object
for the eyepiece and lies at the first principal focus of the
eyelens. In this case, the virtual image viewed through
the negative eyelens will be at infinity and can be viewed
by the eye without accommodation. This is the zero
diopter setting (fig 5-14) for this instrument, meaning that
all rays from any point source emerge from the eyepiece
parallel. If the eyelens is moved in or out, the emergent
rays either will converge or diverge so that the instrument
cap be adjusted for farsighted or nearsighted eyes or for
distance. The image viewed through the eyepiece is
erect because the emergent rays are bent further away
from the axis instead of recrossing it as in a Keplerian or
astronomical system employing a positive eyelens.
Compare figure 5-13 with figure 5-6.
5-6
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Figure 5-13. Galilean system.
(2) This is the only telescopic system in
which the diameter of the objective controls the field of
view (width of visible observed area), as the objective is
both field stop and entrance window (fig 5-15). This
system is, therefore, limited to small fields and low power
and usually is designed for 2 or 3 power.
Figure 5-14. Relations of elements in Galilean telescope
(zero diopter setting)
5-7
TM 9-258
Figure 5-15. Objective diameter limits field in Galilean system
b. Uses. This system is quite inexpensive
because of its simplicity and is used in field glasses. It is
used in opera glasses because of its very short length
and light weight. Note that is provides an erect image
without lens or prismatic erectors in a system shorter
than a modern astronomical system of equal power. It
is used frequently as a reduction view finder on cameras
by reversing it with the positive element used as the
eyelens and the negative as the objective. Because of its
low power limited field, and lack of real image, plane, it
has no application in military instruments in this country.
Section IV. FUNCTIONS OF COLLECTIVE LENSES AND EYEPIECES
rays would miss the erectors without it and leave the
outer portions of the field with less illumination than the
central portion. The clear aperture required in the lenses
in the erecting system is reduced so that smaller lenses
can be used or the collective lens can be considered to
increase the field, if the erectors are of sufficient size and
the eyepiece has sufficient apparent field.
5-12.
Functions of Collective or Field Lenses.
a. Primary Functions. This element (usually
plano-convex) may be placed in a system with the plano
side coincident with the image plane (focal plane) of the
objective. If the principal plane of this lens lies in the
image plane of the objective, this lens does not alter the
power of the instrument but reduces the power if placed
closer to objective (fig 5-16).
The collective lens
provides balanced illumination of the entire field. Rim
5-8
TM 9-258
Figure 5-16. Function of collective lens.
b. Secondary Functions. This lens also shortens the
eye reflief (distance from eyelens to eye) of the
instrument by shifting the plane of the exit pupil (in which
the eye must be located to view the full field) closer to
the instrument. It is not used, therefore, in rifle scopes or
gunsights where long eye relief is essential to’ avoid
injury from the recoil.
5-13.
Functions of Eyepiece or Ocular.
a. General. An eyepiece in its simplest form
may consist of a single lens (simple magnifier) which
presents to the eye an enlarged virtual image of the real
image formed either by the objective or the erectors as
illustrated in figure 5-17. The usual apparent field
provided by an eyepiece is 400 to 500. A wide angle
eyepiece is one fully corrected to the edges and of
sufficient aperture so that in a system of given eye relief
a wide field of up to 75. may be provided. Because
magnification is angular, this apparent field limits the true
field of the instrument for any certain power. For
example, a 4-power instrument with an eyepiece
providing a 400 apparent field will have a true field of
100. This will be discussed fully in paragraphs 5-21
through 5-31.
Figure 5-17. Function of eyepiece.
b. Function of Elements. In a simple eyepiece
using two lenses, an eyelens next to the eye provides all
or most of the magnification depending on the location of
the filed lens relative to the real image. The field lens (fig
5-18), if in the image plane, has no effect on
magnification but acts as a collective lens providing
balanced illumination of the field. Without the field lens
principal rays from the edge of the field would miss the
eyelens and leave the edge of the field dark. With the
field lens in the image plane, however, any dust on its
surface lying in the image plane will be in sharp focus.
For this reason, unless there is an etched reticle on this
surface, the field lens usually is displaced slightly from
the image plane thus contributing to the magnification.
Another reason for not placing the field lens in the focal
plane is that this is the position normally occupied by the
reticle in those instruments emplying reticles with the
reticle pattern etched on glass or a
NOTE
The power of an eyepiece is inversely
related to its focal length; i.e., highpower eyepiece is one of short focal
length. It is thus possible to replace an
eyepiece with one of shorter focal length
and increase the power of the
instrument.
5-9
TM 9-258
crosshair mounted on a reticle holder ring. If the eyelens
and the field lens are of the same power (focal length)
and the field lens lies in both the image plane and the
first focal plane of the eyelens, the image will be viewed
at infinity (without requiring accommodation by the eye).
NOTE
A field lens in the image plane shortens
eye relief by shifting the plane of the exit
pupil without affecting its size (para 527b). The field lens also increases the
apparent field of the eyepiece.
Figure 5-18. Function of field lens in eyepiece.
Section V. ADJUSTMENTS
5-14.
Diopter Movement.
a. Zero Setting (Normal). In a well-adjusted
instrument the beams of light from each point in a distant
field or target will emerge from the eyepiece (of focal
length fe) as a beam of parallel rays. This is a zero
diopter setting (para 10-1), as diagramed in figure 5-19,
and will feed the eye with the same parallel light as that
received from distant points by the unaided eye. The
normal or emmetropic eye, therefore, can view the field
through the instrument without accommodation.
Figure 5-20. Minus setting for shortsighted eye
c. Plus Setting (Farsighted ). Farsightedness
or hypermetropia results in image formation in back of
the retina. Movement of the eyepiece away from the
recticle and the real image under observation, will cause
rays emerging from the eyepiece to coverge so that the
farsighted eye can focus the image clearly. This is a
plus diopter setting and is illustrated in figure 5-21.
Figure 5-19. Zero diopter setting.
b. Minus Setting (Shortsighted).
The
shortsighted or myopic eye will form an image in front of
the retina. Movement of the eyepiece toward the reticle
and the real image under observation, will cause
emergent rays from the eyepiece to diverge and correct
for the deficiency of the eye. This is a minus diopter
setting and is diagramed in figure 5-20.
Figure 5-21. Plus setting for farsighted eye.
d. Low-powered Instruments.
If
the
instrument is less than 4x, the eyepiece can be fixed
focus for satisfactory use. This means adjusting the
5-10
TM 9-258
eyepiece to a single focus setting at -0.75 to -1.0 diopter.
A single focus setting will be satisfactory to any observer
whose eye correction is not too great. In a low-power
instrument, the objective is short focal length; therefore,
the image plane shift is small when target distance
changes unless the target is moved very close.
e. High-Powered Instruments. In high-power
instruments (more than 4x) the diopter movement is
useful because the eye relief frequently is too short
(excepting in rifle scopes and gunsights) to permit view
of full field while wearing spectacles. If the user’s
defective sight is not caused by astigmatism (is either
farsighted or nearsighted) and the correction, required is
within the range of the ’diopter movement, the diopter
adjustment permits adjustment of the eyepeice to correct
for his defective vision. This eliminates the necessity for
spectacles while using the instrument. In a high-power
instrument not having a reticle, the diopter movement
permits focusing the eyepiece on the different image
planes, resulting when the instrument is used at all
ranges, thus focusing the instrument for the different
ranges. An example of this is the observation telescope
which can be used from approximately 50 feet to infinity.
f.
Diopter Scale. The diopter scale on an
eyepiece is calibrated in units indicating the movement of
the eyepiece necessary to effect a change of 1 diopter in
the emergent light. For example, at plus 1 the angular
convergence is the same as with a 1-power lens (focal
length of 1 meter) and the rays will focus or cross at 1
meter behind the eyepiece. At minus 1 the angular
divergence is such that the rays, if extended back into
the instrument, would cross at 1 meter forward of the
eyepiece.
5-15. Dioptometer. The dioptometer (fig 5-22) is a
precision instrument useful in determining accurately the
diopter setting of another instrument. It is placed
between the eye and the instrument to be checked. The
focusing sleeve of the dioptometer is then adjusted until
the field is in sharp focus. The diopter setting of the
instrument being checked is then read from the focusing
sleeve (usually calibrated in 0.1 diopter) of the
dioptometer. Set at zero diopter the dioptometer can be
used to check-the accuracy of the zero diopter setting in
the collimating telescope itself which is commonly
available for checking zero diopter setting.
Figure 5-22. Dioptometer
5-16.
Collimation.
a. Definition. Collimation is the alinement of
the optical and mechanical axes of an instrument and
necessitates a reticle in the collimating telescope so it
can be used as a telescopic sight. (for reticles, see para
5-19). Alinement can be obtained by sighting through a
fixed collimating telescope and the instrument to be
collimated, and adjusting accordingly.
b. Lens Decentration. Lens decentration is
utilized frequently in collimating optical systems,
especially binocular instruments, which usually employ
objectives mounted in eccentric rings (para 4-18) to
permit movement in any direction across the axis. If a
lens is decentered (moved across the axis of the system)
it will cause an image shift in the same direction. The
optical center seldom coincides with the exact
geometrical center of a lens so that an image shift
frequently will result from revolving a lens in its cell. In
fact, disturbing any lens in a collimated system will
necessitate collimation of the system again as any lens
which fits freely enough to facilitate removal will almost
surely not go back in its exact position.
c. Prismatic. If prisms are employed in a
system, collimation also can be affected either by sliding
prism parallel to the plane of reflection (causing an
image shift in same direction) or by tipping prism and
plane of reflection (shifting image in same direction).
5-11
TM 9-258
Section VI. FUNCTIONS OF TELESCOPIC SIGHTS
designed to measure angular distance between two
points (stadia lines in a transit or grid lines in a military
sight). As it is in the same focal plane as a real image, it
appears superimposed on the target and can be viewed
by the eye with the same accommodation required for
viewing the target or field.
b. Location. In an optical system employing a
lens erecting system, there are two possible reticle
locations (fig 5-23). It may be placed either in the image
plane of the objective or at the focal point of the eyepiece
(image plane of errectors). If the erecting system
increases the magnification and the reticle is in the
image plane of the objective, the reticle lines will appear
wider on the target than if placed at the focal point of the
eyepiece. For this reason, the reticle in a low-power rifle
scope usually is placed in front of the erectors and in
high-power target-type rifle scopes, it is at the focal point
of the eyepiece. With a lens erecting system, the reticle
may be placed at the front or rear of the erecting system,
or reticles may be placed at both points, and their
patterns are then observed superimposed upon each
other. The preferable position with this type of erecting
system is in front of the system as the objective and
reticle then form a unit and any shift of the erecting
system does not disturb the alinement of these elements.
With a prism erecting system, the reticle usually is
placed in back of the erecting system.
5-17. Definition.
Terrestrial telescopes having a
reticle in a focal plane in the optical system are
commonly known as telescopic sights or military
telescopes.
Such a reticle can be placed in an
astronomical telescope, as in the collimating telescope,
but a telescopic sight is universally considered to be an
instrument having an erecting system.
5-18. Advantages.
A
telescopic
sight
is
advantageous in that it permits the viewing of reticle and
target in the same optical plane and does not require
precise alinement of the eye with respect to the line of
sight. Alinement of the eye need be only within the exit
pupil diameter. In comparison, open rifle sights require
the eye to attempt an actual impossibility; the focusing
simultaneously on rear sight, front sight, and target. The
eye, furthermore, must be in perfect alinement with all
three.
5-19. Functions of Reticles.
a. Application. Reticles (para 4-9) are used in
fire-control instruments for superimposing markings or
a predetermined pattern of range and deflection
graduations on a target. A reticle in its simplest form is a
post or picket, or it may consist of two intersecting lines;
then the line of sight through their intersection will be in
the center of the field of view. It represents the axis of
the bore of the weapon when adjusted for short range
firing or is fixed at a definite angle to the bore of the
weapon for long range firing. A reticle is used as a
reference point for sighting or aiming, or it may be
Figure 5-23. Reticle location
of the principle of universal focus through a small
aperture (familiar to most marksmen). In such a system,
it is impossible to have both reticle and target image in
sharp focus simultaneously. Note also that the reticle
actually is outside the system. A reticle cannot be placed
within this system as no real image plane
5-20. Galilean-Type Rifle Sight.
In this Britishdevised system, a long focal length objective (bearing a
reticle to its front surface) is mounted in place of an open
front sight. A negative eyelens and aperture replace the
conventional peepsight. The aperture provides sharper
definition
and
limited
parallax
by
use
5-12
TM 9-258
exists. Also, the negative eyelens alone would minify the
reticle and require eye accommodation to focus on it so
that it would be impossible to see both field and reticle
simultaneously with the same accommodation. Although
this system is of basic optical interest, it has no practical
military application in this country.
Section VII. OPTICAL FACTORS IN TELESCOPE DESIGN
This formula is applicable to all types of telescopes.
5-21. Magnification or Power of Telescope. Power
in a telescope can be determined by dividing the
diameter of the entrance pupil by the diameter of the exit
pupil.
Power = AP
EP
where AP is the diameter or aperture of the entrance
pupil and EP is the diameter of the exit pupil (fig 5-24).
NOTE
Power in optical instruments is denoted
by the letter x. For example, a 6 x 30
binocular is 6-power and has an
entrance pupil or objective size of 30
millimeters.
Figure 5-24. Entrance and exit pupils.
5-22.
Entrance Pupil.
a. Function. The entrance pupil diameter is
limited by the clear aperture diameter of the objective
lens. This clear aperture is limited by the inside diameter
of either the lens cell or the retaining ring as indicated in
figure 5-24. The entrance pupil can be viewed as such
from the objective end of the instrument.
b. Measurement. The entrance pupil can be
approximated by measuring directly across the objective
with a transparent metric scale. Usually the above
method is sufficiently accurate for most practical
purposes.
5-23.
Exit Pupil.
a. Location. The diameter of the bundle of
light (figs 5-24 and 5-25) leaving an optical system is
determined by the size of the exit pupil. If an instrument
is held at arm’s length, the exit pupil is seen as a virtual
image of the aperture stop and will appear as a bright
disk of light (fig 5-25). The position of the exit pupil can
be determined by directing the telescope towards an
illuminated area, such as the sky, and by holding a piece
of translucent paper or ground glass behind the eyepiece
in the place where the emergent beam is smallest and
most clearly defined. This disk of light formed at the exit
pupil is called the eye circle or Ramsden circle.
5-13
TM 9-258
Figure 5-25. Exit pupil-virtual image of objective.
b. Plane.
In figure 5-25, rays 1 and 2
originating at opposite limits of the true field (para 5-25)
cross at a common point in the objective lens and
recross back of the eyepiece in the plane of the exit
pupil. In the exit pupil plane must be the point of rotation
of the observer’s eye if the observer is to see the full
field.
c. Diameter. The diameter of the exit pupil can
be measured by pointing the telescope toward a light
source (out a window) and inserting a piece of translucent
material in the plane of the exit pupil (para 9-3). The
diameter of the image then can be measured on the paper.
This diameter also can be approximated by holding the
telescope away from the eye and measuring across the
eyelens the diameter of the bright disk seen through the
eyelens or it can be measured quite accurately by using a
dynameter (para 5-24).
5-24. Ramsden Dynameter.
a. Basic Use. The Ramsden dynameter
(fig 5-26) is used in measuring the position and diameter of
the exit pupil. The dynameter is essentially a magnifier or
eyepiece with fixed reticle. The eyepiece and the reticle
move as a unit within the dynameter tube. When in use,
the dynameter is placed between the eye and the eyepiece
of the instrument and focused until the bright disk of the exit
pupil is sharply defined on the dynameter reticle. The
diameter of the exit pupil is then measured directly on the
dynameter reticle (usually graduated in 0.5 millimeters) and
the eye distance or eye relief is read from the scale on the
dynameter tube.
Figure 5-26. Ramsden dynameter.
b. Purpose of Measurement. Measurement of
exit pupil and entrance pupil diameters of an instrument
is useful in determining the power of an unknown, or for
example, a foreign instrument by the use of the equation:
Power = AP; (para 5-21).
EP
If the power is known and the diameter of the exit pupil is
desired (and a dynameter is unavailable), it is usually
easier to measure approximately the entrance pupil and
compute the diameter of the exit pupil.
5-25. True and Apparent Fields of Telescope.
a. Definitions. The true field of view in a
telescope is the width of the target area or field that can
be viewed. It is expressed as either angular true field or
linear true field (figs 5-27 and 5-28). The former is the
angle at the objective included by the two extreme
principal rays which will enter the observer’s eye. The
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TM 9-258
latter is the width of field at 1,000 meters. This is for
military instruments. Commercial distance is 100 yards
(fig 5-28). The apparent field of view is the angle at the
eye included by these two extreme principal rays.
Figure 5-27. Field limit.
Figure 5-28. Linear true field.
b. Determining True Field of Telescope. True
field can be determined by sighting through the
instrument on some target at the edge of the field. The
angular movement of the instrument required to shift the
position of the target to the opposite edge of the field is
the angular true field of the instrument (para 9-5).
c. Determining Apparent Field of Telescope.
The apparent field of the instrument can be measured by
repeating the above procedure while reversing the
instrument. It will be found that the angle required will
approximate the true field multiplied by the power of the
instrument. Threrefore,
Apparent Field
Power = —————————— (a close approximation)
True Field
5-26. Factors Determining the Field of View of
Telescope.
a. Apertures. The diameter of the effective
aperture of entrance pupil (para 5-21) of the objective
lens is not a determining factor in any telescope except
in the Galilean system (unsuitable for sighting
instruments). The field of view is limited by the optical
possibilities of the eyepiece, a consideration of the
aperture diameter, and the extent of the apparent field
for any given eye relief. Eye relief is the distance from
the rear surface of the eyelens (para 4-4b ) to the plane
of the exit pupil (figs 2-34 and 5-25). The center of
rotation of the observer's eye should be situated in this
plane. For a given eye relief, apparent field (and thus
true field) is related directly to the aperture diameter of
the eyepiece.
For this reason, military telescopic
gunsights frequently have huge eyepieces to provide a
large field at the long eye relief required in such an
instrument. In a binocular or observation instrument
designed to be used at any eye relief as short as 8
millimeters, an eyepiece of small aperture diameter will
provide a large field, when used in a system of such
short eye relief.
b. Relation to Eye Relief. When eye relief is
reduced, the aperture of the eyepiece required to provide
the same angle is also reduced. The field of
5-15
TM 9-258
view is thus inversely related to eye relief. If the eye
relief is shortened and the aperture of the erectors is
increased accordingly, the field of view will be increased
without requiring any increase in eyepiece aperture. If
the size of the/eyepiece is increased and erectors are of
corresponding aperture, the field of view will be
increased for the same eye relief.
NOTE
Coversion of angular field to linear field:
LF = 2aR, where LF equal Linear Field, R equals
E
Range, "a" equals Angular True Field
measured from the axis, and E equals 1
Radian
which
equals
57.3°
(approximately).
5-27. Factors Determining Eye Relief of Telescope.
a. Definition. Eye relief is the distance from the rear
surface of the eyelens to the plane of the exit pupil in
which the eye must be positioned to view the full field
(para 2-34). Eye relief can be, and usually is, short in
high-power observation instruments. In a binocular, it is
quite short. Eye relief must be long in rifle scopes or
gunsights where recoil is to be considered. Its location
will depend on the location of the real image
(0' in figs 5-29, 5-30 and 5-31) of the aperture stop
(objective) formed by the erectors. The closer this is to
the eyepiece the greater is the eye relief.
Figure 5-29. Eye relief-symmetrical erectors with real images at focal points of errectors and image 0’
of objective between erectors.
Figure 5-30. Eye relief-objective image outside erectors.
Figure 5-31. Eye relief-collective lens in system.
b. Effects of Field Lenses. A field or collective
lens in the focal plane of the objective will shift this image
forward and shorten the eye relief (the stronger the lens
the farther the shift). For this reason, a collective lens
usually is not used in a rifle scope.
It
would make the eye relief too short in most cases for the
recoil. As mentioned in the note attached to paragraph
5-13b, the field lens in the eyepiece shortens eye relief
by shifting the plane of the exit pupil without affecting its
size.
5-16
TM 9-258
c. Power of Eyepiece. Eye relief also is
inversely related to the power of the eyepiece; i.e.,
replacing a low-power eyepiece with one of higher power
will reduce the eye relief.
5-28. Light Transmission or Ilumination.
a. Pupils.
The illumination of the image
depends on the amount of light received by the objective
and the specific intensity (whether bright daylight or
twilight) of these light rays. The amount of light received
is determined by the diameter of the entrance pupil (clear
aperture, abbreviated as AP) of the objective. The
amount of light entering the eye is limited either by the
exit pupil of the instrument or the pupil of the eye,
whichever is smaller.
(1) An S x 30 (8-power, 30-mm diameter)
glass in daytime (eye pupil diameter of 5 millimeters),
since amount of light entering the instrument is
proportional to area of entrance aperture and thus to the
2
square of its diameter, picks up (30/5 ) or 36 times as
much light as the naked eye. The area of the retinal
2
image in the eye, however, is 8 or 64 times as large as
with the unaided eye. Thus, we have 36 times as much
light distributed over an area 64 times as great, so
illumination on the retina is only 36/64 or 56.3 percent of
that with the unaided eye.
(2) Consider now an 8 x 40 binocular. In
2
this case the amount of light entering the eye is (40/5 or
64 times that entering the unaided eye. There is now 64
times as much light falling on an area 64 times as large
(power is 8, thus the comparative area of the retinal
2
image is 8 ). In this case, illumination is the same as
with the unaided eye.
(3) In an 8 x 50 glass, illumination will not increase with a
5-millimeter eye pupil because the iris of the eye will stop
and the extra light from the extra 10 millimeters of
diameter of the objective. Since,
Entrance Pupil
Power = Exit Pupil
(para 5-21).
the exit pupil of this 8 x 50 glass is 50/8 or 6.25
millimeters; therefore, the eye pupil of 5 millimeters will
not admit all this light.
b. Maximum Illumination.
For maximum
illumination at any given light intensity, exit pupil of
instrument must equal entrance pupil of eye under the
same condition.
Also, with any instrument, retinal
illumination never is greater than with the unaided eye.
Opaque foreign material such as dust or lint on any
optical surface, except one in a real image plane, will
reduce the illumination of the system.
5-29. Night Glasses. When subjected to intense
illumination(brilliant daylight), the entrance pupil of the
eye may be stopped down to 2 millimeter diameter.
When used under this illumination, an optical instrument
need have an exit pupil only 2millimeter diameter. When
the eye is subjected to very faint illumination, as at night,
the pupil opens to a diameter of 8 millimeters. Thus, for
use at night an optical instrument must have an exit pupil
of at least 8 millimeters provide the pupil of the eye with
all the light it will admit. The relationship between the
size of the exit pupil and the power of an instrument is
important. For example, a 7 x 50 binocular is suitable for
nighttime use because if the clear aperture of the
objective (50) is divided by the power of the binocular (7)
(para 5-21), the exit pupil diameter is determined to be
7;15 millimeters. This is able to pass approximately all
the light the eye pupil can use at its widest aperture.
Now a 7 x 35 binocular provides ample illumination in
daytime because its exit pupil diameter is 5 millimeters,
which is larger than the eye’s 2-millimeter opening under
intense illumination and ample for its 5-millimeter
opening under moderate illumination but insufficient for
the 8-millimeter eye opening at night.
5-30. Resolving Power of Optical Systems.
a. Definition. The numerical measure of ability of an
optical system to distinguish fine detail is called resolving
power. It is further defined as the reciprocal of the
smallest angular separation (angular limit of resolution)
between two points which can be distinguished or
resolved as separate points. Resolving power = 1/α,
where α = least angular separation in radians (fig 5-32).
As the angle becomes smaller between two points that
can be resolved, the resolving power increases and
definition, therefore, is better.
Figure 5-32. Angular limit of resolution.
5-17
TM 9-258
b. Controlling Factor. The angular limit of
resolution can be proved to be related inversely to the
diamter of the lens. According to Dawes’ rule (an
approximation): a (in minutes of arc) = 1/Do (in fifths of
an inch, fig 5-32); thus it is advisable to have large.
objectives for sharp definition.
The observation
telescope has such an objective of large diameter. The
targot shooter uses a telescope of 1 1/4-inch diamter
objective or larger. The 7 x 50 (7-power, 50-millimeter
diameter) binocular provides good resolution because of
the large diameter objective and for this reason is better
in daytime than a 7 x 35.
c. Normal
Magnification.
Normal
magnification is that power of a telescope giving an exit
pupil diameter just equal to that of the eye usually
standardized at 2 millimeters (Some authorities,
however, consider the eye to provide the best resolution
at an aperture of 4 to 5 millimeters.) For example:
D (Objective)
Normal magnification = ————————
C (Pupil of Eye)
or, normal magnification = 25.4 Do over 2 to 4,
where Do is in inches.
Normal magnification is one of three factors limiting the
magnifying power obtainable with a given objective lens.
The other two factors are the resolving power of the
given lens (RP = 1/α para 5-30a) based on diffrection
effects, and the quality of the lens (precision of grinding
and polishing and absence of aberrations) measurable
by labratory methods (fig 5-32).
d. Other Resolving Factors. The foregoing
equations are in agreement if the entrance pupil of the
eye is 2 millimeters. The plus 2 to 4 results in empty
magnification, an increase in image size of fine details so
the eye can more readily see them, but without any
increase in telescopic resolving power.
For best
resolution, the power of the eye piece in the system is
determined by the formula:
Focal Length Eyepiece = Diameter Eye
Focal Length Objective
Pupil X
Diameter of Objective
NOTE
All values in this formula are in millimeters.
Although the resolving power of the human eye
normally is equal to 1 minute of arc, for long
continued observation this becomes 2 or 3
minutes (due to fatigue). This, for continous
observation an instrument of greater power is
needed to provide the same definition obtainable
with a lower power telescope used for short
intervals. Transparent foreign material such as
grease or fingerprints on a lens will impair
definition (resolving power). Opaque foreign
material on the eye-lens either may impair
definition or blot out small portions of the field.
5-31. Optical Glass Used in the Design of Military
Instruments.
a. Common Types.
Although there are
numerous types of optical glass in the design of optical
systems for military instruments, five types in common
use are as follows:
Index of
Critical angle
Name
refraction
in degrees
Boro silicate crown
1.5170
41
Barium crown
1.5411
40
Baryta light flint
1.5880
39
Ordinary flint
1.6170
38
Dense flint
1.6490
37
It will be noted that as the index of refraction increases,
the critical angle decreases.
b. Specific Uses. Boro silicate crown normally
is used for the collective lenses and dense flint for the
dispersive lenses in objectives, erecting lenses, and
eyepieces. In the Kellner eyepiece (A, fig 4-5), boro
silicate crown is used for the field lens, barium crown for
the collective element, and either light flint or ordinary
flint for the dispersive element of the achromatic doublet
eyelens. Prisms, generally, are made of boro silicate
crown. For wide-angle instruments, however, baryta light
flint is used. Reticles are usually made of baryta light
flint because it etches best and windows usually are of
boro silicate crown.
5-18
TM 9-258
CHAPTER 6
PRINCIPLES OF LASERS
from the partially mirrored end of the tube.
(4) Liquid-type lasers consist of solutions
such as coumarine and rhodamine red. In this type of
laser, the liquid-laser materials are stimulated to
emission by irradiating the lasing liquid or dye solution
with another laser beam.
b. Although the heart of a laser is a light wave
amplifier, the device produces a beam when it oscillates.
The laser is, therefore, a special kind of oscillator, but it
will optically behave in the same manner as ordinary
light. For this reason, only gas lasers and solid-state
rod-type lasers, are described in detail.
6-3.
Gas Lasers.
a. A gas laser is structurally a simple device,
and in many ways it is similar to neon electric signs. A
gas laser (fig 6-1) consists of a thin glass tube about 1
foot long, and filled with a low-pressure mixture of helium
and neon gases. A pair of electrodes, a negative
cathode and a positive anode, are mounted near the
ends of the tube. These electrodes are connected to a
high voltage, direct current power supply. The electric
field produced between the two electrodes breaks down
the column of gas, instantly transforming it from a poor
conductor of electricity into a relatively good conductor.
A continuous electric glow discharge takes place within
the glass tube, and produces a continuous electric
current flow between cathode and anode through the
partially ionized column of gas.
6-1.
General.
Simply defined, a laser (Light
Amplification by Stimulated Emission of Radiation) is a
light-emitting body with feedback for amplifying the
emitted light.
The laser is a unique, and highly
specialized source of light; the beam of light it produces
has three significant characteristics, namely, laser light is
monochromatic, highly collimated, and coherent.
Although light possessing the first two properties can be
produced by some conventional light sources, only laser
light possesses all three properties. In addition, the laser
beam is a powerful and very intense light source.
6-2.
Types of Lasers.
a. There are essentially four types of lasers:
(1) Solid state rod-type lasers use
materials such as a ruby rod of about 1 centimeter in
diameter and 15 centimeters long as an elementary light
emitter or generator.
(2) Semiconductor
diode-type
lasers
use material such as gallium arsenide and consist of a
junction formed by p-type material and an n-type
material. In this type of laser, stimulation to laser
emission occurs by passing a current through the
junction.
(3) Gas-type lasers use helium-neon,
argon, carbon dioxide, nitrogen and Xenon. In this type
of laser, stimulation to laser emission occurs by passing
a current through the gas. The current causes the gas to
ionize and radiate. The radiation oscillates within a tube
provided with mirrored ends and then discharges
Figure 6-1. Gas laser.
6-1
TM 9-258
b. The tube emits a reddish-orange neon glow
because the electric discharge causes countless
collisions among the gas atoms that excite them to high
energy states; the atoms randomly fall back to their
normal energy state, and, in so doing, emit a bundle of
light energy photons. In a conventional neon sign, these
up and down energy shifts take place at random within
the glowing gas column.
c. The mixture of gases in the laser tube has
been carefully selected so that more neon atoms are in
high energy states than in low energy states when the
discharge occurs; it is the neon gas that is responsible
for lasing. That is, as the discharge excites the helium
atoms, they collide with the neon atoms and transfer
energy to the neon atoms. This transfer of energy to the
neon atoms raises their energy state.
Stimulated
emission of photons can now take place within the
column of glowing gas. This occurs when an excited
neon atom drops to a lower energy level and emits a light
wave. As the light wave speeds past other excited
atoms, it stimulates them to emit their photons. This
process of amplification continues, since one photon
entering the column of gas at one end causes other
photons to be emitted. The reflecting mirrors at the front
and the rear of the glass tube cause any light waves
emitted in the direction of the tube to bounce
back and forth along the gas column establishing a
continuous lasing action.
d. The electric discharge continuously pumps
the neon gas atoms to higher energy states, and, at the
same time, light waves bouncing between the mirrors
stimulate the excited atoms into emitting their packets of
light energy. As a result, a steady stream of coherent,
monochromatic light is generated within the column. The
laser tube’s front mirror is designed to be partially
transmitting, i.e., it is designed to reflect 99% of the light
that hits it, and allow the other 1% to pass through. This
small portion of the laser light generated within the tube
passes through the front mirror as a narrow light beam;
its wavelength is 633 nm, and it is deep red in color.
6-4.
Solid State Rod Lasers.
a. A ruby rod solid state laser consists of an
elementary light emmitter or generator that is illuminated
by a high-intensity light (A, fig 6-2). The high-intensity
light, such as that from a photoflash lamp, makes the rod
fluoresce with a pink color. The fluorescence persists as
long as the photoflash light persists. This effect is not a
laser radiation, but just another optical characteristic of
the emitter. The ruby rod produces a characteristic pink
color because it is made of aluminum oxide (sapphire)
containing 0.05 % chromium.
Figure 6-2. Solid-state rod lasers.
6-2
TM 9-258
b. In laser radiation, the ends of the ruby rod
are highly polished so that light can pass through almost
without absorption. A mirror is placed at each end and
aligned perpendicularly to the principal axis of the rod.
When the rod is illuminated with an intense photoflash
light, it emits a fluorescent light which reflects back and
forth between the two mirrors. This increase in intensity
of the light is produced by the oscillation of the ruby light
within an optically resonant cavity formed by the rod and
the two reflecting surfaces of the mirrors.
c. The removal of the laser energy, or
radiation, from the resonant cavity is accomplished by
making the rear reflecting mirror 100% reflective and the
front reflecting mirror partially reflective. This allows
some of the laser light generated within the resonant
cavity to pass through as a laser beam of the same
diameter as the ruby rod
d. During the optical pumping of the ruby rod
by the flashlamp, some of the chromium in the ruby rod
become excited. This causes their electrons to move
away from the atoms and position themselves at higher
energy levels from which they spontaneously fall back to
their normal energy states. During this transition, each
of these electrons produces a photon of light. These
photons now oscillate by reflecting from one mirror
surface to the other within the resonant cavity. On their
way to the mirror, some of the photons collide with one or
more atoms which are in an excited state due to the
flashlamp pumping, and interact with them to produce a
photon or photons identical in energy and frequency with
the initial photon. The newly formed photons continue to
interact with other excited atoms, producing more
photons, and these photons continue interacting to
produce still more photons. When a threshold energy of
the total photonic energy within the resonant cavity is
attained, a pulse, consisting of a very intense laser beam
formed by photon waves, bursts out of the partially
reflective end of the ruby rod.
e. In practical applications the highly polished ends of
the ruby rod can be mirrored or coated with a dielectric
material, such as magnesium fluoride or cerium dioxide
(B, fig 6-2). When this is done one end is fully coated
and the other is partially coated so that the emitted laser
light can pass through it. This laser beam, as it emerges
from the ruby rod, has a slight divergence.
6-5.
Laser Optics.
a. The extremely tight beam produced by the
laser, with final tightening obtained by external. optical
systems, gives it the ability to travel extreme distances
with little divergence. The laser output can be focused to
a parallel beam where spreading is expected to be less
than one foot per mile of travel. A beam of light from a
ruby laser would suffer so little divergence that, for
example, it would be concentrated in an area ten miles
across when it reached the moon; an ordinary search
light with the same intensity would place a beam over
25,000 miles wide on the moon.
b. Brightness may approach millions of times
that of the sun, on a relative bandwidth basis. Spectral
narrowness allows good signal to background ratios to
be realized. Compared to microwave systems, laser
devices will allow construction of equipment with an
antenna only inches across.
6-3
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CHAPTER 7
INFRARED PRINCIPLES
discontinuous spectrum source, a line source, or a band
source.
e. An intense source of infrared radiation can
be devised using the maser (Microwave Amplification by
Stimulated Emission of Radiation) principle. This type of
source is very directional, is concentrated within a narrow
spectral interval, and possesses a high degree of
coherence.
7-3.
Thermal Radiation.
The electromagnetic
energy that is emitted from the surface of a heated body
is called thermal radiation. This radiation consists of a
continuous spectrum of frequencies extending over a
wide range. The spectral distribution and the amount of
energy radiated depend chiefly on the temperature of the
emitting surface. Careful measurements show that at a
given temperature there is a definite frequency or
wavelength at which the radiated power is maximum,
although this maximum is very broad. In addition, the
frequency of the maximum is found to vary in direct
proportion to the absolute temperature. This rule is
known as Wien’s law.
At room temperature, for
example, the maximum occurs in the far-infrared region
of the spectrum, and there is no perceptible visible
radiation emitted. But at higher temperatures, the
maximum shifts to correspondingly higher frequencies.
Thus at about 5000C and above, a body glows visibly.
7-4.
Infrared Detectors.
a. The central element in any infrared
detection system is the detector. This is the device
which transduces the energy of the electromagnetic
radiation falling upon it into some other form (in most
cases, electrical). The mechanisms used to perform this
function are broadly classified as either photon detectors
or thermal detectors.
b. When radiation of wavelength less than a critical
value falls upon the surface of certain materials,
electrons are emitted from the surface. Photo tubes
using this photoemissive effect are very widely used.
However, the nature of the effect is such that
photoemissive detectors operate in the ultraviolet, visible,
or very near infrared. The photoemissive effect, also
termed the external photoelectric effect or simply the
photoelectric effect, was discovered by Hertz in 1887 and
explained by Einstein in 1905. The image converter is
the only type of detector which has found much
application in infrared technology, but its use is
7-1.
General. Infrared light is considered to be
radiant energy in the band of wavelengths between about
0.76 and 100 microns. The portion of the band between
0.76 and 3 microns is sometimes referred to as near
infrared light, and the portion between 3 and 100 microns
has been called far infrared light. Most of the infrared
light band overlaps the heat radiation band of
electromagnetic radiation. By definition, infrared pertains
to or designates those radiations, such as are emitted by
a hot body, with wavelengths just beyond the red end of
the visible spectrum.
7-2.
Infrared Radiation.
a. The term infrared radiation is used to
describe electromagnetic radiation whose wave-length
lies just beyond the red end of the visible spectrum, and
the beginning of the region that can be detected by
microwave radio techniques.
The distinction, for
example, between infrared astronomy and radio
astronomy is, therefore, an arbitrary one based entirely
on differences in detection techniques.
b. A study of source characteristics is
important because of the help such knowledge provides
in the choice of detectors and the design of optical
elements. In many applications the radiation source is
not subject to control. This is most often true for passive
systems where the objects are detected by their natural
radiation. It also may be true for those active systems
that use a source for the illumination of a scene so that
objects may be detected by the reflected radiation.
c. The natural division or grouping of infrared
sources depends upon the nature of the wavelength
distribution of the emitted energy. One type of source
emits radiation over a very broad and continuous band of
wavelengths. A plot of its emission versus wavelength is
a smooth curve which usually passes through only one
maximum. This type is called a continuous spectrum
source, or simple a continuous source.
d. Another type of source is one which
radiates strongly is some relatively narrow spectral
intervals, but, in other wavelength intervals, the source
does not radiate at all. A plot of emission versus
wavelength reveals a series of emission bands or lines.
The curve is discontinuous and the source is called a
7-1
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limited to the very near infrared. The most elemental
form of photoemissive cell is the vacuum photocell,
consisting of a photo cathode and an anode in an
evacuated enclosure.
c. The photon effect most widely used in infrared
detection is photoconductivity. The reason for this is that
photon effects in general are faster and more efficient
than thermal effects, and photoconductivity is the most
simple of the photon effects. Photoconductive detectors
as a class are easier to produce and easier to use than
detectors based upon the other photon effects. The
photoconductive detector (fig 7-1) is placed in the input
circuit of an amplifier.
In order to detect the
change in conductivity of a photoconductor upon
exposure to radiation, it is also necessary to supply a
bias battery and a load resistor. The photoconductor of
dark resistance R is placed in series with a load resistor
RL and a bias battery Vb. Infrared radiation, which is
modulated or chopped, is absorbed by the
photoconductor, thereby causing its resistance to
decrease with a corresponding increase in the current
flowing in the circuit and a greater portion of Vb to appear
across RL. The voltage appearing across RL is coupled
through capacitor c to the input circuit of the amplifier,
and represents the radiation absorbed by the
photoconductor.
Figure 7-1. Photoconductive detector.
d. Another internal photo effect of interest in
known as the photovoltaic effect. As the name implies,
the action of photons in this case produces a voltage
which can be detected directly without need for bias
supply or load resistor. In addition to the photovoltaic
detector, the operation of the photodiode and the
phototransistor are based upon this effect. That is, as
long as radiation falls upon the p-n junctions within the
body of a semiconductor, electronhole pairs will be
formed and separated by the internal field at the junction.
If the semiconductor ends are short circuited by an
external conductor, a current flow in the circuit. On the
other hand, if the ends are open circuited by a high
impedance voltage measuring device, a voltage will exist
as long as radiation falls upon the p-n junction.
e. Thermal detectors make use of the heating
effect of radiation. They are simply energy detectors and
their responses are dependent upon the radiant
power which they absorb. The most well known forms of
thermal detectors are the bolometer and the
thermocouple. The bolometer has seen widespread
military use, while the thermocouple has found extensive
commercial application.
f.
A bolometer is a radiant power detecting
device, the operation of which is based upon measuring
the temperature change in resistance of a material due
to the heating effect of absorbed radiation (fig 7-2). The
simplest form of bolometer is a short length of fine wire
whose resistance at a given temperature is known.
Radiation allowed to fall upon the wire is partially
absorbed, causing a rise in temperature. The resistance
change due to the change in temperature is a measure
of the radiant power absorbed. Because bolometers are
power detectors, they are capable of detecting radiation
of all wavelengths and are widely used as detectors of
microwave power.
7-2
TM 9-258
Figure 7-2. Bolometer circuit.
g. Bolometers may be of three types: metal,
semiconductor, and superconductor.
Metal and
semiconductor bolometers are operated at ambient
temperatures, whereas the superconducting bolometer
must be cooled to temperatures near absolute zero. A
form of metal bolometer used in microwave work
consisting of an encapsulated platinum wire is known as
a barretter; semiconductor bolometers are known as
thermistor bolometers, thermistor denoting thermally
sensitive resistors. Unlike a barretter the resistance of a
thermistor drops as its temperature rises.
This
characteristic has caused semiconductors to be much
more widely used than metals from bolometers. A
superconducting bolometer has the advantage over
thermistor and metal bolometers of reduced thermal
noise, reduced heat capacity, and a step resistancetemperature curve. On the other hand, the problems
associated with low temperature operation and precise
temperature control are severe.
h. The radiation thermocouple was one of the
earliest infrared detectors. It consists of a junction of two
dissimilar metals.
Absorbed radiation causes the
junction temperature to rise, and the heating of the
junction generates a small flow of electric current. This
action is responsible for generating a voltage which is
proportional to the temperature rise and, therefore, is
proportional to the intensity of the radiation. A widely
used form of the thermocouple is the radiation
thermopile; it consists of a parrellel array of
thermocouples. The thermopile has a higher response
than the thermocouple because of the use of multiple
junctions. However, its response time is long and it is not
suitable for ac amplification techniques. The fragile
construction makes it of little use in applications where it
would be subject to vibration and shock.
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CHAPTER 8
TYPICAL FIRE CONTROL INSTRUMENTS
Section I. TELESCOPE
panoramic types. Telescopes are incorporated in some
periscopes, others are used in constructing binoculars
and rangefinders.
8-2.
Telescope (Rifle).
a. The telescope illustrated in figure 8-1 is
typical of those designed for use with the rifle for direct
fire.
8-1. General. Telescope is the standard nomenclature
designation for instruments of a number of different
types. Some typical telescopes illustrated and described
in this section are designed to be used for observation;
others are designed to aim weapons. Of these some
telescopes are of the straight tube type; some
observation telescopes are of the prism-offset type; other
telescopes are of the articulated, elbow and
Figure 8-1. Telescope (Rifle)—assembled view, reticle pattern.
b. The telescope is secured in a rear sight
mount which is in turn secured to the rifle so that the
telescope and mount move with the rifle in azimuth and
elevation. The gunner aims with that part of the reticle
which represents the desired deflection and range. The
reticle may be illuminated for night operation.
c. This telescope has a slender tube and
enlarged eyepiece with a 2.8X magnification and
approximately 70 3 minutes field of view.
d. The telescope body is a piece of cold-
drawn seamless steel tubing about 1 inch in diameter
with an adapter shrunk onto the rear end. The front end
of the tube is enlarged to about 1½ inches to
accommodate the objective lens assembly.
8-3.
Telescope (Sniper’s Sighting Device).
a. This telescope (fig 8-2) is a straight tubetype telescope with a fixed focus and is designed for
direct sighting. It is typical of those designed for use as a
sniper's sighting device with rifles to aid in obtaining
more accurate fire.
8-1
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Figure 8-2. Telescope (Sniper’s Sighting Device)—assembled view.
b. The telescope is characterized by a large bright field
of view; has an elevation range knob calibrated from 0 to
900 meters, in 50-meter intervals, causing a total change
in elevation of 12.1 mils. A windage knob, having a
range of 20 minutes either right or left, is used to correct
drift caused by the wind.
The telescope has a
magnification of 2.2 power.
c. The telescope is equipped with a sunshade to shade
the objective and prevent reflections and an eyeshield to
position the observer’s eye at the proper eye distance.
8-4.
Telescope (Observation).
a. The observation telescope (fig 8-3) is
typical of those designed for observation purposes by the
infantry in observing the effectiveness of artillery fire.
Figure 8-3. Observation telescope-assembled view with
tripod.
8-2
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b. The telescope image is erected by means of
two Porro prisms and magnified by the lenses in the
eyepiece, as shown in figure 8-4.
Figure 8-4. Observation telescope-optical elements and optical diagram.
The telescope is of 20 x power with a field of
c.
view of 20° 12 minutes, is approximately 14 1/2 inches
long, is focused by turning the knurled focusing sleeve
and is mounted on the tripod which supports the
instrument about 11 inches above the ground. With this
tripod the telescope can be swung around to any desired
position. An elevating
8-5.
the
screw is provided for elevating or depressing the forward
end of the telescope.
d. The telescope is strapped in position in a
cradle which in turn is mounted on the corresponding
tripod. The tripod folds into a compact unit and fits into a
carrying case for ease and convenience in
transportation.
Section II. ARTICULATED TELESCOPES
mounted coaxially with the gun.
General.
b. The optical system diagram of one type of
a. The articulated telescope is a component of
articulated telescope is illustrated in figure 8-5.
direct free control system and is generally
Figure 8-5. Articulated telescope - optical system diagram.
8-3
TM 9-258
system. It is the main part of the secondary, direct, fire
control system used by the gunner and is mounted
coaxially with the gun.
8-6.
Articulated Telescope (Tank).
a. The articulated telescope illustrated in figure
8-6 is typical of those designed to supplement the
primary and more sophisticated tank fire control
Figure 8-6. Articulated telescope (Tank) - assembled view.
b. The optical system of this articulated
telescope is shown in figure 8-5. The telescope has an
8-power magnification with a field of view of 71/2 and is
used for viewing the target during daylight and periods of
artificial illumination.
7.62 mm machinegun, and the secondary fire control
instruments for conventional ammunition. The telescope
attached to the mount, is coaxially connected with the
152 mm gun/launcher and embodies and articulate joint
so that the eyepiece remains stationary and available to
the gunner throughout the range of gun elevation and
depression. One of two powers, 8X or 12X, may be
selected by the gunner.
8-7.
Articulated
Telescope
(Self-Propelled
Vehicle).
a. The articulated telescope illustrated in figure
8-7 and its associated mount are the primary fire control
instruments for the missile system and the
Figure 8-7. Articulated telescope ( Self-Propelled Vehicle)-assembled view.
8-4
TM 9-258
b. The optical system of this articulated
target image. After passing through the prisms in the
articulated joint, the inverted image is erected and
telescope forms an inverted image of the viewed target.
projected by the power lens erecting system through the
One of the two reticle patterns is projected into the
offset prisms to the eyepiece.
optical train of the telescope by the beamsplitter and
appears
superimposed
on
the
Section III. ELBOW TELESCOPES
c. There are elbow telescopes of a number of
8-8.
General.
designs differing in components of the optical system,
a. The elbow telescope is used wherever it is
method of mounting, and other details depending upon
necessary to direct the line of sight through a 90ƒ angle.
the purpose of the instrument. The specific instruments
This is desirable where the range of the angle of
described herein are typical of those designed for
elevation of the instrument is considerable. Telescopes
different purposes.
of this type are used to permit observers to assume the
most convenient positions for the operation of their
8-9.
Typical Elbow Telescopes.
instruments. Such telescopes are employed in the
a. The elbow telescope illustrated in figure 8-9
pointing of recoilless rifles, towed and self-propelled
is typical of those designed for direct sighting in elevation
howitzers and guns for tracking the target and fire, and
as a part of a two-sight, two-man system of the howitzer.
for other purposes.
It has 3X magnification, 13° 20 minutes field of view, and
b. An elbow telescope is essentially the same
has a combination reticle graduated for use with two
as a straight tube telescope except that an erecting
types of ammunition. The elbow telescopes of this
prism (e.g., Amici) may be used in the elbow to erect the
series are of the fixed focus type, are not provided with
image and also produce the 90ƒ deviation. This
filters, have two reticle illuminating windows, an
eliminates the lens erecting system and shortens the
instrument light adapter, and are identical except for
optical system. The schematic diagram of one type of
differences in reticle patterns (fig 8-9).
elbow telescope is illustrated in figure 8-8.
Figure 8-8. Schematic diagram of elbow telescope.
8-5
TM 9-258
Figure 8-9. Elbow telescope (Howitzer)- assembled view, reticle patterns.
b. The elbow telescope illustrated in figure 8-10
is typical of those designed for use as a component of
various sight units for pointing the weapon in azimuth
and elevation for both direct and indirect fire. The reticle
may
be
illuminated
for
night
operation. It has a 3X magnification and a field of view
of 12° 12 minutes, a removable mounting ring, is of the
fixed focus type, and is not provided with filters.
8-6
TM 9-258
Figure 8-10. Elbow telscope (Sight Units)-assembled
view, reticle pattern.
c. The elbow telescope illustrated in figure 8-11
is the basic instrument used for laying the towed howitzer
in elevation for direct fire. It is mounted and bore-sighted
in a mechanism which is integral with the upper part of
the elevation quadrant. The instrument is basically
similar in function to other direct fire telescopes now in
use except for a reticle presentation of multiple ballistic
data and use of a moveable range marker than can be
set to range values for direct fire. This elbow telescope
has a field of view of 142 mils and a 8X magnification.
Figure 8-11. Elbow telescope (Towed Howitzer)assembled view.
d. The elbow telescope illustrated in figure 8-12
is the direct fire telescope used for positioning the
howitzer on targets visible from the vehicle.
8-7
TM 9-258
Controls for laying the howitzer on line-of-sight targets
are the same as those utilized for indirect fire, since the
telescope and mounts are bolted to the howitzer mount.
Correction for cant is achieved by rotating the telescope
within its mount and
thereby erecting the reticles with the aid of an illuminated
cross-level. The howitzer is then elevated and the target
alined to the proper range mark on the reticle. It is a 4
power elbow-type instrument with a field of view of 10°.
Figure 8-12. Elbow telescope ( Self Propelled Howitzer)- assembled view.
Section IV. PANORAMIC TELESCOPES
raising or lowering of the line of sight to any required
General.
angle. By the combination of these motions, the line of
a. The panoramic telescope is a type of firesight can be directed on any aiming point.
control instrument employed to aim a gun or howitzer in
c. The different types and models of panoramic
azimuth. It is used in conjunction with a telescope mount
telescopes are of the same basic design. They differ in
to which it is assembled. The telescope mount travels
the components of their optical systems, the angle at
in azimuth with the weapon, carrying the line of sight of
which the eyepiece is mounted with relation to the upright
the telescope: The telescope provides the mechanisms
body of the instrument, the indexing of the scales,
for setting the azimuth angle while other associated
provisions for setting in deflection, method of mounting,
equipment supplies the elevation angle of site (vertical,
reticle patterns, manner of adjustment, lighting, and other
angle.).
details. A number of these differences are determined by
b. The characteristic feature of the panoramic
the characteristics of the materiel with which the
telescope is that it maintains an upright image regardless
instrument is used.
of whether the line of sight is directed forward, to the
d. An objective prism, a rotating Dove prism,
side, or to the rear of the observer (para 4-7d (4) and
and an Amici prism (fig 8-13) comprise the erecting
(5)). The upper or rotating head of the instrument and
system of the great majority of panoramic telescopes. A
the line of sight may be rotated 360ƒ or through any
Pechan erecting prism assembly may be used in place of
desired angle in the horizontal plane without requiring the
the rotating Dove prism.
observer to change his position. The mount provides for
the
8-10.
8-8
TM 9-258
Figure 8-13. Panoramic telescope (towed Howitzer) - assembled view, optical elements, and optical diagram.
double-convex eyelens.
A reticle with appropriate
pattern for the type of materiel used is mounted in the
focal plane of the objective between the Amici prism and
the eyepiece. The model designation of the telescope
indicates the reticle pattern. The reticle is illuminated by
a lighting system for night use. An open sight on the side
of the rotating head enables the observer speedily to pick
up the designated target.
8-11.
Panoramic Telescope (Towed Howitzer).
a. The panoramic telescope illustrated in figure
8-13 is typical of those designed for use on a telescope
mount primarily to point a gun or howitzer in azimuth for
indirect fire and may also be used for direct fire. It has a
magnification of 4 power and a field of view of 10°. An
azimuth scale and micrometer are provided for setting
deflections. A throwout lever permits rapid traversing
without the use of the azimuth knob. The 900 objective
prism in the head of the telescope may be rotated in
elevation a limited amount, if necessary, to bring the
aiming point into the field of view.
b. The erecting system is comprised of an
objective prism, rotating Dove prism, and Amici prism
(figs 4-11 and 4-22). A plane glass window in the
rotating head protects the optical system from dirt, dust,
and other foreign matter; it particularly protects the
objective prism from scratches and breakage.
A
compound objective is mounted between the rotating
prism and the Amici prism. The eyepiece is of the
Kellner type with an achromatized doublet field lens and
8-12. Panoramic
Telescope
(Self-Propelled
Howitzer).
a. The panoramic telescope illustrated in figure
8-14 is the basic instrument used in laying the weapon in
azimuth and is mounted directly on the telescope mount.
It is a 4 power, fixed-focus telescope with a 170 mil field
of view. It is equipped with a mechanical counter device
and a gunner's aid counter is integral with the instrument.
Included also is a reset counter which can be set to show
a reading 3,200 mils when the telescope is alined with
the aiming stakes and the weapon is parallel to the base
line. A gunner's aid counter mechanism, which permits
azimuth
8-9
TM 9-258
b. The 90ƒ head prism, objective lens, reticle
and erector lens are included in a single assembly which
permits no relative movement between the reticle and
the head 90° prism as the cab traverses in azimuth.
corrections for factors peculiar to the individual weapon
and its emplacement, can be entered easily into the
instrument and is an integral part of the counter
mechanism.
Figure 8-14. Panoramic telescope (Self Propelled
Howitzer)- assembled view.
Section V. PERISCOPES
operation of the periscope is that double reflection of
8-13. General.
light from two paralleled mirrors, each placed at a 45ƒ
a. The basic purpose of the periscope is to
angle and with their reflecting surfaces facing each other
raise the line of vision in order that targets may be seen
(fig 8-15), forms a normal erect image of the object.
from entrenchments, or from behind obstructions, or out
of
enclosed
vehicles.
The
principle
of
8-10
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Figure 8-15. Periscope (Tank Observation)- assembled view, optical elements, and diagram of principle of operation.
8-11
TM 9-258
b. Solid 90ƒ glass prisms can be used instead of
plane mirrors (fig 8-16 to reflect the light through
an angle of 90° twice and to displace the line of vision
vertically.
Figure 8-16. Optical diagram and optical elements of periscope.
when struck by a projectile and not jam or injure the
periscope body. Spare heads are furnished for a
replacement. When the periscope is in place, only the
head projects through the tank.
b. The optical system (fig 8-15) consists of two
plane mirrors, two vertical windows, and two horizontal
windows. The mirrors are mounted at 450ƒ angles,
parallel to and facing each other. One mirror is mounted
in the head, the other in the elbow. The vertical windows
are mounted in the front of the head and in the rear of
the elbow. The horizontal windows, of thicker glass, are
mounted in the bottom of the head and in the top of the
elbow.
Gaskets around the edges of the
8-14.
Periscope (Tank Observation).
a. The periscope illustrated in figure 8-15 is
typical of those designed for use as an observation
instrument, to view outside objects from the inside of a
tank. It has a rectangular body with a removable top or
head and a removable bottom or elbow. The top surface
of the head and the bottom surface of the elbow slant at
the same 45ƒ angle. Clamps attached to the head and
elbow engage latches operated by eccentric assemblies
in the periscope body. They hold the head and elbow
firmly on the body yet permit ready removal. Plastic
material is used in the construction of the head to permit
it
to
shatter
into
small
fragments
8-12
TM 9-258
d. The head assembly contains a 90ƒ prism for
directing the forward line of sight through the window of
the body assembly. The body assembly contains the
aforementioned window and another 90° prism that
displays the field of view to the driver. This periscope
provides a 500 horizontal and 140 vertical field of view.
windows provide a seal against the entrance of dust and
moisture. Crosslines are etched on the horizontal
windows for rough sighting in observation.
c. The periscope is supported in either a
viewing or a retracted position in a periscope holder
which is secured to the body of the vehicle. The holder
permits rotation of the periscope in elevation and
azimuth.
d. This periscope is 11 inches long, 6 1/2
inches wide, 3-1/8 inches in thickness, and has an offset
of 8-7/8 inches.
8-16.
Periscope (Tank).
a. The periscope illustrated in figure 8-18 is a
monocular-type optical sighting instrument that serves as
the main component in the fire control system of the
machinegun on the tank.
8-15.
Periscope (Self-Propelled Vehicle).
a. The periscope illustrated in figure 8-17 is a
unity-power daylight viewing device employed by the
driver of the vehicle. Three periscope stations are
provided within a notable armored hatch which afford the
driver the maximum of safety and vision of the terrain
during tactical maneuvers.
Figure 8-18. Periscope (Tank)- assembled view
Figure 8-17. Periscope (Self-Propelled
Vehicle)assembled view.
b. The eyepiece of the periscope, which
contains the eyelens, center lens and field lens (fig 8-19)
is fixed and remains stationary relative to the vehicle.
The prism rotates the line of sight from 15ƒ depression to
60ƒ elevation in maintaining the line of sight parallel to
the line of fire of the machinegun throughout the gun's
full range. Connection from the gun to the periscope is
made
b. Each periscope consists of two separate
units, a head assembly and a body assembly. Both are
coupled together by a common mount.
c. Two 90ƒ prisms and an optical instrument
window (fig 8-16) comprise the optical components of
this periscope.
8-13
TM 9-258
through a quick-release sight link assembly, movement
of which is transmitted to the prism through tapeconnected
pulleys.
The
sight
link
assembly is not part of the tank periscope, but is issued
as equipment with the periscope.
Figure 8-19. Periscope (Tank) - diagram of optical system.
8-14
TM 9-258
c. Means for making azimuth boresighting
adjustment is provided by azimuth adjustment pin at the
mounting flange of the sight. Elevation adjustment is
provided at the connecting arm to which the sight link
assembly connects. A rubber eyeshield, mounted over
the periscope’s eyepiece, serves to protect the eyelens
and prevent injury to the machinegunner.
8-17.
Periscope (Infrared).
a. The periscope illustrated in figure 8-20 is an
infrared viewing device of the binocular type used in night
driving of vehicles. Invisible infrared rays are projected
forward from headlamps at the bow of the vehicle to
"illuminate" the field of view. The periscope converts the
infrared image to a visible image which is viewed through
conventional eyelenses. For best vision at average
speeds the range of the periscope is focused at 18
yards.
Figure 8-20. Periscope ( Infrared) - assembled view.
b. This periscope has a magnification of 1
power, a field of view of 26.8° and a focal point of
18 to 20 yards.
8-15
TM 9-258
Section VI. BINOCULARS
vision and to provide a compact instrument (fig 8-21).
General.
The optical axis of each scope must be parallel to the
a. A military binocular or field glass consists of
hinge throughout the entire movement of the
two terrestrial prism offset-type telescopes pivoted
interpupillary range; otherwise each eye will see a
about a hinge which provides adjustment for
different field resulting in a double image, if deviation is
interpupillary distance for use with both eyes. The left
great, and headache will result from eyestrain in bringing
telescope of many models contains a reticle (H and I, fig
the two fields into coincidence.
4-25). An erecting system is required in each of the
halves of the instrument; prisms are used as erectors to
increase stereoscopic
8-18.
8-16
TM 9-258
Figure 8-21. Binocular (General Use)- assembled view, optical elements, and optical diagram.
8-17
TM 9-258
b. The power or magnification of the binocular,
like that of the monocular telescope, depends upon the
focal lengths of the objective and eyepiece groups. The
true field of view depends upon the design of the lenses
and the power. The brightness of image depends upon
the size of the objective. The amount of the exit pupil that
can be used depends upon the size of the pupil of the
eye of the observer which varies from about 2 millimeters
for very brilliant illumination to about 4 or 5 millimeters in
daytime, and about 8 millimeters for very faint
illumination. The design of an instrument of this type
determines its suitability for a specific purpose. A
binocular with large objectives and exit pupils is generally
better suited for observation at night and other conditions
of poor visibility. A binocular of this type is often referred
to as a "night glass."
c. Binoculars usually are designated by the
power of magnification and the diameter of the
objectives. Thus, a 6 x 30 binocular magnifies 6
diameters and has objectives which are 30 millimeters in
diameter. This designation usually is stamped on the
instrument.
d. Binoculars permit the use of and increase
the radius of stereoscopic vision. The observer views
the object from the two objectives which are more widely
separated than his eyes, while the magnification
provided by the instrument increases his range of vision.
For example, if the distance between the lines of sight of
his eyes is doubled by the use of prism binoculars and a
6-power instrument is used, the radius of stereovision is
increased from a normal of approximately 500 meters to
approximately 6,000 meters (500 meters times 2 times
6).
e. The binocular hinge is equipped with a scale
which indicates in millimeters the interpupillary distance
(distance between the pupils of the eyes). When the
proper setting for the observer has been determined, any
binocular may be adjusted at once for the correct
interpupillary distance.
f.. All of the binoculars covered in this manual
are constructed for separate focusing; that is, each
eyepiece can be focused independently of the other by
turning the diopter scale in a plus or minus direction.
The scale, which is calibrated in diopters, indicates the
correction required for the corresponding eye. Once the
proper setting for diopter adjustment has been
determined for each eye, the setting may be applied
immediately on future use.
8-19.
Binocular (General Use).
a. The binocular illustrated in figure 8-21 is
typical of those designed for use as a general purpose
instrument by all services for observation and
approximate measurement of small angles. It has a
magnification of 6 power, the objective diameter is 30
millimeters, and the field of view is 80 30 minutes. Each
of. the prism offset-type telescopes has an achromatic
objective and a Kellner-type eyepiece with a compound
eyelens and a plano-convex field lens. A porro prism
erecting system is employed in each telescope. The
eyepieces are individually adjustable from plus 4 to
minus 4 diopters to meet eyesight variations. Graduated
diopter scales permit prefocusing of both eyepieces.
The interpupillary distance adjustment also is provided
with graduations to permit presetting.
b. A reticle is included in the optical system for
the left eye (I, fig 4-25). The binocular is equipped with a
filter and a carrying strap. This model has improved
waterproofing.
8-20.
Battery Commander’s Periscope.
a. The
battery
commander’s
periscope
illustrated in figure 8-22 is a binocular instrument which
consists of two periscope-type telescopes joined at the
top by a hinge mechanism, to permit adjustment of the
distance between the eyepieces. The eyepiece being
about 12 inches below the line of sight. The BC
periscope is not hinged at the bottom (as were earlier
model BC’s periscopes) and is typical of those designed
to be used only for periscopic observation, with the
telescopes in a vertical position.
8-18
TM 9-258
Figure 8-22. Battery commander’s periscope - assembled view, reticle pattern.
b. The BC periscope employs 90ƒ prisms at the
top with their mates at the bottom to complete a Porro
prism erecting system (para 4-5c) (thus eliminating the
lens erecting system) and also to effect a vertical
displacement of the line of sight.
c. The instrument has a magnification of 10
power and a field of view of 60. It is used for observation
and for measuring angles. The right telescope contains
a reticle, calibrated in mils (1/6400 part of a circle), for
measuring
angles
in
elevation and azimuth. One lamp provides illumination
for the reticle. Another lamp, on a flexible cable, is used
as a hand light to read the instrument scales and
micrometers.
d. Each periscope is provided with filters;
amber, red, neutral (smoke), or a clear window may be
moved into position between the objective lens and
eyepiece of either telescope.
Section VII. RANGE FINDERS
General.
angle of convergence is indicative of the range. The
a. The range finder is essentially two periscopic
compensator lenses (27 and 28, fig 8-23) provide
telescopes or sights (either with one eyepiece or two)
angular deviation of the right line of sight for ranging.
with two lines of sight through prisms or reflectors at
ends
converging
on
the
target.
The
8-21.
8-19
TM 9-258
*PART OF BONDED PORRO REFLECTOR ASSEMBLY
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
LEFT END HOUSING ASSEMBLY OPTICAL COMPONENTS
End housing window
12. Right coincidence reticle window
End housing penta reflector
13. Collimator lens
End housing correction wedge
14. Right coincidence reticle
Range finder end window
15. First collective lens
Filter
16. Main boresight (gunlaying) reticle
Porro reflector (part of bonded porro reflector assembly)
17. Second collective lens
Porro reflector (part of bonded porro reflector assembly)
18. First erector lens
Objective lens
19. Correction wedge
Corrective wedge
20. Correction wedge
Corrective wedge
21. Correction wedge.
Right reticle lamp
RIGHT END HOUSING ASSEMBLY OPTICAL COMPONENTS
22.
End housing window
23.
End housing penta reflector
24.
End housing correction wedge
RIGHT MAIN HOUSING ASSEMBLY OPTICAL COMPONENTS
Range finder end window
41. Beam splitter prism
Filter
42. Eyepiece field stop
Compensator lens
43. Field lens
Compensator lens
44. Eye lens
Range scale prism
45. Combining prism
Porro reflector (part of bonded porro reflector assembly)
46. Left ocular prism
Porro reflector (part of bonded porro reflector assembly)
47. Left reticle lamp
Prism
48. Left coincidence reticle window
Objective lens
49. Left coincidence reticle
Porro prism
50. First 90-degree prism
Reticle lens
51. Second erector lens
ICS wedge
52. Second 90-degree prism
Reticle lens
53. Reticle lamp
Correction wedge
54. Diffusion disk
Collimator lens
55. Auxiliary boresight (gunlaying) reticle
Right ocular prism
56. Reticle mirror
Figure 8-23. Optical system schematic.
8-20
TM 9-258
b. The purpose of a range finder is to find the
range of an object or target. This instrument measures
distance by triangulation.
c. The range finder contains two optical
systems which permit a single observer to view the target
from points some distance apart. This distance serves
as the known leg of the triangle. It is termed the base
line (fig 8-24) and is one form of designation of a range
finder, that is, a 1-meter base instrument. One of the
two angles from which the target is observed is fixed at
900;
the
other
angle is variable, depending upon the distance of the
target from the instrument. The angle to which the 90ƒ
and the variable angle converge is termed the parallactic
angle. It becomes larger as the distance to the observed
object becomes less. Because of the extreme shortness
of the base line in comparison with the range to be
computed, the utmost accuracy and precision are
required in the determination of the angles at the two
ends of the base line.
Figure 8-24. Fundamental triangle of range finder.
d. In the stereoscopic range finder, by turning
the range knob, the target is made to appear in the same
distance plane as the central measuring mark (pip) of the
stereo reticle, when "stereoscopic contact" is
established.
e. In the coincidence range finder, the two
optical systems are focused into a single eyepiece
assembly. When the range knob is rotated, the left and
right fields of -view are superimposed and "coincidence"
of the two images of the selected target is established.
f. A very simple and practical type of
stereoscopic range finder, based on still another
principle, consists of a binocular telescope having a
scale in each ocular. The two scales will fuse into a
single scale, in relief, having the appearance -f a row of
dots receding to infinity. These dots are seen alongside
the scene observed and every object in the scene
appears to be located in the same plane as one of the
dots.
Distance to rapidly moving objects can be
determined approximately very quickly by this method,
while a slower
method could not be used to advantage with a very
rapidly moving target.
8-22. Range Finder, Tank (Typical Coincidence
Type).
a. General. The range finder illustrated in
figure 8-25 is typical of the coincidence-type instruments
designed for use as the principal ranging device in the
primary sighting system of tanks, and is used for direct
fire operation. This range finder is designed to provide
automatic and continuous ranging information through an
output shaft to the ballistic computer. For sighting
systems that do not include a ballistic computer, range
information may be read from the range scale. Accuracy
of the range finder is inherently high because of its long
base length, 79 in., and 10-power magnification. The
range finder may also be used, if the gunner’s periscope
is out of commission, for boresighting the gun. Either the
left or right end of the range finder may be used for
boresighting.
8-21
TM 9-258
Figure 8-25. Range finder , tank (typical coincidence type).
b. Characteristics. This typical coincidencetype range finder measures 85 x 14 x 12 inches overall,
weighs 149 pounds including end housing assemblies,
has a range of 500 to 4400 meters, and a field of view of
4°.
NOTE
The key letters shown in parenthesis
in c and d below, refer to figure 8-23.
c. Theory of Operation.
(1) Light from a target enters the range finder
through the right and left end housings. There is a
definite parallactic (convergent) angle between the light
entering through one end housing window from the target
and light entering through the other end housing window
from the target. This parallactic angle varies with target
distance, being larger for close targets than for distant
targets. The optical system (fig 8-23i utilizes this
parallactic angle variation as a means of measuring
target range. This is accomplished by displacing a
specially designed lens a calibrated distance in order to
deviate the light through the parallactic angle and
establish coincidence of the target at the eyepiece.
(2) Light rays, upon entering each housing
window, are reflected 90 degrees toward the center of
the range finder. These light rays enter the telescope
optical systems and, after passing through the lenses
and being reflected by the prisms, leave the instrument
by way of the single eyepiece. Light rays from the
internal collimator optical system are projected to the
telescope optical systems by means of reflectors.
Images formed at the eyepiece diaphragm are a
composite of a target image and the image of the
internal collimator optical system coincidence reticles for
both the right and left side optical systems.
d. Functional Description.
(1) General. For purposes of description, the
left optical system, the right optical system, reticle
patterns, and reticle illumination system of the range
finder are treated separately. Component parts of each
system are individually described to define their
functional purpose.
NOTE
The key numbers shown in parentheses
refer to figure 8-23.
(2) Left optical system. The left optical
system consists of an end housing assembly, a main
sighting system, and a collimating system. A target
image picked up at the end housing assembly is
transmitted into the main sighting system. A coincidence
reticle image projected from the collimating system is
also projected in this main sighting system. These
images merge with the gunlaying reticle and are
transmitted to the eyepiece which is common to both the
left and right optical systems.
(a) End housing assembly. The target image is
picked up by the end housing assembly and turned 90
degrees for introduction into the left main sighting
system.
1. The end housing window (1) is a plano plate,
that seals the end housing against dirt and moisture.
2. The end housing penta reflector (2) provides
a constant deviation of 90 degrees. The penta reflector
is adjustable to correct for tilt and deviation.
3. The end housing corrections wedge (3) on
the inboard end of the housing may be used to correct
deviation due to initial misalignment of the penta reflector
or housing. The wedge also serves as a window, which
seals the end housing against dirt and moisture.
4. The end face of the housing casting is
precision machined. Alinement keys are used to position
the end housings to the main housings.
(b) Left main sighting system. The left main
sighting system carries the target image from
8-22
TM 9-258
the left end housing assembly to the eyepiece. The left
main sighting system is housed in both left and right
main housing castings.
1. The range finder end window (4) is a plano
plate, which seals the left sighting system against dirt
and moisture.
2. The filter (5) increases contrast between the
target image and the illuminated coincidence reticle (49).
A matched filter (26) for the right main sighting system is
engaged simultaneously with the left filter by a control
lever on the right main housing.
3. A porro reflector assembly (6) and (7)
consisting of a partial reflector (7) and a full reflector (6)
bends the coincidence reticle image 180 degrees to
superimpose it on the target image. The partial reflector,
in the line of sight of the main sighting system, picks up
the coincidence reticle image from the full reflector,
which picks up the image from the left collimating
system. The path of the target image is not affected by
the partial reflector. Approximately 65 percent of the
incoming field light is transmitted through the partial
reflector, 30 percent is lost by reflection and 5 percent is
lost by absorption. Less than 30 percent of the light from
the collimating system is transmitted into the line of sight;
70 percent of the light passes through the partial reflector
and is lost. However, collimator light intensity can be
varied by a rheostat.
4. The left objective lens (8) receives light from
the target and coincidence reticle, and provides a focus
adjustment. Light rays from the left objective lens (8) are
converged upon the first collective lens (15) and are
brought into focus at the main boresight (gunlaying)
reticle (16). Moving the collective lens adjusts the focal
length to the’ desired angular values of the reticle
calibrations. The left objective lens and the first collective
lens function together as a single lens.
5. The main boresight reticle assembly
contains the main boresight (gunlaying) reticle (16).
Because it is in the focal plane of the objective lens (8)
and the first collective lens (15), this reticle is
superimposed on the target image. Control knobs move
the boresight reticle vertically and horizontally to aline it
with the target image for boresighting, range estimation,
or both. The main boresight reticle assembly has a
collective lens (15 and 17) on each side of the reticle
(16) to increase the field or peripheral light.
6. A composite image of the reticles and target,
formed by the second collective lens (17), is located at
the focal plane of the first erector lens (18).
7. The target and reticle images are positioned
vertically
and
horizontally
by
a
series
of
three correction wedges (19, 20, and 21). The vertical
calibration mechanism optical components consist of two
correction wedges (19 and 20). A gear located between
the two wedges causes these wedges to rotate in
opposite directions.
Light rays entering the first
correction wedge (19) are bent towards the thickest part
of this wedge. Due to the relationship of the thick
sections, a vertical vector resultant is formed which
optically raises or lowers the light rays. The horizontal
calibration mechanism consists of a single correction
wedge (21). This correction wedge receives the light
rays from the second correction wedge (20) of the
vertical calibration mechanism. As the horizontal wedge
is rotated, light rays are deviated in a horizontal direction.
8. The first 90-degree prism (50) reflects the
target and reticle images into the second erector lens
(51).
9. The second erector lens is used to set the
focus during the assembly of the range finder, and to
bring the focal plane of the left main sighting system to
the eyepiece field stop (42).
10. The second 90-degree prism ’52) bends the
light rays into the left ocular prism (46).
11. The left ocular prism (46) is a penta prism
that reflects the target image 90 degrees into the
combining prism (45).
12. This combining prism displaces the light rays
parallel to itself. The light rays are then reflected into the
eyepiece assembly by the rhomboidal prism component
of the combining prism.
assembly.
The eyepiece
(c) Eyepiece
contains a field stop and an eyepiece assembly. The
field stop (42) is a diaphragm for the eyepiece assembly.
It restricts extraneous or stray light for a sharp circular
field of view. The eyepiece assembly has a field lens
(43) and an eye lens (44), which together magnify the
image.
(d) Left collimating system. The left collimating
system projects the left coincidence reticle into the left
main sighting system so that the coincidence reticle
appears in the field of view.
1. The left coincidence reticle (49) is
illuminated and projected into the collimator lens (13).
2. The collimator lens (13) gathers the
diverging light from the reticle assembly and directs it,
with the coincidence reticle image, into the porro reflector
assembly (6 and 7).
3. The collimator assembly contains two
correction wedge cell assemblies, each containing a
wedge (9 and 10) that alines the coincidence reticle in
deflection and elevation so that it is projected in an exact
relationship to the target image as seen in the main
sighting system.
8-23
(3) Right Optical System. The right optical
system consists of an end housing assembly, a main
sighting system, and a collimating system. A target
image picked up at the end housing assembly is
transmitted into the main sighting system. A coincidence
reticle (14), projected from the collimating system, is also
projected in this main sighting system. An auxiliary
gunlaying reticle is located in this system, and, if
illuminated, is projected into the main sighting system.
These images are transmitted to the eyepiece which is
common to the left and right optical systems.
(a) End housing assembly. The end housing
assembly for the right optical system is identical to that
for the left optical system in (2) (a) above.
(b) Right main sighting system. The right main
sighting system carries the target image from the right
end housing assembly to the eyepiece. The right main
sighting system is housed in the right main housing
casting.
1. The range finder end window (25) is a plano
plate and seals the right sighting system against dirt and
moisture.
2. The filter (26) increases contrast between
the target image and the illuminated coincidence reticle
(14). A matched filter (5) for the left main sighting
system is engaged simultaneously with the right filter by
a control lever on the right main housing casting.
3. The compensator lenses (27 and 28)
provide angular deviation for ranging.
4. The porro reflector assembly (30 and 31) in
the right main sighting system is similar to that of the left
main sighting system, except for the halving adjustment.
The halving adjustment tilts the porro reflector to change
the elevation of the coincident reticle (14) in relation to
the target image.
5. The 90-degree prism (32) bends the target
image into the objective lens (33).
6. The objective lens (33) converges the target
image and the coincidence reticle image to a focal point
at the eyepiece field stop (42) through the beam-splitter,
the, right ocular,, and the combining prisms.
7. The beam-splitter prism (41) reflects the
image 90 degrees into the right ocular prism (40) and
superimposes the image of the auxiliary boresight reticle
(55) on the target image. It also dissipates part of the
light in the right main sighting system to balance the
transmitted light with that in the left main sighting system.
8. The right ocular prism (40) reflects the target
image into the right angle prism component of the
combining
prism
(45).
At
this
TM 9-258
point, the light combines with the reflected light from the
left field of view and proceeds to the single eyepiece.
The right
(c) Right collimating system.
collimating system performs the same function as the left
collimating system in (2) (d) above.
1. The right coincidence reticle (14) is
illuminated and projected into the collimator lens (39).
2. The collimator lens (39) gathers the
diverging light from the reticle assembly and directs it
with the coincidence reticle image into the porro reflector
assembly (30 and 31).
3. The ICS corrections wedge mount assembly
for the right collimating system is similar to the collimator
assembly for the left collimating system, except that the
ICS wedge (36) for deflection correction can be rotated
by the ICS knob. This moves the coincidence reticle (14)
in a horizontal plane.
(d) Auxiliary boresight (gunlaying) reticle
The auxiliary boresight (gunlaying) reticle
system.
system can be used for auxiliary sighting and range
estimation if the left optical system becomes inoperative.
It can also be used to check range finder alinement by
checking its coincidence with the left main boresight
reticle (16). The system consists of two assemblies: an
auxiliary boresight (gunlaying) reticle bracket assembly
and an auxiliary boresight (gunlaying) lens assembly.
1. The auxiliary boresight (gunlaying) reticle
bracket assembly has a reticle lamp (53), a dark field
cemented auxiliary boresight (gunlaying) reticle (55), and
reticle mirror (56) to reflect the reticle image into the
system.
2. The auxiliary boresight reticle lens assembly
has two reticle lenses (35 and 37). Both reticle lenses
are adjustable longitudinally along the optical axis for
proper focusing of the reticle image at the eyepiece field
stop.
In addition, reticle lens (37) is adjustable
transversely by means of the auxiliary boresight knobs to
provide horizontal and vertical movement of the reticle in
the field of the right optical system.
3. The porro prism (34) turns the light rays 180
degrees into the beam-splitter prism (41).
(4) Reticle Patterns.
(a) General. Four reticle patterns are used in
the range finder. There is one pattern for gunlaying (A,
fig 8-26), one for auxiliary gunlaying (B, fig 8-26), and
two-coincidence reticle patterns (C, fig 8-26). The
coincidence reticles are provided to aline the optical
systems (left and right) to each other. An auxiliary reticle
is projected into the right optical system in the event of
damage to the left optical system.
8-24
TM 9-258
Figure 8-26. Range finder reticle patterns.
(b)
Coincidence
reticle.
The
right
coincidence reticle (14, fig 8-23) in the right optical
system is composed of a vertical line located on the
vertical axis of the coincidence reticle pattern (C, fig 826) and a horizontal line located at the top and to the left
of the vertical line. This pattern forms the lower vertical
and left horizontal bars of the coincidence reticle pattern.
The left coincidence reticle (49, fig 8-23) is located in the
coincidence cell assembly of the right main housing. The
left coincidence reticle pattern (C, fig 8-26) is formed by a
vertical line located on a vertical axis of the reticle disk
and a horizontal line located at the bottom and to the
right
of
the
vertical
line.
This
pattern forms the upper vertical and right horizontal
members of the coincidence reticle pattern. The right half
of the coincidence reticle is alined with the left half of the
coincidence reticle to establish a reference calibration of
the instrument when the target image is brought into
coincidence.
(c)
Main boresight (gunlaying) reticle. The
main boresight (gunlaying) reticle (16, fig 8-23) is located
in the focal plane of the objective lens system, which
results in the target image and the reticle image
(B, fig 8-26) being superimposed. The aiming point of the
pattern is a boresight cross, 2 mils x 2 mils, coinciding
with the intersection of the vertical and horizontal
geometric
8-25
TM 9-258
axes. Two lines and two spaces, located on the
horizontal axis on both sides of the boresight cross
along the horizontal axis, provide a zero elevation and
lead reference representing a field 40 mils in width. The
spaces located immediately adjacent to each 1-mil arm
of the boresight cross are 4 mils wide. Each line and
space located beyond the 4-mil space represents 5
mils. Elevation and depression reference is provided by
a pair of symmetrically placed broken lines of 2 mils
below the center of the boresight cross. Each of these
lines consist of a line, a space, and a line of 5 mils
each. A 1-mil space and a 2-mil line arranged above
and below the ends of the vertical member of the
boresight cross on the vertical axis provide a zero
reference in azimuth.
reticle is coated to prevent light rays from passing
through the disk except at the lines of the reticle
pattern.
(5) Electrical lighting system. A 24-volt dc electrical
system provides illumination for the reticle and the
range scale. A control panel on the back of the range
finder provides controls for light intensity of the reticles
and scales.
e. Metric System. Since range distances are being
converted to the metric system of measurement rather
than yards, future range finders will measure distance
in meters.
8-23. Tabulated Data. The data listed in table 8-1
includes basic general characteristics of a typical range
finder which are necessary to Army maintenance
personnel for performing their function.
(d) Auxiliary boresight (gunlaying) reticle.
The auxiliary boresight (gunlaying) reticle pattern (A, fig
8-26) is identical to the main boresight (gunlaying)
reticle pattern ((4) (c) above). The reticle disk for this
Table 8-1. General Dimensions and Leading Particulars
Technical data
Value
Length (overall)....................................................
Height (overall).....................................................
Depth (overall) .....................................................
Weight (including end housing assemblies) ........
Base length..........................................................
Range ..................................................................
Magnification........................................................
Field of view.........................................................
Exit pupil ..............................................................
Diopter scale........................................................
84 in.
12 in.
14 in.
149 lb
79 in.
500 to 4,400 meters
10 power
40
0.120 in.
4.00 diopters
Section VIII. SPECIALIZED MILITARY OPTICAL INSTRUMENTS
convenience of operation but its simplicity, ruggedness,
and low cost make it particularly desirable as a sighting
device for mortars.
8-24. Collimator Sight.
a. The collimator sight (fig 8-27) is an ingenious
and relatively inexpensive type of sighting device. The
collimator is inferior to the telescope in effectiveness or
8-26
TM 9-258
Figure 8-27. Collimator sight-optical system and optical diagram
b. The principle of the collimator is that a reticle,
fairly close to the observer, can be optically transferred to
a position infinitely distant from the observer. Parallax
between the reticle and the target is thereby eliminated.
Since there are only two optical elements, a reticle on a
ground glass window and an eyelens, the entire structure
can be housed in a compact tube. The eyelens is called
a collimating lens because it renders rays parallel. It
permits observation of the reticle at infinity or with the
same eye accommodation as required to view the target.
The ground glass window is covered inside with an
opaque coating, excepting an uncoated center cross or
vertical line, through which diffused light can enter to be
viewed
through the eyelens as a cross or vertical line of light.
This cross or line of light is the reticle which is placed in
the principal focal plane of the eyelens for the infinity
adjustment.
c. When employed as the sighting device for a
weapon, the mount of the collimator is provided with
leveling mechanisms and scales which permit the
weapon to be placed at a prescribed elevation with
relation to the line of sight established by the collimator.
Sighting is accomplished by looking into the collimator
and at the target simultaneously or successively to
super-impose the cross or line upon the target.
8-25. Reflector (Reflex) Sight.
A typical optical
system of this kind is illustrated in figure 8-28.
8-27
TM 9-258
Figure 8-28. Reflex sight.
a. Condensing Lens. Light from a lamp or reflected
sunlight is concentrated on the reticle (para 4-9), usually
by using a condensing lens or lens system. A frosted
surface (either one surface of the condensing lens or of
a plane glass plate) may be used to diffuse this light and
obtain even illumination of the reticle. This light then
passes through either a perforated reticle (usually
punched in a metal disk) or illuminates a reticle pattern
etched on glass.
b. Collimating Lens. Light from the illuminated
reticle passes through a collimating lens placed one focal
length from the reticle. The collimated light from the
reticle is ordinarily introduced into the field of view by the
use of a high-reflectance-coated glass plate which
permits light to pass straight through from one side but
reflects most of the light coming from the opposite
direction. A half-silvered mirror may perform the same
function but less efficiently.
c. Parallax. To remove parallax from this type of
optical system, either the collimating lens or the reticle
must be moved until the reticle pattern .s clearly defined
or sharply focused when viewed through the collimating
telescope.
d. Operation. Since the reflected light rays from
the image of the reticle are parallel, due to the action of
the collimating lens, the reticle pattern appears at infinity
which makes it possible for the observer to super-impose
the image on the target and focus on both at once.
e. Use.
The reflex sight is a projection-type
collimator sight used for direct sighting of machine guns.
8-26. Aiming Circle.
a. The aiming circle (fig 8-29) is used to measure
the azimuth and elevation bearing angles of a ground or
aerial target with respect to a preselected base line. The
aiming circle has many of the characteristics of a
surveyor’s transit. Basically, it consists of a telescope
mounted on a mechanism which permits unlimited
azimuth and limited elevation movements. By rotating
two orientating knobs, zero azimuth heading with respect
to magnetic north or any other selected compass
heading can be established. The azimuth orienting
control knobs can be disengaged for rapid movement by
exerting an outward pressure on the knobs.
The
mechanisms are spring-loaded and will reengage when
outward pressure is removed. A locking device secures
the compass after the orienting adjustment, has been
made.
8-28
TM 9-258
Figure 8-29. Aiming circle.
accomplished by transmitting a pulse of laser light,
receiving reflecting light from the target, and converting
the time from transmission to reception into range data.
8-27. Laser Range Finder. The laser range finder
shown in figure 8-30 is a laser electronic device whose
function is to improve the first-round hit capability of the
primary
weapon.
This
function
is
8-29
TM 9-258
1.
2.
3.
4.
5.
Display Unit
Control Unit
Electronics Unit
Receive/Transmit Sight Unit
Auxiliary Power Supply Unit
Figure 8-30. Laser rangefinder.
8-30
TM 9-258
with the gun turret and the telescope sight unit of the
TOW
(tube-launched, optically-sighted, wire-guided)
missile subsystem.
8-28. Helmet Directed Subsystem.
a. The fire control subsystem shown in figure 8-31
is a helmet-directed sighting subsystem.
This
subsystem, which is helicopter mounted, interfaces
8-31
TM 9-258
Figure 8-31. Fire control subsystem, helmet-directed.
8-32
TM 9-258
Legend for figure 8-31:
1. Reflex sight M73
2. Pilot helmet sight (including pilot linkage and
extension cable)
3. Electronic interface
4. Gunner helmet sight including gunner linkage
and extension cable)
extends over the operator’s right eye, and an illuminated
reticle pattern is projected into the optical sight.
Electromechanical linkages sense the helmet sight lines
and generate sight-line signals which are processed by
the electronic interface assembly. Either operator can
command the gun turret or sight unit by means of
operator-select able cockpit switches. Under certain
circumstances, the turret and sight unit can be
commanded simultaneously, one by each operator.
b. The system enables the helicopter pilot and
copilot/gunner to rapidly acquire visible targets and to
direct either the gun turret or the sight unit to those
targets. The helmet-mounted optical sight
8-33
TM 9-258
CHAPTER 9
TESTING OPTICAL PROPERTIES OF INSTRUMENTS
known. It can be determined by measuring the focal
lengths of the elements. The magnifying power can be
approximately determined by this method: Mark a
number of connected squares of equal size in a line on a
distant white wall. Observe the squares with the right
eye looking through the instrument and with the left eye
looking at them naturally. The right eye will receive a
magnified impression of one or two of the squares; the
left eye will receive a natural impression of a number of
them. The two impressions will be fused by the brain
into a single impression of a large square with a number
of smaller ones crossing it in a line. If four natural
squares are seen inside the magnified square, the
instrument has a magnification of approximately 4
diameters or 4 power.
9-5. Measurement of Field of View.
a. The field of view (FOV) of an instrument may be
expressed in either angular or linear measure. The
angular measure is more commonly given and is
expressed in degrees and minutes. The linear measure
is expressed in meters at ranges of 100 meters or 1,000
meters.
b. To determine angular FOV, place the telescope
in a fixture or mount which is provided with an azimuth
scale, a micrometer, and levels. Level the instrument
and direct the telescope so that a convenient vertical
object is at the left edge of the FOV. Read the scale and
micrometer. Traverse the instrument until the object is at
the right edge of the FOV. Again read the scale and
micrometer. The difference between the two readings is
the angular measure of the true FOV (para 5-25). If the
scale and micrometer are graduated in mils, convert to
degrees and minutes.
c. To measure the linear FOV, place two stakes at
a specified distance from the instrument; one at each
edge of the observed FOV. Measure the actual distance
between the two stakes. The linear FOV may be
determined mathematically by the formula: 2 x tangent
of one-half the angular FOV x the range of the stakes.
9-6. Determination of Spherical Aberration. Cut a
mask of black paper that will cover the objective. Cut a
circle one-half the diameter of the objective out of the
center of this mask. Apply the mask to the objective.
Sharply
focus
a
distant
object
on
the
9-1. General. This chapter describes a number of
relatively simple procedures by means of which the
properties and qualities of complete instruments may be
measured or calculated. For the most part, the tests can
be made without special equipment. Laboratory test
methods are purposely avoided.
9-2. Objective Aperture. The objective aperture size is
limited by the clear area provided by the mount of the
objective. It can be measured in millimeters or decimal
parts or fractions of an inch with a scale, rule, or with a
Ramsden dynameter (para 5-24).
9-3. Measurement of Exit Pupil and Eye Distance.
Direct the instrument at an illuminated area. Focus the
exit pupil (fig 2-54) on translucent paper or ground glass.
To focus the exit pupil, move the paper or glass towards
or away from the eyepiece until the exit pupil is seen in
its smallest diameter as a sharp circular shape of even
brightness. Measure the distance of the face of the
paper or glass from the rear surface of the eyepiece lens
with a scale graduated in millimeters or decimals or
fractions of an inch (fig 2-55). Mark the opposite edges
of the exit pupil on the paper or glass and measure its
diameter in millimeters or decimals or fractions of an
inch. The position and diameter of the exit pupil can be
measured exactly by means of a Ramsden dynameter
(para 5-24).
9-4. Magnifying Power.
a. Definition. The magnifying power of an optical
instrument is the ratio of the heights of the retinal images
produced in the observer’s eye with and without the
instrument.
b. Calculation. The magnifying power of an optical
instrument depends upon the relation between the focal
length of the objective and the focal length of the lenses
of the eyepiece, the latter being considered as a single
lens. The magnifying power of a telescope equals to the
focal length of the objective divided by the focal length of
the eyepiece. For example, if the objective has an
equivalent focal length of 8 inches and the eyepiece has
an equivalent focal length of 1 inch, the instrument will
have a magnification of 8 power.
c. Measurement. The magnifying power of an
instrument can be calculated when the equivalent focal
lengths of the objective and the eyepiece are
9-1
TM 9-258
crossline of the instrument. Now, remove the mask and
place the cutout circle on the center of the objective.
Examine the image of the object. Any blurring of the
image may be attributed to unfocused rays in the outer
zones of the objective. Adjust whatever focusing devices
there are available to sharply focus the image. The
amount of movement required is an indication of the
amount of spherical aberration present.
9-7. Determination of Curvature of Image.
Sharply focus the instrument on a point in the center of
the image and note whether or not the points at the edge
of the image are clear and well defined. If the edges of
the image are hazy, continue focusing the instrument
until the edges of the image are sharp and distinct. The
amount of focusing required is an indication of the
curvature of image.
9-2
TM 9-258
CHAPTER 10
MEASUREMENT SYSTEMS EMPLOYED IN OPTICS
The power of other lenses than those of 1-meter focal
length is the reciprocal of the focal length in meters and
varies inversely as the focal length. This means that a
converging lens with a focal length of 20 centimeters or
1/5 meter has a power of +5 diopters, while a diverging
lens with a focal length of 50 centimeters or 1/2 meter
has a power of -2 diopters. The lens with the shortest
focal length has the greatest plus or minus power in
diopters.
c. Prism Diopter. A prism with a power of 1 prism
diopter will deviate light by 1 centimeter at a distance of 1
meter from the prism.
Thus, the prism diopter
designation of a refracting prism is the measurement of
the amount it can bend light.
10-3. Degree System.
a. The degree system is a means of measuring
and designating angles of arcs. The degree is 1/360 part
of a circle or the value of the angle formed by dividing a
right angle into 90 equal parts. Each degree is divided
into 60 parts called seconds.
b. In the use of fire-control equipment, either
degrees or mils (para 10-4) may be used to designate
angles of elevation and azimuth. One degree equals
17.78 mils. One mil equals 0.0560 or 3 minutes 22 1/2
seconds. Refer to the conversion chart (fig 10-1) for the
approximate relative values of given angles of elevation
and azimuth in degrees and mils.
10-4. Mil System.
a. The artillery mil system is a means of angular
measurement that lends itself to simple mental
arithmetic.
It provides accuracy within the limits
demanded by the military forces with distinct advantages
of simplicity and convenience not afforded by any other
method of angular measurement. It is based on an
arbitrary unit of measurement known as the mil. The mil
is exactly 1/6400 of a complete circle.
b. The mil is very nearly the angle between two
lines which will enclose a distance of 1 meter at a range
of 1,000 meters (fig 10-1). Exact computation of the
distance enclosed at 1,000 meters by 1 mil gives the
result of 0.982 meters. Therefore in assuming that 1 mil
encloses 1 meter at 1,000 meters, an error of 0.018
meter or 1.8 cm is introduced. This error is negligible for
all practical purposes in the use of fire-control
instruments.
10-1. Candlepower and Foot Candles.
a. The intensity of illumination on a surface
depends upon the brightness of the light source and the
distance of the surface from the light source. The
brightness of any light source is measured in units of
candlepower. One candlepower is the rate at which a
standard candle emits light. A standard candle was
initially a sperm whale oil candle, 7/8 inch in diameter,
which burned at the uniform rate of 120 grains per hour.
Current standards of candlepower are electric lamps
which may be obtained from the Bureau of Standards.
b. The amount of illumination on any surface
depends upon the distance of the surface from the light
source. A candle 1 foot away from a screen will shed
more intense light on a given area of the screen than it
would if it were 3 feet from the screen. In the latter case,
it would illuminate a larger area with less intensity. The
intensity of illumination is measured in foot candles. A
foot candle is the amount of light intensity received on a
surface 1 foot away from a light source of 1
candlepower.
c. The intensity of illumination or the quantity of
light which is received per unit surface varies inversely
as the square of its distance from the source.
Candlepower
foot candles =
(distance)
2
d. The amount of light falling upon a surface is
measured in lumens. A lumen is the amount of light
falling on an area of 1 square foot at 1 foot distance from
a standard candle (a above). Therefore, total lumens
divided by area equals foot candles of illumination.
10-2. Diopters.
a. General. The lens diopter or simply diopter is
the unit of measure of the refractive power of a lens or
lens system. The prism diopter is the unit of measure of
the refracting power of a prism. Both are based on the
metric system of measurement.
b. Lens Diopter. A lens with a focal length of 1
meter is internationally recognized to have the power of 1
diopter. The power of a converging lens is positive and
that of a diverging lens is negative.
10-1
TM 9-258
Figure 10-1. Artillery mil-degree conversion chart.
10-2
TM 9-258
mils tall at 1,000 meters or 1 mil tall at 2,000 meters.
c. A circle of 1,000 meters radius would have a
circumference of 6,283 meters. A 1-meter portion of the
circumference would be the 1/6283 fractional part of a circle.
By choosing the mil as the 1/6400 fractional part of a circle,
sufficient accuracy is retained for all practical purposes while
the number can be handled easily by mental arithmetic.
d. The mil provides an extremely small unit of angular
measurement that is easily adaptable to the small angles
encountered by the artilleryman. For example, if an object has
an angular width or height of 1 mil, it is 1 meter wide or high at
1,000 meters; 2 meters wide or high at 2,000 meters; and so
on.
In angular height, a man is approximately 2
Artillery mils
NOTE
’The Navy mil and the French infantry mil are
exactly 1 meter at a range of 1,000 meters.
There are 6283.1853 Navy or French
infantry mils in a complete circle.
e. For quick approximate conversion from artillery mils
to degrees, or vice versa, refer to figure 10-1. On this chart
locate the number of degrees or mils to be converted. Directly
across the black line of arc is its equivalent.
f. For more exact conversion from artillery mils to
degrees, minutes, and seconds, use the following listing:
Degrees
Minutes
1....................................................................................... 0
2....................................................................................... 0
3....................................................................................... 0
4....................................................................................... 0
5....................................................................................... 0
6....................................................................................... 0
7....................................................................................... 0
8....................................................................................... 0
9....................................................................................... 0
10....................................................................................... 0
20....................................................................................... 1
30....................................................................................... 1
40....................................................................................... 2
50....................................................................................... 2
60....................................................................................... 3
70....................................................................................... 3
80....................................................................................... 4
90....................................................................................... 5
100 ....................................................................................... 5
200 ....................................................................................... 11
300 ....................................................................................... 16
400 ....................................................................................... 22
500 ....................................................................................... 28
600 ....................................................................................... 33
700 ....................................................................................... 39
800 ....................................................................................... 45
900 ....................................................................................... 50
1,000..................................................................................... 56
2,000..................................................................................... 112
3,000..................................................................................... 168
4,000..................................................................................... 225
5,000..................................................................................... 281
6,000..................................................................................... 337
6,400..................................................................................... 360
For example, to convert 2,569 mils:
2,000 mils =
500 mils =
60 mils =
9 mile =
2,569 mile =
g. For more exact conversion from degrees and
minutes to artillery mils, use this conversion table:
10-3
3
6
10
13
16
20
23
27
30
33
7
41
15
48
22
56
30
3
37
15
52
30
7
45
22
0
37
15
30
45
0
15
30
0
1120
280
30
0°
1430
1440
30’
7’
22’
30'
89'
30'
0"
30"
30"
22.5"
82.5"
22.5"
Seconds
22.5
45
7.5
30
52.5
15
37.5
0
22.5
45
30
15
0
45
30
15
0
45
30
0
30
0
30
0
30
0
30
0
0
0
0
0
0
0
TM 9-258
Degree
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
2
3
4
5
6
7
8
9
10
20
30
40
50
60
70
80
90
100
200
300
360
Minutes
Artillery mils
1..................................................................................... 30
2..................................................................................... 59
3..................................................................................... 89
4.................................................................................. 1.19
5.................................................................................. 1.48
6.................................................................................. 1.78
7.................................................................................. 2.07
8.................................................................................. 2.37
9.................................................................................. 2.76
10.................................................................................. 2.96
02.................................................................................. 2.96
20.................................................................................. 5.93
30.................................................................................. 8.89
50................................................................................ 14.82
0................................................................................ 17.78
0................................................................................ 35.56
0................................................................................ 53.33
0................................................................................ 71.11
0................................................................................ 88.89
0.............................................................................. 106.67
0.............................................................................. 124.44
0.............................................................................. 142.22
0.............................................................................. 160.00
0.............................................................................. 177.78
0.............................................................................. 355.56
0.............................................................................. 533.33
0.............................................................................. 711.11
0.............................................................................. 888.89
0........................................................................... 1,066.67
0........................................................................... 1,244.44
0........................................................................... 1,422.22
0........................................................................... 1,600.00
0........................................................................... 1,777.78
0........................................................................... 3,555.56
0........................................................................... 5,333.33
0........................................................................... 6,400.00
For example, to convert 78°43’:
70° 0' =
1,244.44
8° 0' =
142.22
0° 40' =
11.85
0° 3' =
.89
78° 43' =1,399.40 mils
units of linear measurement of the metric system are
multiples or fractional parts of the meter in the units of
10. It is a system based on decimals which provides
ease of conversion from one to another of the various
units of the system. Units of measurement are provided
from extremely small to the very large, inasmuch as they
include the physical units of measure of the X-ray Unit,
micron, meter, and kilometer.
b. One meter is equal to 39.37 inches. Following is
a listing of metric units with their equivalents in inches,
yards, and miles:
10-5. Metric System.
a. The meter, which is the basis of the metric
system, was intended to be and is very nearly one tenmillionth of the distance from the equator to the pole
measured at sea level on the meridian of the earth. The
meter is now defined as the distance between two lines
on a certain platinum-iridium bar kept in Paris, when the
bar is at 0° C. The value of the meter in wave length of
light is known with very great accuracy. The value for
the wave length of red cadmium radiation under
specified standard conditions is 0.00064384696 mm. All
10-4
TM 9-2520-215-34
10 millimeters=
10 centimeters
10 decimeters
10 meters
10 dekameters
10 hectometers
=
=
=
=
=
=
=
1 millimeter
1 centimeter
1 decimeter
1 meter
1 dekameter
1 hectometer
1 kilometer
c. One inch equals 25.4 millimeters. One foot
equals 30.48 centimeters. One yard equals 0.914 meter.
One mile equals 1.609 kilometers.
For quick
approximate conversion from inches to metric
=
=
=
=
=
=
=
0.03937 inch
0.3937 inch
3.937 inches
1.0936 yards
10.936 yards
10.936 yards
0.6214 mile
system units, or vice versa, refer to the conversion table
(fig 10-2). On this table, locate the measurement to be
converted. Directly across the black line is its equivalent.
10-5
TM 9-258
Figure 10-2. Metric unit-inch conversion table.
10-6
TM 9-258
d. For more exact conversion and for conversion
of large units, use the following factors to convert:
From
To
Multiply
Millimeters ..................................Inches...................................................... Millimeter by 0.03937
Inches.........................................Millimeters ............................................... Inches by 25.4
Meters ........................................Inches...................................................... Meters by 39.37
Meters ........................................Yards....................................................... Meters by 1,0936
Inches.........................................Meters ..................................................... Inches by 0.0254
Yards ..........................................Meters ..................................................... Yards by 0.9144
Kilometers ..................................Miles ........................................................ Kilometers by 0.6214
Miles ...........................................Kilometers ............................................... Miles by 1.609
10-7
TM 9-258
APPENDIX A
REFERENCES
A-1. Publication Indexes. The following indexes should be consulted frequently for latest changes or revisions of
references given in this appendix and for new publications relating to materiel covered in this manual:
Index of Army Motion Pictures, Film Strips, Slides, and Phono-Recordings.
DA Pam 108-1
Military Publications:
Index of Administrative Publications........................................................................................... DA Pam 310-1
Index of Blank Forms ................................................................................................................. DA Pam 310-2
Index of Graphic Training Aids and Devices .............................................................................. DA Pam 310-5
Index of Supply Manuals; Ordnance Corps................................................................................ DA Pam 310-6
Index of Technical Manuals, Technical Bulletins (Type 7, 8,...................................................... DA Pam 310-4
and 9), Lubrication Orders, and Modification Work Orders.
Index of Doctrinal, Training, and Organizational Publications.................................................... DA Pam 310-3
A-2. Other Publications.
Elements of Optics and Optical Instruments ...................................................................................... ST 9-2601-1
Military Symbols
...................................................................................................................... FM 21-30
Military Terms, Abbreviations, and Symbols: Authorized Abbreviations ............................................ AR 320-50
and Brevity Codes.
Dictionary of United States Army Terms ............................................................................................ AR 320-5
Military Training FM 21-5
Techniques of Military Instruction....................................................................................................... FM 21-6
A-3. Textbooks.
A College Textbook of Physics by Arthur L. Kimbal-Henry Holt and Co., New York - 1939. The
Principles of Optics by Arthur C. Hardy and Fred H. Perrin - McGraw-Hill Book Co., Inc., New
York and London - 1932.
A-1
APPENDIX B
GLOSSARY
system which causes the image to be imperfect. See
astigmatism, chromatic aberration, coma, curvature of
image, distortion, and spherical aberration.
Absorption - The act of process by which an object
or substance "takes up" or "soaks up" all the colors
contained in a beam of white light except those colors
which it reflects or transmits. Because of absorption,
objects appear to have different colors; white objects
have little absorption; other objects having varying
powers of absorption appear to have different colors and
different shades of brightness. For example, an object
which appears red has absorbed all the color of the
spectrum except the color red. This is known as
selective absorption or selective reflection. A piece of
ruby glass placed between the eye and a source of white
light allows only red light to pass through because it
absorbs all other colors.
This is called selective
absorption or selective transmission (fig B-1).
B-1. General. This glossary contains definitions of a
number of specialized terms found in the study of optics,
in the use of sighting and fire-control equipment, and
with reference to the eyes and vision. It includes
definitions of most of the more unusual words used in
this manual. The majority of the italicized words in this
manual will be found in this glossary. When in doubt as
to the meaning of a word, refer to this glossary.
B-2. Glossary.
Abbe prism - Two right-angle prisms joined to form
one prism (fig 4-16). Used as component of one form of
the Abbe prism system (figs 4-10 and 4-21).
Abbe prism erecting system - A prism erecting
system in which there are four reflections to erect the
image. It is composed of two double right-angle prisms
which bend the path of light 360ƒ, displacing but not
deviating the path of light (fig 4-21).
Aberration - Any defect of a lens or optical
Figure B-1. Absorption-Selective transmission.
Accommodation - Automatic adjustment of the
crystalline lens of the human eye for seeing objects at
different distances. The process whereby the crystalline
lens is adjusted to focus successive images of objects
located at various distances from the eye. See limits of
accommodation.
Achromatic - Without color.
Achromatic lens - A lens consisting of two or more
elements, usually made of crown and flint
glass, which has been corrected for chromatic
aberration. See compound lens.
Acuity - Keenness; sharpness. See stereo-acuity
and visual acuity.
Adapter - A tube, ring, or specially formed part
which serves to fit or connect one part with another, or
mount an element of smaller diameter into a part of
larger diameter.
Aiming circle - An instrument for measuring
B-1
TM 9-258
horizontal and vertical angles and for general
topographic work.
Aiming point - The point on which the gunner sights
when aiming the gun. This point is not necessarily the
target itself.
Ametropia - Any abnormal condition of the seeing
power of the eyes, such as farsightedness
(hypermetropia),
nearsightedness
(myopia),
or
astigmatism. An ametropic eye is one which does not
form distinct images of objects on its retina.
Amici prism - Also called "roof prism" and "roofangle prism." A form of roof prism designed by G. B.
Amici, consisting of a roof edge formed upon the long
reflecting
face
of
a
right-angle
prism
(fig 4-11). Used as an erecting system in elbow and
panoramic telescopes. It erects the image and bends
the line of sight through a 90° angle.
Anastigmat - A compound lens corrected for
astigmatism. Angle The amount of rotation of a line
around the point of its intersection with another,
necessary to bring it into coincidence with the second
line.
Angle of azimuth - An angle measured clockwise in
a horizontal plane usually from north (fig B-2). The north
used may be True North, Magnetic North, or Y-North
(Grid North).
Figure B-2. Angle of azimuth.
Angle of convergence - Angle formed by the lines of
sight of both eyes in focusing on any point, line, corner,
surface, or part of an object Also referred to as
convergence angle (figs 3-8 and 3-9).
Angle of deviation - The angle through which a ray
of light is bent by a refracting surface; the angle between
the
extension
of
the
path
of
an
incident ray and the refracted ray (fig 2-24).
Angle of elevation - The angle between the line of
site (line from gun to target) and line of elevation (axis of
bore when gun is in firing position) (fig B-3). The
quadrant angle of elevation is the angle between the
horizontal and the line of elevation. Angle of elevation
plus angle of site equals quadrant angle of elevation.
B-2
TM 9-258
Figure B-3. Angle of elevation, angle of site, and quadrant angle.
Angle of incidence - The angle between the normal
(perpendicular) to a reflecting or refracting surface and
the incident ray (figs 2-13 and 2-24).
Angle of reflection - The angle between the normal
to a reflecting surface and the reflected ray (fig 2-13).
The incident ray, reflected ray, and normal all lie in the
same plane.
Angle of refraction - The angle between the normal
to the refracting surface and the refracted ray (fig 2-24).
The incident ray, refracted ray, and normal all lie in the
same plane.
Angle of site - The vertical angle between the
horizontal and the line of site (line from gun to
target) (fig B-3).
Angstrom unit - A unit of measure equaling one tenmillionth part of a millimeter.
Angular - Composed of or measured by angles.
Antiglare diaphragm - See diaphragm.
Aperture - An opening or hole through which light or
matter may pass. In an optical system, it is equal to the
diameter of the largest entering beam of light which can
travel completely through the system. This may or may
not be equal to the aperture of the objective (fig B-4).
B-3
TM 9-258
Figure B-4. Apertures.
Aperture of objective - The diameter of that part of
the objective which is not covered by the mounting.
Aperture stop - The diaphragm which limits the size
of the aperture. See diaphragm.
Aplanatic lens - A lens which has been corrected for
spherical aberration, coma, and chromatic aberration.
Apochromatic lens - A lens, usually consisting of
three components of different kinds of glass (two crown
glass elements, one flint glass element), which has been
corrected for chromatic aberration with respect to three
selected colors or wavelengths of light.
Apparent field of view - The angular size of the field
of view of an optical instrument, as seen through the
instrument by the eye (fig 2-51). See field of view.
Aqueous humor - The transparent liquid which
is contained between the cornea and the crystalline lens
of the eye (fig 3-2).
Arc - A part of the circumference of a circle.
Artillery - mil See mil.
Asthenopia - Weakness or rapid fatigue resulting
from use of the eyes indicated by headache or pain in
the eyes. Often referred to as weak sight or eyestrain.
Astigmatism
a. An aberration or defect of a lens which
causes a point of the object off the axis to be imaged as
a short line or pair of short lines. When two lines are
formed, each is at a different distance from the lens and
is at an angle to the other, and the lens has two points of
principal focus (B, fig B-5). A sharp image cannot be
secured at either focal point and the best results are
obtained at a point between the two focal points in a
plane known as the circle of least confusion (fig 2-64).
B-4
TM 9-258
Figure B-5. Astigmatism
b. A defect of the human eye causing straight lines
in a certain direction to appear blurred and distorted
while lines in another direction may be well defined. This
defect results when rays from a point are not brought to
a single focal point on the image on the retina but a short
line is formed. It is caused by unsymmetrical surfaces of
the cornea and the crystalline lens of the eye (A, fig B-5).
Astigmatizer - A cylindrical lens which may be
rotated into the line of sight of a range finder to cause the
effect of astigmatism, to stretch a ray of light from a
single point to form a line.
Axis (plural, axes) - A straight line passing through a
body and indicating its center. See axis of bore, principal
axis, and optical axis.
Axis of bore - The center line or straight line
B-5
TM 9-258
through the center of the bore of a gun (fig B-6). The line
of
sight
of
the
sighting
instrument
of
a
gun
is
adjusted
parallel
to
the
axis
of
bore.
Figure B-6. Axis of bore and boresights-boresighting.
Axis of lens - See principal axis.
Azimuth - An angle measured clockwise in a
horizontal plane from a known reference point frequently
one of the norths: True North, Magnetic North, or YNorth (Grid North). (See angle of azimuth). Also used
as an adjective to indicate reference to movement in an
horizontal plane as azimuth mechanism or azimuth
instrument. To traverse the carriage of a weapon in
azimuth means to rotate the weapon from side to side in
a horizontal plane.
Azimuth instrument - A telescopic instrument used
for measuring horizontal angles.
Azimuth mechanism - Any mechanical means
provided for turning an instrument in azimuth (in
horizontal plane). It may contain a worm and wormwheel
to give accurate, smooth movement.
Backlash - A condition wherein a gear, forming part
of a gear train, may be moved without moving the next
succeeding
or
preceding
gear.
It
is due to space between the teeth of the meshing gears.
Balance - See orthophoria.
Balsam - See Canada balsam.
Base length - In range finders, the actual optical
length of the instrument. It is approximately equal to the
distance between the centers of the end windows. It is
the base of the range triangle by means of which the
range is computed (fig 8-23).
BC periscope (Battery Commander’s Periscope) - A
binocular telescope used for observing artillery fire.
Beam - With reference to light, a shaft or column of
light; a bundle or rays. It may consist of parallel,
converging, or diverging rays.
Binocular - Pertaining to vision with both eyes. Also,
a term applied to instruments consisting of two
telescopes utilizing both eyes of the observer.
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TM 9-258
Case I pointing (or firing) - Direct pointing, laying, or
fire; gun pointing in which direction and elevation are set
with sight of telescope pointed at the target.
Case II pointing (or firing) - Combined direct and
indirect pointing, laying, or fire; gun pointing in which
direction is set with a sight or telescope pointed at the
target and the elevation with an elevation quadrant,
range quadrant, or range disk.
Case III pointing (or firing) - Indirect pointing, laying,
or fire; gun pointing in which direction is set with an
azimuth circle or with a sight or telescope pointed at an
aiming point other than the target; the elevation is set
with an elevation quadrant, range quadrant, or range
disk.
Cataract - A diseased condition of the human eye in
which the cornea or crystalline lens becomes opaque
resulting in blindness.
Cell - A tubular mounting used to hold a lens in its
proper position. The lens may be held in the cell by
burnishing or by a retaining ring.
Center of curvature - The center of the sphere of
which the surface of a lens or mirror forms a part. Each
curved surface of a lens has a center of curvature
(fig B-8). The curved surfaces may be convex or
concave.
Blind spot - The part of retina, inner coat of eyeball,
not sensitive to light where the optic nerve enters.
Bore sight - A sighting device consisting of a breech
element and a muzzle element which, inserted in a gun,
is used to determine the axis of the bore and the
alinement of other sighting equipment with the axis of the
bore. A bore sight may consist of a metal disk with a
peephole for the breech and a pair of crosslines for the
muzzle (fig B-6).
Boresight - To adjust the line of sight of the sighting
instrument of a gun to the axis of the bore (fig B-6).
Brightness of image - A term used to denote the
amount of light transmitted by an optical system to give
definition to the image seen by the observer.
Burnishing - The process of turning a thin edge of
metal over the edge of a lens to hold it in place in its cell
(fig B-7). This eliminates the use of a retaining ring.
Burnished optics are usually procured as assemblies,
inasmuch as it is difficult to replace their component
parts.
Figure B-7. Burnishing
Figure B-8. Centers of curvature
Centric Pencil - Oblique pencil or cone of light which
passes through the center of a lens at a considerable
angle to the principal axis.
Choroid - The opaque middle coat or tunic of the
human eye. It is a deep purple layer made up of blood
vessels; it supplies nourishment to the eye tissues and
shuts out external light (fig 3-2).
Chromatic aberration - An aberration (deviation from
normal) of a lens which causes
Calcite - See Iceland spar.
Canada balsam - A clear, practically colorless sap
of fir, solidifying to a transparent resin used in cementing
optical elements together particularly the components of
a compound lens.
It is used because it has
approximately the same index of refraction as glass.
Candlepower - A unit of measure of the brightness
or power of any light source. One candlepower is the
rate at which a standard candle, initially made of sperm
whale oil, 7/8 inch in diameter, burning at the rate of 120
grains per hour, emits light. Current standards of
candlepower are electric lamps.
B-7
TM 9-258
Color blindness - An eye condition of not being
capable of proper discrimination of colors. In the most
common type, dichromatism, red-green blindness, these
two colors appear gray.
Coma - An aberration of a lens which causes
oblique pencils of light from a point on the object to be
imaged as a comet-shaped blur instead of a point. This
aberration is caused by unequal refraction through the
different parts of the lens for rays coming from a point
which lies a distance off the axis (fig 2-65).
Compass North (Magnetic North) - The direction
indicated by the north-seeking end of the needle of a
magnetic compass. This direction is different usually
from either True North or Y-North (Grid North),
depending upon one’s position upon the earth’s surface.
Compensator - See measuring wedge.
Compound lens - A lens composed of two or more
separate pieces of glass. These component pieces or
elements may or may not be cemented together. A
common form of compound lens is a two element
objective; one element being a converging lens of crown
glass and the other being a diverging lens of flint glass.
The surfaces of the two elements are ground to eliminate
aberrations which would be present in a single lens (fig
4-3).
Concave - Hollowed to form a shallow cavity and
rounded like the inside of a sphere.
Concave lens - See diverging lens.
Concave-convex lens - A lens with one concave
surface and one convex surface.
Concentric - Having the same center. Circles
differing in radius but inscribed from a single center
point.
Cone - One of the two types of light-sensitive
elements or visual cells in the retina of the eye which
permit sight. The cones are associated with daylight
vision and give clear sharp vision for the seeing of small
details and the distinguishing of color. The other type of
visual cell is termed the rod (fig 3-5).
Conjugate focal points (conjugate foci) - Those pairs
of points on the principal axis of a concave mirror or a
convergent lens so located that light emitted from either
point will be focused at the other as shown in figure B-9
where D and D- are the conjugate focal lengths of a lens.
Shifting the source of light along the axis will cause a
shifting of the second corresponding conjugate focal
point. Likewise related points in object and image are
located optically so that for every point on the object
there is a corresponding point in the image.
different colors of wavelengths of light to be focused at
different distances from the lens resulting in colored
fringes around the borders of objects seen through the
lens (fig 2-60).
Ciliary body or muscle - The muscle in the eyeball
which is capable of increasing or decreasing the
curvature of the crystalline lens to decrease or increase
the focal length of the lens system of the eye. Used in
accommodation (focusing) of the eye (fig 3-4).
Circle of least confusion - A focal plane between the
two focal points of a lens affected by astigmatism where
the most clearly defined image can be obtained. See
astigmatism.
Coated optics - Optical elements which have been
coated with a thin chemical film, such as magnesium
fluoride, to reduce reflection and thereby increase light
transmission.
Coincidence - Occupying the same place; agreeing
as to position; corresponding. In the coincidence range
finder, two optical systems are focused into a single
eyepiece assembly. When the range knob is rotated, the
left and right fields of view are superimposed and
coincidence of the two images of the selected target is
established.
Coincidence prism - A compound prism, consisting
of a system of small prisms cemented together, used as
an ocular prism assembly in a coincidence range finder
to bring the image from the two objectives to a single
eyepiece for viewing.
Coincidence range finder - A self-contained
distance measuring device operating on the principle of
triangulation. Images, observed from points of known
distance, are alined to determine the range. See
coincidence.
Collective lens - A convex or positive lens used in
an optical system to collect the field rays and bend them
to the next optical element. It prevents loss of rim ray
light. Sometimes used to denote a convergent or convex
lens.
Collimating telescope - A telescope with an outer
cylindrical surface that is concentric with its optical axis.
It contains a reticle, usually straight crosslines. It is used
to aline the axis of the optical elements to the
mechanical axis of the instrument (collimation) in other
telescopes, to focus eyepieces, and to set diopter scales
at infinity focus.
Collimation - The process of alining the axis of the
optical elements to the mechanical axis of an instrument.
Collimator - A sighting instrument occupying a
position between the open sight and the telescopic
instrument (fig 8-27). It has an eyelens, and reticle of
cross or vertical line pattern.
B-8
TM 9-258
Figure B-9. Conjugate focal points (conjugate foci).
Constant deviation - A certain amount of deviation
given to the line of sight or optical axis of one of the
internal telescopic systems of a range finder in order that
the correction wedge may adjust the system by supplying
a plus, neutral, or minus correction, as required.
Converge - As applied to the eyes or binocular
optical instruments, to direct them so that the two lines of
sight meet at a common focus and form an angle. To
direct light rays or lines to a small area.
Convergence - See angle of convergence.
Convergent lens - See converging lens.
Converging lens - Also known as convergent lens,
positive lens, convex lens, collective lens. A lens that will
converge light. One surface of a converging lens may be
convexly spherical and the other plane (planoconvex),
both may be convex (double convex) or one surface may
be convex and the other concave (convexo-concave
meniscus converging). A converging lens is always
thicker at the center than at the edge (fig B-10).
Figure B-10. Converging lenses.
B-9
TM 9-258
Converging meniscus lens - See meniscus
converging lens.
Convex - Rounded and bulging outwardly as the
outer surface of a sphere.
Convex lens - See converging lens.
Convexo-concave lens - A lens with one convex
and one concave surface.
Cornea - The transparent, slightly bulging front
surface of the eyeball through which all light enters
(fig 3-2). It serves as one of the lenses of the eye.
Corrected lens - A compound lens, the various
surfaces of which have been so designed with respect to
each other that the lens is reasonably free from one or
more aberrations.
Correction wedge - In range finders a rotating
wedge-shaped element used to precisely divert the line
of sight to correct errors in the optical system caused by
temperature variations. In range finders, serves to
supply a measuring factor correction for the constant
deviation given one of the telescopes. The measuring
factor correction is read on a scale.
Correction window - End correction window. These
are optical wedges of very small angles. They admit
light, seal out dirt and moisture, and are so mounted that
they may be rotated to compensate for the accumulated
errors in the entire system. Two are used as end
windows on some range finders.
Critical angle - That angle at which light, about to
pass from a medium of greater optical density, is
refracted along the surface of the denser medium (fig 234). When this angle is exceeded, the light is reflected
back into the denser medium. The critical angle varies
with the index of refraction of the substance or medium.
Cross-eye - See strabismus.
Crown glass - One of the two principal types of
optical glass; the other type is flint glass. Crown glass is
harder than flint glass, has a lower index of refraction,
and lower dispersion. See compound lens.
Crystalline lens - The flexible inner lens of the eye
which provides accommodation for sharply focusing near
and distant objects (fig 3-2).
Curvature of field - See curvature of image.
Curvature of image - An aberration of a lens which
causes an image to be focused in a curved plane instead
of a flat one (A, fig 2-68).
Cylindrical lens - A lens having for one of its
surfaces a segment of a cylinder. Lenses ground in this
manner are used in spectacles to correct astigmatism of
the eyes and in some of the coincidence range finders to
cause astigmatism by stretching points of light into lines
of light.
Dark-adapted - A term applied to the adjustment of
the visual cells of the retina of the eye for better vision
under
conditions
of
poor
lighting.
It
applies to the process whereby the rods of the retina
take over the major portion of the act of seeing.
Definition - Sharpness of focus: distinctness of
image.
Deflection - A turning aside from a straight course.
The horizontal clockwise angle from the line of fire to the
line of sight to the aiming point. A small horizontal
(traverse) angle by which a gun is aimed slightly away
from its target to allow for factors such as wind and drift.
See drift.
Degree - A unit of measurement equal to the angle
between radii subtended by 1/360 of a circle.
Depth perception - The ability to see in depth or
three dimensions. In addition to stereoscopic vision, light
and shade, perspective, convergence of the visual axes
of the two eyes, one object or part concealing another,
atmospheric haze, and apparent size are factors which
aid depth perception.
Deviation - Turning aside from a course; deflection;
difference. In optics, the bending of light from its path.
In gunfire, the distance from a point or center of impact
to the center of a target.
Dialyte - A type of compound lens in which the inner
surfaces of the two elements are ground to different
curvatures to correct for aberrations and the dissimilar
faces cannot be cemented together (fig 4-3). Gauss
objective (para 4-3b).
Diaphragm - A flanged or plain ring with a limited
aperture placed in an optical system at any of several
points to cut off marginal rays of light not essential to the
field of view. Diaphragms are used as: field stops, to
reduce aberrations; aperture stops, to limit the aperture
or light-gathering power of the instrument; and antiglare
diaphragms, to eliminate reflections from the sides of the
tube and consequent glare in the field of view. Lens cells
or the sides of the tube may act as diaphragms. The
rays eliminated by diaphragms are those which would
cause aberrations or glare and ghost images by
reflection inside the instrument.
Dichromatism - See color blindness.
Diffusion - The scattering of light by reflection or
transmission. Diffuse reflection and transmission result
when light strikes an irregular surface such as a frosted
window or the surface of a frosted light bulb. When light
is diffused, no definite image is formed.
Diopter - A unit of optical measurement which
expresses the refractive power of a lens or prism. In a
lens or lens system, it is equivalent to the reciprocal of
the focal length in meters. For example, if a lens has a
focal length of 25 centimeters, this figure converted into
meters would equal 1/4 meter. In as much as the
reciprocal of 1/4 equals 1 divided by 1 which equals 4,
such a lens would be said to have
B-10
TM 9-258
Dispersive lens - See diverging lens.
Distortion - The aberration of a lens or lens system
which causes objects to appear misshapen or deformed
when seen through the lens or lens system. This defect
is known as "barrel-shaped" distortion when the center of
the field of view is enlarged with respect to the edges
and "hourglass" or "pincushion" distortion when the
edges are enlarged with respect to the center (fig 2-67).
Wavy lines are sometimes produced by improperly
shaped or polished windows, prisms, and mirrors. When
caused by a lens, it is due to varying magnification of
different parts of the image.
Diverge - In a lens, to deviate the light outward from
a common center in different directions. As applied to
the eyes, the action of the pupils being directed outward
(Walleye vision) or in not being brought to a common
focus.
Divergent lens - See diverging lens.
Diverging lens - Also known as divergent lens,
negative lens, concave lens, dispersive lens. A lens
which causes parallel light rays to spread out. One
surface of a diverging lens may be concavely spherical
and the other plane (planoconcave), both may be
concave (double concave) or one surface may be
concave and the other convex (concavo-convex
meniscus diverging). The diverging lens is always
thicker at the edge than at the center (fig B-11).
a power of 4 diopters. The lens with the shorter focal
length has the greater power in diopters. See prism
diopter.
Diopter movement - A term applied to adjustment of
the eyepiece of an instrument to provide accommodation
for eyesight variations of individual observers.
Diopter scale - A scale usually found on the
focusing nut of the eyepiece of an optical instrument. It
measures the change in refracting power of the eyepiece
in diopters to introduce a correction to compensate for
the-nearsightedness or farsightedness of the individual
observer. It permits presetting of the instrument if the
observer knows his diopter correction.: Dioptometer Precision instrument used in determining the diopter
setting of another instrument.
Dioplopia - See double vision.
Discernible difference of convergence angles - The
differences in the angles of view from the two eyes to
objects or parts of objects. These differences, although
small, permit persons with normal, two-eyed vision to
distinguish which of two objects is farther away, making
stereoscopic vision possible without resorting to the other
factors which aid depth perception.
Dispersion - In optics, the separation of a beam of
white light into its component colors as in passing
through a prism (fig 2-51). See spectrum.
Figure B-11. Diverging lenses.
Diverging meniscus lens - See meniscus diverging
Double right-angle Abbe prism - See Abbe prism.
Double vision a. A malfunction of a binocular instrument
causing two images to be seen separately instead of
being fused. It is caused by the optical axes of the
lens.
Double concave lens - A lens with two concave
surfaces (fig 4-1).
Double convex lens - A lens with two convex
surfaces (fig 4-1).
B-11
TM 9-258
two telescopes not being parallel or not converging to a
point. In minor cases, the eyes will adjust themselves to
compensate for the error of the instrument until the
images are superimposed and only one object is seen.
This may cause eyestrain, eye fatigue, and headaches.
b. An eye disorder, known as diplopia, which
results in double vision of a single object. It is usually
caused by one eye failing to converge or diverge in
unison with the other eye.
Doublet - A compound lens consisting of two
elements with inner surfaces curved identically and
cemented together (fig 4-3).
Dove prism - Used as a rotating prism. A form of
prism designed by H. W. Dove. It is used to invert the
image in one plane without deviating or displacing the
axis of the rays of light. Used as the rotating prism in the
conventional type of optical system of panoramic
telescopes (figs 4-13 and 4-22).
Drift - The amount by which a projectile will deviate
horizontally from its proper path, due to rotation caused
by the rifling in the bore of the gun, and reaction of the air
(fig B-12).
Figure B-12. Drift.
Dynameter- A Ramsden dynameter is a small
eyepiece or magnifier equipped with a reticle scale and
used in the precise measurement of the exit pupil and
eye distance (eye relief) of other optical instruments.
Eccentric mounting - A type of lens mounting
consisting of eccentric rings which may be rotated to shift
the axis of the lens to a prescribed position (fig 4-36).
Eccentric ray - One of the rim rays which pass
through a lens remote from its center.
Effective aperture of objective - See aperture of
objective.
Elbow telescope - A refracting optical instrument for
viewing objects in which the line of sight is bent 90
degrees by means of a prism.
Electromagnetic spectrum - A chart or graph
showing the relation of all known electromagnetic wave
forms classified by wavelength.
The visible light
spectrum occupies an extremely minute portion of the
electromagnetic spectrum (fig 2-3).
Elementary lens equation - A law giving the
quantitative relation between the distance of the object,
the image, and the principal focus of the lens (para 224.e).
Emergent ray - In optics, the term applied to a ray of
light leaving an optically dense medium as
contrasted with the entering or incident ray (fig 2-24).
Emmetropia - Normal refractive condition of the
eyes. A normal eye is termed emmetropic.
End correction window - See correction window.
End play - Movement of a shaft along its axis. A
type of lost motion common to worm and wormwheel
assemblies. The error lies in looseness in the bearings
at the ends of the shaft or in the ball cap and socket.
The result is that the worm can be rotated a small
amount without causing rotation of the wormwheel.
Entrance pupil - The clear aperture of the objective.
Equilibrium, equipoise - Muscular balance of the
eyes.
Erect - To change an image from an inverted to a
normal position. To both revert and invert.
Erect image - The image produced by an optical
system which is seen with its upper part up. The normal
erect image appears as it is seen by/the normal eye.
The reverted erect image is seen with the right side on
the left.
Erect type - The image produced by one type of
coincidence range finder in which the image appears
B-12
TM 9-258
normal erect when in coincidence except for the
presence of the halving line.
Erecting system - Lenses or prisms, the function of
which is to erect the image, that is, to bring the image
upright after it has been inverted by the
objective. An erecting system may consist of one or
more lenses (figs B-13 and 4-20), each of which is called
an erector, or of one or more prisms (figs 4-21 and 4-11).
Figure B-13. Lens erecting system.
Erector - One of the lenses of a lens erecting
system (fig B-13).
Esophoria - A condition of the eyes in which the
lines of sight tend to turn inward.
Etching - The marking of a surface by acid, acid
fumes, gas, or a tool. A process extensively used in the
manufacture of reticles.
Exit pupil - The diameter of the bundle of light
leaving an optical system. The small circle or disk of
light which is seen by looking at the eyepiece of an
instrument directed at an illuminated area (fig 2-53). Its
diameter is equal to the entrance pupil divided by the
magnification of the instrument.
Exophoria - A condition of the eyes in which the
lines of sight tend to turn outwardly.
Extra-foveal vision - Vision in parts of the retina
other than fovea.
Eye distance or eye relief - The distance from the
rear surface of the eyelens to the plane of the exit pupil
(fig 2-55). The center of rotation of the observer’s eye
should be situated in this plane. The center of rotation of
the eye is about 6 millimeters behind the vertex of the
cornea.
Eyeguard - See eyeshield.
Eyelens - The lens of an eyepiece which is nearest
to the eye (fig B-14). See eyepiece. Various types of
lenses are used for this purpose.
B-13
TM 9-258
Figure B-14. Eyelens and fields lens of eyepiece.
Eyepiece - An optical system used to form a virtual,
erect, enlarged image of the real image formed by the
objective (fig B-14). The optical system of the eyepiece
usually consists of two lenses, an eyelens and a field
lens, but the eyepiece may contain another lens (figs 4-5,
4-6 and 4-7).
Eye relief - See eye distance.
Eyeshield or eyeguard - A shield of rubber plastic,
or metal to protect the eyes of the observer from stray
light and wind and to maintain proper eye distance.
Far point - The farthest point of clear vision of the
unaccommodated eye. For the normal eye, the far point
is infinity. See infinity.
Farsightedness - See hypermetropia.
Field glass - A type of compact binocular.
Field lens - One of the leneses of an eyepiece. It is
the lens which is nearest the image upon which the
eyepiece is focused. It serves to shorten the eye relief
and adds to brightness of the edge of the field (fig B-14).
Field of view - The open or visible space or angle
commanded by the eye. It
is the maximum angle of view that can be seen at one
time. In an instrument, the true field of view is the actual
angle of view of the instrument, the maximum angle
subtended at the objective by any objects which can be
viewed simultaneously. The apparent field of view is the
size of the field of view angle as it appears to the eye; it
is approximately equal to the magnifying power of the
instrument times the angle of the true field of view
(fig 2-52).
Field stop - See diaphragm.
Filters or ray filters - Colored glass disks with plane,
parallel surfaces placed in the path of light through the
optical system of an instrument to reduce glare and light
intensity and likewise in observing tracer fire and
detecting outlines of camouflaged objects. They are
provided as separate elements or as integral devices
mounted so they may be placed in or out of position as
desired (fig 4-27).
Finite - Having limits, as opposed to infinite, or
without limits.
Fire control - The determination and regulation of
the direction of gunfire. More specifically, it refers
B-14
TM 9-258
Fluorescence - Phenomenon whereby light of one
wavelength is absorbed by a material, and then
reemitted as light of a different wavelength. Fluorescent
light sources require generally external white light to
cause them to become luminous and glow with a colored
hue. They cease to become luminous when the esciting
agent is removed.
Focal length - The distance from the principal focus
(focus of parallel rays of light) to the surface of a mirror
or the optical center of a lens (figs 2-44, 2-45, 2-46 and
B-15)
to the observation and calculations necessary for
determining the correct aiming of a weapon system.
Fixed focus - The term applied to instruments which
are not provided with means for focusing.
Such
instruments, generally, have a wide range of
accommodation which permits them to be used by the
majority of observers. Collimator is an example.
Flint glass - One of the two principal types of optical
glass, the other being crown glass. Flint glass is softer
than crown glass, has a higher index of refraction, and
higher dispersion. See compound lens.
Figure B-15. Focal length of lens and mirror.
Focal plane - A plane through the focal point
perpendicular to the principal axis of a lens or mirror (fig
2-44).
Focal point - The point to which rays of light
converge or from which they diverge when they have
been acted upon by a lens or mirror (figs 2-46 and 2-62).
A lens has many focal points, depending upon the
distance of the object from the lens.
Focus a .To adjust the eyepiece of a telescope so that
the image is clearly seen by the eye or to adjust the
lens of a camera so that a sharp, distinct image is seen
on the ground glass.
b. The process of adjusting the distances between
optical elements to obtain the desired optical effect.
c. Same as focal point.
Focusing nut - A threaded nut in the center of which
the eyepiece of a telescope is attached to permit the
eyepiece to be moved in or out to accommodate the
instrument to eyesight variations. It
B-15
TM 9-258
usually carries a diopter scale to permit presetting of the
instrument (fig 4-38).
Focusing sleeve - A knurled sleeve which is rotated
to shift the positions of the erectors with relation to the
objective and eyepiece to focus the instrument or to
change its magnification.
Foot-candles - A unit of measurement of the intensity
of illumination. A foot-candle is the amount of light
intensity received on a surface 1 foot away from a light
source of one candle power, such as a standard candle
of sperm whale oil, 7/8 inch in diameter, burning at the
rate of 120 grains per hour.
Fovea or fovea - centralis A tiny area of cones in the
center of the macula or the yellow spot of the retina of
the eye. It is the area of the eye responsible for the
clearest vision. It is about 0.25 millimeter in diameter,
and contains cones only.
Freedom from defects - Optical glass which is
homogeneous and free from bubbles or striae (localized
variations in the index of refraction).
Freedom from distortion - An image that is a true
reproduction of the object.
Frequency - In light or other wave motion, the
number of crests of waves that pass a fixed point in a
given unit of time, usually 1 second.
Front surface mirror - An optical mirror on which the
reflecting coating is applied to the front surface of the
mirror insead of to the back.
Fuse - In stereoscopic vision, the action of seeing as
one image the images seen by the two eyes.
Fusion - The mental blending of the right and left eye
images into a single, clear image by stereoscopic action.
Galilean telescope - One of the first telescope
models, utilizing a positive objective lens and diverging
eyelens (figs 5-13 and 5-14).
Gauss - objective See dialyte.
Gear train - Two or more gears meshed together so
that rotation of one causes rotation of the others.
Geometrical center - As applied to lenses, the
physical center of the lens as determined by
measurement; sometimes referred to as the mechanical
center to distinguish it from the optical center.
Ghost image - A hazy second image caused by
reflection sometimes seen in observing through a
telescope.
Gimbal joint - A mechanism which permits rotation
around two perpendicular axes or for suspending an
object so that it will remain vertical or level when its
support is tipped.
Grid declination - The angle between True North
and Y-North.
Grid North - See Y-North.
Gunner’s quadrant - An instrument with a
graduated arc, used in range adjustment. It measures
the angle of elevation of the gun.
Halving line - The line which divides the two half
images in a coincidence type range finder. The two
halves of the images produced by the two objectives of
the instrument must be brought to a point where they
match or coincide above and below the halving line.
Hard coat - The term applied to the coating of
magnesium fluoride on coated optics to differentiate
between other softer coatings which are less durable.
Height of image adjuster - A glass plate with plane
surfaces which is tipped one way or the other in the line
of sight in one of the internal telescopes of a range
finder. It deviates the light slightly upward or downward
to adjust the image of one eyepiece to the same height
as the image visible in the other eyepiece.
Heterophoria or muscular imbalance - Any
disturbance of the power, strength, or nervous system of
the eye muscles which does not permit both eyes to
function together in a normal way. It may occur in any of
a number of different forms such as esophoria in which
the lines of sight tend to turn inwardly, esophoria in which
they tend to turn outwardly, and hyperphoria in which the
line of sight of one eye tends to be above that of the
other.
Homogeneity - Being uniform throughout.
Homogeneous - Uniform throughout; composed of
similar parts.
Horizontal travel - Rotation of an instrument (or the
line of sight of an optical instrument) in a horizontal
plane; traverse; movement in azimuth.
Hypermetropia - An eye defect in which the image
tends to be focused beyond the retina with the result that
the image is blurred and indistinct, farsightedness. It is
caused by the refracting surfaces of the eye being too
slightly curved, by the eye being of insufficient depth, or
by the decline in the power of accommodation in viewing
nearby objects. It is corrected by a converging lens (fig
3-11).
Hyperopia - See hypermetropia.
Hyperphoria - A condition of the eyes in which the
line of sight of one eye tends to be above that of the
other.
Hyperphoric displacement - Action of the eyes necessary
to obtain fusion in observing through a stereoscopic or
binocular instrument when the image seen by one eye is
above that seen by the other eye.
Iceland splar, calcite - A mineral that has the faculty
of polarizing light.
Illuminated - Lighted by another source than
B-16
TM 9-258
itself, as opposed to a luminous body, which is a light
source.
Image - A reproduction or picture of an object
produced by light rays.
An image-forming optical
element produces an image by collecting a beam of light
diverging from an object point and trans forming it into a
beam which converges toward, or
diverges from, another point. If the beam converges to a
point, a real image is produced (fig 2-47); if the beam
diverges, a virtual image is produced at its apparent
source (fig 2-47). A real image can be thrown upon a
screen (fig 2-47); a virtual image exists only in the mind
of the observer.
Figure B-16. Virtual image produced by diverging lens .
Figure B-17. Virtual image produced by diverging lens .
Image plane - The plane in which the image lies or is
formed. It is perpendicular to the axis of the lens and is
at the focal point. A real image formed by a converging
lens would be visible upon a screen placed in this plane
(fig 2-46).
Imbalance - See Heterophoria.
Incandescence - The state of a body in which its
temperature is so high that it gives off light.
Examples: the sun or the filament of an electric lamp.
Incidence - The act of falling upon, as light upon a
surface.
Incident ray - A ray of light which falls upon or strikes
the surface of an object such as a lens or mirror. It is
said to be incident to the surface (figs 2-13 and 2-24), as
contrasted with the ray leaving an optically dense
medium or emergent ray.
Index (plural, indices)
a. An arrow or mark against which a
B-17
TM 9-258
Infinite - Without limit, as opposed to finite, meaning
with limits.
Infinity - Distance having no end or limits. Greater than
any assignable distance or quantity. In optics, the term
is used to denote a distance sufficiently great so that light
rays coming from that distance are practically parallel to
one another. Infinity is indicated by the symbol ∞
Infrared radiation - Electromagnetic radiation whose
wavelength lies just beyond the red end of the visible
spectrum, and the beginning of the region that can be
detected by microwave radio techniques.
Interpupillary - Between the pupils of the eyes.
Interpupillary adjustment - The adjustment of the
distance between the eyepieces of a binocular
instrument to correspond with the distance between the
pupils of the eyes of an individual.
Interpupillary distance - The distance between the
centers of the pupils of the eyes. It is generally stated in
millimeters. It varies with the individual.
Inversion - Turned over; upside down so that top
becomes bottom and vice versa.
It is the effect
produced by a horizontal mirror in reflecting an image.
Invert-type Image - The type of image observed in
certain coincidence range finders. When in coincidence,
the upper half image appears to be the mirrored
reflection of the lower half image.
Inverted image - Turned over; upside down. An
image, the top of which appears to be the bottom of the
object and vice versa. Usually refers to the effect of a
mirror, a prism, or lens upon the image.
Inversion is the effect of turning upside down. A plane
mirror held under an object will produce an inverted
image. The normal inverted image is seen upside down,
with the right side on the right. The reverted inverted
image is seen upside down with the right side on the left.
Iris The colored part of the eye about the pupil.
Depending upon the intensity of light, the iris causes the
pupil to dilate or contract.
Jack screw - A screw threaded through one part and
pressing against another. A jack (portable machine) in
which a screw is used for lifting or for exerting pressure.
Laser - Acronym for light amplification by stimulated
emission of radiation.
Law of reflection - The angle of reflection is equal to
the angle of incidence; the incident ray, reflected ray, and
normal all lie in the same plane (fig 2-13).
Law of refraction - When light is passing from an
optically lighter medium to an optically denser medium,
its path is deviated, toward the normal; when passing
into an optically less dense medium,
calibration, graduation, or scale is set to indicate the
extent to which a mechanism is adjusted; something that
points out.
b. The ratio of one dimension or quantity to
another.
Index of refraction - A number applied to a
transparent substance which denotes how much faster
light will travel in a vacuum than through that particular
substance. It is equal to the ratio between the speed of
light ’in a vacuum to the speed of light in a substance. It
determines the relation between the angle of incidence
(i) and the angle of refraction (r) when light passes from
one medium to another (figs 2-24 and B-18). The index
between two media is called the relative index, while the
index when the first medium is a vacuum is called the
absolute index of the second medium. The relative index
is the ratio of the sine of the angle of incidence to the
sine of the angle of refraction, or the speed of light in the
first medium to the speed of light in the second medium.
If the first medium is air, this relation is expressed as = sin i = Speed of light in air
n sin r Speed of light in other medium
(Snell’s law)
where n equals very nearly the absolute index of
refraction, i equals the angle of incidence, and r equals
the angle of refraction.
Figure B-18. Index of refraction
NOTE
The index of refraction expressed in tables is the
absolute index, that is, vacuum to substance at a
certain temperature, with light of a certain
wavelength.
Examples: vacuum, 1.00; air,
1,000293; water 1.333; ordinary crown glass,
1.517. Since the index of air is very close to that
of vacuum, the two are often used
interchangeably as being practically the same.
B-18
TM 9-258
its path is deviated away from the normal (fig 2-24).The
amount of deviation is determined by means of the
equation for the index of refraction for the two media
involved. See index of refraction.
Law of relative size of object and image - The size of
the image is to the size of the object as the image
distance from the lens or mirror is to the object distance.
Law of reversibility - If the direction of light is
reversed, it will travel in the opposite direction over the
same path despite the number of times it is reflected or
refracted.
Lens (plural, lenses) - A transparent object, usually a
piece of optical glass, having two polished surfaces of’
which at least one is curved, usually with a spherical
curvature. It is shaped so that rays of light, on passing
through it, are made to converge or diverge. The fact
that lenses either converge or diverge rays provides one
means of classification, the type of curvature provides
another (fig 4-1), the corrections made in the lens
provides still another. The term lens may be employed
to mean a compound lens which can consist of two or
more elements.
See achromatic lens, anastigmat,
aplanatic lens, apochromatic lens, astigmatizer,
collective lens, compound lens, concavo-convex lens,
converging
lens, convexo-concave lens, corrected lens, cylindrical
lens, dialyte, diverging lens, double-concave lens,
double-convex lens, doublet erector eyelens, field lens,
meniscus-converging lens, meniscus diverging lens,
minus lens, objective, orthoscopic lens, plano-concave
lens, planoconvex lens, plus lens, and triplet.
Lens cell - A number of lenses mounted as a unit in
a tubular mounting frame.
Lens diopter - See diopter.
Lens equation, elementary - The equation giving the
relation between the distances from the optical center of
the lens of the object (Do), the image (Di), and the
principal focus (F). These values are related as
1
plus
Do
1
Di
equals
1
F
Lens erecting system - See erecting system.
Lens retaining ring - See retaining ring.
Lens system - Two or more lenses arranged to work in
conjunction with one another (fig B-19). When in proper
alinement, the principal axes will be coincident and the
system can be referred to as a "centered lens system."
Figure B-19. Optical system of erecting telescope .
Lenses of the eye - The refractive elements of the
eye. See cornea and crystalline lens.
Light-adapter - A term applied to the adjustment of
the visual cells of the retina of the-eye for the clearest
and most distinct ’sight under good lighting conditions. It
applies to the process whereby the cones of the retina
take over the major portion of the act of seeing.
Light ray - The term applied to the radii of waves of
light to indicate the direction of travel of the light. Light
rays are indicated by arrows and lines (fig 2-5).
Limiting angle of resolution - The angle subtended by
two points which are just far enough apart to permit them
to be distinguished as separate points. It is commonly
used as a measure of resolving power.
Limits of accommodation The distances of the
nearest and farthest points which can be focused clearly
by the eyes of an individual. Usually varies from 4 to 5
inches to infinity. See accommodation.
Line of elevation In artillery, the line of the axis of the
bore of a gun when it is in firing position (fig B-3).
B-19
TM 9-258
Line of position - See line of site.
Line of sight - Line of vision; optical axis of a
telescope or other observation instrument. Straight line
connecting the observer with the aiming point; the line
along which the sights are set in a gun, parallel to the
axis of the bore (fig B-6).
Line of site A straight line extending from a gun or
position-finding instrument to a point, especially a target
(fig B-3). Usually called line of position.
Figure B-20. Linear field
Lost motion - Motion of a mechanical part which is
not transmitted to connected or related parts. It is the
cumulative result of backlash and end play.
Lumen - The amount of light falling on an area of 1
square foot, at 1 foot distance from a standard candle.
See standard candle.
Luminescence - Giving off light at a temperature
below that of incandescence. The radiating of "cold" light
as seen from fireflies or luminous paint.
Macula or yellow spot - A small area of the retina of
the eye which is highly sensitive to light and which is the
zone of the clearest and most distinct vision except for a
much smaller area at its center known as the fovea or
fovea centralis. The macula is about 2 millimeters in
diameter, and contains principally cones and only a few
rods (fig 3-2).
Magnesium fluoride - See hard coat.
Magnetic azimuth - Azimuth measured from
Magnetic North.
Magnetic declination - The angle between True
North and Magnetic North
Magnetic-grid declination - The angle between
Magnetic North and Grid North (Y-North).
Magnetic needle - A magnetized needle used in a
compass. When freely suspended, it will assume a
position parallel to the earth’s magnetic lines of force
which connect the magnetic poles.
Magnetic North - The direction of the earth’s north
magnetic pole as indicated by the north-seeking end of a
magnetic needle.
Magnification - The increase in the apparent size of
an object produced by an optical element or instrument.
Magnifying power - The ability of a lens, mirror, or
optical system to make an object appear larger (fig B21). If an optical element or optical system makes an
object appear twice as high and twice as wide, the
element or system is said to have a magnification of 2power. The power of an optical instrument is the
diameter of the entrance pupil divided by the diameter of
the exit pupil, the focal length of the objective divided by
the focal length of the eyepiece, or the apparent field of
view divided by the true field of view
.
B-20
TM 9-258
Figure B-21. Magnifying power .
Marginal rays - Rays of light near the edge of a lens.
Maser - Acronym for microwave amplification by
stimulated emission of radiation.
Measuring wedge - An optical element employed in a
range finder to deviate or displace light entering the
variable angle end of the instrument. One type consists
of a single, perpendicular wedge which is moved in
alinement with the path of light to shift or displace the
light (fig 4-18). The other type consists of one or two
pairs of circular wedges, each pair being capable of
rotating equally and simultaneously in opposite directions
about the axis of the path of light to produce variable
deviation (fig 4-19). The wedges of both types are
compound elements corrected for color.
Medium (plural, media) - Any substance or space.
Meniscus - A lens the surfaces of which are curved
in the same direction.
Meniscus converging lens - A lens the surfaces of
which are curved in the same direction; one surface is
convex, the other is concave. The convex surface has’
the greater curvature or power (fig 4-1). Spectacle
lenses of this type are used to correct farsightedness,
hypermetropia, or hyperopia (fig 3-11).
Meniscus diverging lens - A lens the surfaces of
which are curved in the same direction; one surface is
convex, the other is concave. The concave surface has
the greater curvature or power (fig 4-1). Spectacle
lenses of this type are used to correct nearsightedness,
myopia (fig 3-10).
Micrometer - A mechanism for measuring small
angles or dimensions. On optical instruments it is the
"fine" scale.
Micrometer scale - An auxiliary scale used to read
small angles or dimensions.
Micromicron - A unit of measure equalling onemillionth part of a micron.
Micromillimeter or millimicron - Terms applied to unit
of measure equal to one-millionth part of a millimeter or
one-thousandth part of a micron.
Microna - A unit of measure equaling one--millionth
part of a meter or one-thousandth part of a millimeter.
Mil (Artillery Mil) - A unit of angular measurement
used in Army military calculations. It is 1/6400th of a
complete circle or very nearly the angle between two
lines which will enclose a distance of 1 meter at a range
of 1,000 meters or 2 meters at a range of 2,000 meters
(fig 10-2). 17.78 mils is approximately equal to 1 degree.
NOTE
The Navy mil and the French infantry mil
enclose exactly 1 meter at a range of 1,000
meters. There are 6283.1853 Navy or French
infantry mils in a complete circle.
Minus lens - A diverging lens. A lens with negative
focal length (focal point toward the object).
Minute - A unit of measurement of an angle equal to
one-sixtieth of a degree.
Mirror - A smooth, highly polished surface for
reflecting light. It may be plane or curved. Usually a thin
coating of silver or aluminum on glass constitutes the
actual reflecting surface. When this surface is applied to
the front face of the glass, the mirror is termed a front
surface mirror.
Monochromatic - Composed of one color.
Monocular - Pertaining to vision with one eye.
A term applied to optical instruments requiring the use of
only one eye
B-21
TM 9-258
Mount - A device for attaching or supporting an
optical instrument. It may be may not be provided with
means for elevating and traversing the instrument,
calibrations to indicate degree of movement, and means
and gages for leveling. This term is applied also to
means of securing an optical element in an instrument.
Muscular imbalance - See heterophoria.
Myopia - An eye defect which does not permit
distant objects to be seen clearly, nearsightedness.
It is the result of eyes that do not conform to standard
measurements, having too much optical depth or too
great curvature of surfaces. The image tends to be
focused before it reaches the retina and is indistinct. It is
corrected by a diverging lens (fig B-22).
Figure B-22. Nearsightedness (myopia) .
Navy mil - See mil.
Near point - Closest point of clear vision with the
unaided eye.
Nearsightedness - See myopia.
Negative lens - See diverging lens.
Newton’s rings - A series of concentric colored or
bright and dark circles seen when positive and negative
(convergent and divergent) surfaces of nearly the same
curvature are pressed together.
It is caused by
interference of light rays. It is used to test lenses. The
more nearly the surfaces are matched, the greater the
distance between the rings and the larger and more
regular the circles (fig 2-70).
Night blindness - A condition of the eyes (and
possibly the health) of a person which does not permit
him to see as well at night or with poor illumination as the
average person.
Night glasses - An optical instrument for use at night
having an exit pupil at least 7 millimeters.
Normal - An imaginary line forming right angles with
a surface or other lines, the perpendicular. It is used as
a basis and reference line for determining angles of
incidence, reflection, and refraction. In figure B-23, the
normal is perpendicular or at right angles to all lines lying
in the plane and passing through point 0.
Figure B-23. Normal and oblique .
Normal erect - See erect image.
Normal inverted - See inverted image.
North - See Magnetic North, True North, Y-North
(Grid North).
Object - The principal thing imaged by the optical
system. It may be part of large object, an entire object
plus its immediate surroundings, or a number of objects.
It is a general term used for the sake of convenience.
B-22
TM 9-258
Object distance - The distance of an object from the
eyes or from an optical system.
Objective - The lens in an optical system which
receives light from the field of view and forms the first
image. It is the lens farthest from the eye and nearest
the object viewed.
Objective cap - A protective device, usually made of
leather, which is placed over the objective end of the
instrument when the instrument is not in ’use (fig 4-37).
Objective prism - A right-angle prism employed in
some types of instruments to bend light 90° before it
enters the objective system (fig 4-22).
Oblique Slanting; neither at right angles nor parallel (fig
B-23)
Ocular 1. A term sometimes applied to the eyepiece.
2. Pertaining to, connected with, or used for the
eyes or eyesight.
Ocular prism - The prism employed in a range finder
to bend the line of sight through the instrument into the
eyepiece.
Off-carriage instruments - Those instruments used in
artillery which are not mounted on the carriage of the
weapon such as the aiming circle, BC periscope, and
range finder.
Offset - See prism offset.
Oldsightedness - See presbyopia.
On-carriage instruments - Those instruments used in
artillery which are mounted directly on the carriage of the
weapon such as panoramic and elbow telescopes and
their associated telescope mounts.
Ophthalmic - Pertaining to the human eye.
Optic nerve - Nerve connecting the eye and the optic
centers of the brain.
Optical - Pertaining to vision and the phenomenon of
light.
Optical axis - The line formed by the coinciding
principal axes of a series of optical elements comprising
an optical system; in other words, an imaginary line
passing through the optical centers of the system.
Optical center - Point in a lens on a line connecting
all focal points, peculiar in that it is a point through which
the rays of light pass and emerge parallel to their path of
incidence (fig 2-45). It is in the geometric center of the
thickest part of a converging (convex) lens and of the
thinnest part of a diverging (concave) lens.
Optical characteristics - Those properties of an
optical system which it possesses by reason of its optical
or visual nature such as field of view, magnification,
brightness or image, and freedom from distortion.
Optical - element A part of an optical instrument
which acts upon the light passing through the instrument
such as a lens, prism, or mirror.
Optical glass - Glass carefully manufactured to
obtain purity, transparency, homogeneity, and correct
optical properties. See crown glass and flint glass.
Optical properties - In optical glass, those properties
which pertain to the effect of the glass upon light such as
index of refraction, dispersion, homogeneity , and
freedom from defects.
Optical surface - A reflecting or refracting surface.
Optical system - A number of lenses, or lenses and
prisms, or mirrors, so arranged as to refract, or refract
and reflect light, to perform some definite optical function
(fig B-19).
Optics - That branch of physical science which is
concerned with the nature and properties of light, related
instruments, and vision.
Orbit - The socket of the human eye.
Orient - In fire control, to find the proper bearing; to
fix the position of a gun, with reference to a
predetermined target point.
Orientation - Determination of the horizontal and
vertical location of points and the establishment of
orienting lines, adjustment of the azimuth circle of a
weapon or instrument to read required azimuths, and
adjustment of the vertical quadrant angle of elevation
(angle of site plus angle or elevation) to required values.
Orthophoria - A term used with reference to the eyes
indicating that both eyes are equally and properly
controlled by eye muscles with the result that, with the
eyes at rest, they focus and converge upon a distant
object.
Orthoscopic eyepiece - An eyepiece which has been
corrected for distortion, particularly that type of eyepiece
having a triple element cemented field lens and a
planoconvex or meniscus converging eyelens
(paras 2-24i, 4-4k, and B, fig 4-7). See eyepiece.
Othoscopic lens - A lens which has been corrected
for distortion, giving an image in correct or normal
proportions, with a flat field of view (para 2-24i).
Panoramic telescope - A telescope so designed that
the image remains erect and the position of the \fs20
eyepiece is unchanged as the line of sight is pointed in
any horizontal direction.
Parabolic mirror - A concave mirror which has the
form of a special geometrical surface; a paraboloid of
revolution. Light rays emanating from a source located
at the focal point of the paraboloid are reflected as
parallel rays. Most searchlights are of this design. All
light rays which are parallel and strike the mirror directly
from the front are reflected towards the focal point
(fig 2-21).
Parallactic angle - In range finding, the angle at the
target subtended by the base length. The
B-23
TM 9-258
impression registered upon the retina and the optic nerve
remaining for about 1/15 second after the stimulus has
been removed.
Perspective - Appearance in terms of distance.
Example, railroad tracks which appear to converge as
they are seen receding in the distance. Appearance in
relief, in three dimensions.
Phosphorescence - The glowing or giving off of light
without sensible heat; the property to shine in the dark
due to emission of light caused by chemical reaction, as
fireflies.
Photon - A quantum (elemental unit) of radiant
energy.
Plane - A surface which has no curvature; a perfectly
flat surface.
Planoconcave lens - A lens with one surface plane;
the other concave.
Planoconvex lens - A lens with one surface plane;
the other convex.
Plumbline - A vertical line. When used for adjusting
instruments, it is a weight (plumb bob) hanging on a
string.
Plus lens- A converging (convex) lens.
Point of fixation - The object on which the optical
instrument is focused; an object on which the observer’s
eye is concentrated.
Point of principal focus - The point to which parallel
rays of light converge or from which they diverge when
they have been acted upon by a lens or mirror (figs 2-19,
2-44, and 2-46). A lens has two points of principal focus,
one on each side of the lens. A lens has many focal
points depending upon the distance of the object from
the lens (fig 2-47).
Polarized light - In optics, the light which vibrates in
one direction only (in a single plane).
Plane of polarization is the plane in which the polarized
light vibrates. Light is polarized by natural materials
(Iceland spar) or a manufactured product known as
"Polaroid" or when being reflected, at a glancing angle
from a smooth surface.
Polarizing filter - A light ray filter which polarizes the
light passing through an instrument.
Porro prism - One of two identical prisms (fig B-24)
used in the Porro prism erecting system (fig 4-9). It is a
right angle prism with the corners rounded to minimize
breakage and simplify assembly. See porro prism
erecting system (fig 4-9).
baseline of the range finder forms the base of a triangle
and its apex is the target (fig. 8-28). Corresponds to the
angle of convergence in stereoscopic vision (fig. 3-8 and
3-9).
Parallax - Any apparent displacement of an object
due to the observer’s change of position such as in A,
figure 2-72, where the observer will see either the pole
number 4, or pole 7 positioned opposite the mountain, if
he changes his position. This illusion of a relative
displacement of the poles with respect to the mountain is
a deception, inasmuch as neither the poles nor the
mountain actually move.
Optically, parallax in a
telescope with a reticle is any apparent movement of the
reticle in relation to distant objects in the field of view
caused by movement of the head of the observer. This
condition exists when the image in the telescope lies in
one focal plane and the reticle lies in another
(B, fig 2-72).
Paraxial pencil - A narrow group of light rays along
the axis.
Paraxial ray - Any ray parallel to the axial ray in a
pencil of light. A ray in the immediate neighborhood of
the optical axis of a lens or mirror or close to its center.
Patina - A film. Surface mellowing or softening. A
film or oxide formed on copper and bronze.
Pechan or Z-Rotating prism - Used as a rotating
prism. It is used to invert the image in one plane without
deviating or displacing the axis of the rays of light. Used
as the rotating prism in the conventional type of optical
system of panoramic telescopes (B, fig 4-13).
Pencil of light - A narrow group of light rays coming
from a point source or converging toward a point. A
pinhole opening produces a pencil of rays.
Pentaprism - A five-sided prism used to bend light
through a constant angle, usually 90 degrees, without
producing inversion or reversion if reflection takes place
in the vertical or horizontal plane (fig.4-14).
Peripheral - On the circumference.
Near the
boundary or edge of the field of an optical system; the
outer fringe.
Periscope - An optical instrument designed to
displace the line of sight in a vertical direction usually
upwards. Used to permit observation over the top of a
fortification, a barricade, or out of a tank (fig 8-19).
Persistence of vision - The mental effect of an
B-24
2. In a telescope, power is the number of
times the instrument megnifies the object viewed. For
example, with a 6-power instrument, an object 600 yards
away is enlarged six times or it appears as it would to the
naked eye if it were at a distance of only 100 yards.
Presbyopia - A defect of vision due to advancing
age, "oldsightedness". The crystalline lens no longer is
capable of accommodating for nearby objects and these
objects cannot be seen distinctly. It is corrected by the
use of convex lenses.
Principal axis - A straight line about which an optical
element or system is symmetrical. A straight line
connecting the centers of curvature of the two refracting
surfaces of a double convex lens. In a mechanical
sense, a line joining the centers of the two surfaces of a
lens as it is placed in the mount.
Principal focal plane - Plane perpendicular to the
axis through the point of principal focus (figs 2-44 and
2-46).
Principal focal point - See point of principal focus.
Principal focus - See point of principal focus.
Prism - A transparent body with at least two polished
plane faces inclined toward each other from which light is
reflected or through which light is refracted. When light
is refracted by a prism, it is deviated or bent toward the
thicker part of the prism (fig B-25). See Abbe prism
erecting system, Amici prism, coincidence prism, Dove
prism, objective prism, Pechan prism, pentaprism, Porro
prism, rhomboidal prism, right-angle prism, and triple
mirror.
Figure B-24. Porro prism .
Porro prisms erecting system A prism erecting
system, designed by M. Porro, in which there are four
reflections to completely erect the image. Two Porro
prisms are employed (fig 4-9). The line of sight is bent
through 3600, is displaced, but is not deviated.
Positive lens - See converging lens.
Power 1. In a prism or a lens, power is a measure of
ability to bend or refract light. It is usually measured in
diopters. See diopter and prism diopter.
Figure B-25. Prism .
Prism diopter A unit of measure of the refracting
power of a prism. One diopter is the power of a prism to
deviate a ray of light by one centimeter at a distance of
one meter from the prism.
Prism erecting system - See erecting system.
Prism offset - The term applied to certain telescopes
having a characteristic offset due to the
mounting of the prism erecting system in the body of the
instrument.
Prismatic - Pertaining to a prism or the effects
produced by prisms.
Probable error - An allowance based on average
errors made by a large group of range finder
B-25
TM 9-258
operators. It will be exceeded as frequently as it is not.
Protractor An instrument for measurng or laying off
angles. It consists of a graduated arc or
semicircle, sometimes containing a radial arm (fig B-26).
The graduations can be Artillery mils, Navy mils, or
degrees. See mil.
Figure B-26. Protractor .
Pupil - The dark center of the eye. It is the aperture
through which light enters the eye.
Quadrant 1. A quarter of a circle: a sector, arc, or angle
of 90 degrees.
2. An instrument for measuring or setting
vertical angles such as a gunner’s quadrant or a range
quadrant.
Quadrant angle of elevation - The angle between the
horizontal and the line of elevation or axis or bore
(fig. B-3). See angle of elevation and angle of site.
Quanta or Quantum - See photon.
Radian - An angle included within an arc equal in
length to the radius of a circle. It is equal to 57°, 17
minutes, and 44.8 seconds.
Radiant energy - Energy that is radiated or thrown off
in all directions by its source. Light is a form of radiant
energy. Energy, by definition, is capacity for performing
work.
Ramsden dynameter - See dynameter.
Range Distance. - The horizontal distance from a
gun to its target; the distance from an observer to a
designated object.
Range usually is measured in
meters.
Range finder - An optical instrument used to
determine the distance of an object or target by
triangulation which is performed mechanically and
optically.
Range quadrant - An instrument used to set range or
measure vertical angles of elevation in laying a gun.
Ray - See light ray.
Ray filter - See filters.
Real image - See image.
Rectilinear - In a straight line. When applied to a
lens, it indicates that images of straight lines produced by
the lens are not distorted.
Rectilinear propagation - Straight line travel; refers to
the fact that light travels in a straight line when traveling
through a medium of constant optical density.
Reduction - The decrease in the apparent size of an
object produced by an optical element or instrument.
Reflected ray - The ray of light leaving a reflecting
surface representing the path of light after reflection (fig
2-13).
Reflection - Light striking a surface and returning or
"bouncing back" into the medium whence it came.
Regular reflection (figs 2-13 and 2-15) from a plane
polished surface, such as a mirror, will return the major
portion of the light in a definite direction lying in the plane
of the incident ray and
B-26
TM 9-258
the normal. See angle of reflection. Regular reflection
will form a sharp image. Diffuse reflection (fig 2-16)
occurs when the surface is irregular and the reflected
light diverges from each point as if it were a separate
reflecting surface. Diffused rays are scattered, go in
many directions, and will not form a distinct image.
Refracted ray - The ray of light passing through and
leaving a refracting surface representing the path of light
after refraction (fig 2-24).
Refracting power - The power of a lens or lens
system to converge or diverge light. See diopter.
Refraction - The bending of light which occurs when
a ray of light passes obliquely from one medium to
another of different optical density.
See angle of
refraction and angle of deviation.
Refractive index - See index of refraction.
Regular reflection - See reflection.
Relief - Effect of stereoscopic or three dimensional
vision; solidity or depth; sharpness of outline due to
contrast of the object standing out, from a background.
Resolution - In optics, the ability of a lens system to
reproduce an image in its true sense. Forming separate
images of two objects or points very close together.
Resolving power - A measure of the ability of a lens
or optical system to form separate images of two
objects or points close together. No lens or optical
system can form a perfect image of a point; it will appear
as a small disk surrounded by concentric circles. If two
points are so close together that the disks overlap, the
points cannot be distinguished separately; they are not
resolved. The measure of the resolving power is the
angle subtended at the optical center of the lens by two
points which are just far enough apart to permit
resolution into two separate images. This angle is
termed the limiting angle of resolution.
Retaining ring - A thin ring threaded on the outside
surface. It is screwed into a tube, cell, or other body
member of an optical instrument to retain or hold a lens
or other part in fixed position.
Reticle - Marks or patterns placed in the focal plane
of the objective of an optical instrument which appear to
the observer to be superimposed upon the field of view
(fig B-27). They are used as a reference point for
sighting or aiming; to measure angular distance between
two points; to determine the center of the field; or to
assist in the gaging of distance, determining leads, or
measurement. The reticle may be a pair of crosslines
composed of fine wire or may be etched on a glass plate
with plane parallel surfaces. If it is etched on glass, the
entire piece of glass is referred to as the reticle. (Also,
see figs 4-24 and 4-25).
Figure B-27. Reticle patterns superimposed on image of object .
Retina - The light-sensitive inner coat or tunic of the
eyeball upon which the image is formed by the lenses of
the eye. It contains the visual cells called the cones and
rods (figs 3-2 and 3-5).
Reversion - Turned the opposite way so that
right becomes left and vice versa. It is the effect
produced by a vertical mirror in reflecting an image.
Reverted erect - See erect image.
Reverted image - An image, the right side of which
appears to be the left side of the object and vice versa.
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TM 9-258
Reverted inverted - See inverted image.
Rhomboidal prism - A prism with two pairs of parallel
sides forming right angles and two 45ƒ slanting or
oblique parallel ends. It will displace the path of light
entering its ends without changing the direction of light.
It does not invert or revert the image (fig 4-12).
Rhomboidal prisms may be rotated to divert the lines of
sight to permit interpupillary adjustment of the eyepieces
of a binocular instrument.
Right-angle prism - A type of prism used to turn a
beam of light through a right angle (900) (fig 2-36). It will
invert (turn upside down) or revert (turn right or left),
according to position of the prism, any light reflected by
it. This prism is likewise used to turn a beam of light
through an angle of 180ƒ (fig 2-43) when either a normal
erect image or inverted, reverted image is produced,
depending upon the position of the prism with respect to
the object and the observer.
the line of sight to a limited degree. See correction
wedge, measuring wedge.
Rouge - A material for polishing optical glass made
principally from red oxide of iron. The name is also
applied to polishing materials which are not red, such as
black rouge and white rouge.
Sclera - The outer of the three coats or tunics of the
eyeball. It is tough, white, and flexible and is the white
portion of the eye normally seen. The slightly protruding
transparent portion of the center front of the eye, the
cornea, is part of this coat (fig 3-2).
Second - A unit of measurement of an angle equal to
one-sixtieth of a minute or 1/3600th part of a degree.
Selective absorption - See absorption.
Selective reflection - See absorption.
Selective transmission - See absorption.
Separator - Also known as spacer. A hollow tubular
part used to separate two lenses at a definite distance
(fig 4-35).
Sighting instruments - Devices or instruments
designed to aid in the pointing of a weapon, observation
and azimuth or range determination, and looking over
obstacles.
Sine - The sine of an angle is the side of a rightangle triangle opposite the angle divided by the
hypotenuse (the long side opposite the right angle).
Site Location - See angle of site.
Soft coat - A term designating the soft coating
applied to coated optics to differentiate between the
harder and more durable coating of magnesium fluoride
known as hard coat.
Spacer- See separator.
Spectrum - The band of colors produced by a prism
in dividing white light into its components.
The rainbow is an example, dispersion produces this
spectrum. See electromagnetic spectrum.
Spherical aberration - The aberration of a lens which
results when rays of light which pass through a lens near
its edge are converged to a point nearer the lens than
those rays passing through near the center (fig. 2-55).
The effect is poor definition of the image.
Split field - The field of view as seen when observing
through a coincidence range finder. It is formed by
uniting halves of the images produced by two objectives.
The half images are separated by the halving line.
Squint - See strabismus.
Stadia scale - Graduations on a reticle which in
conjuction with a rod of definite length can be used to
measure distances.
Standard candle - Initially a sperm whale oil candle,
7/8 inch in diameter, burning at the rate of 120 grains per
hour. Current standards of candle power are electric
lamps.
Figure B-28. Right-angle prism .
Rod - One of the two types of light-sensitive
elements or visual cells in the retina of the eye which
permit sight. The rods are associated with night vision
and sight under other conditions of weak light. They also
detect motion. For seeing in weak light, they are
stimulated by a substance known as visual purple. The
other type of visual cell is termed the cone (fig 3-5).
Roof-angle prism or roof prism - See Amici prism.
Rotating prism - See Dove prism and Pechan prism.
Rotating wedge - A circular optical wedge mounted
to be rotated in the path of light to divert
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TM 9-258
Standard letters - Letters of specific size which are
viewed at specified distance (20 feet) to test the
keenness of sight.
Stereoacuity - Keenness or sharpness of sight in
discerning depth or three dimensions.
Stereovision See stereoscopic vision.
Stereogram - A card containing two pictures taken
from different angles (corresponding to the spacing
between the pupils of the eyes), which when viewed
through a stereoscope present a single image with the
illusion of depth or three dimensions.
Stereopsis - See stereoscopic vision.
Stereoscope - A binocular instrument used to view
stereograms. See stereogram.
Stereoscopic contact - A term applied to the action of
bringing the target into the same apparent distance plane
as the central measuring mark (pip) of the reticle in the
use of a stereoscopic range finder.
See stereoscopic range finder.
Stereoscopic effect - The sense of depth, relief, or
solidity resulting when an object is viewed by both eyes,
due to the fact that each eye views the object from a
slightly different angle (fig 3-7).
Similar effect produced when viewing a stereogram.
See stereogram.
Stereoscopic power - The gain in stereoscopic effect
afforded by a magnifying binocular instrument as
compared with the ability of the naked eyes. This power
will vary with the separation of the objectives and the
power of the instrument.
Stereoscopic range finder - A self-contained distance
measuring device operating on the principle of
triangulation, Images, observed from points of known
distance by the eyes, result in a sense of depth, and are
made to appear in the same distance plane as the
central measuring mark (pip) of the stereo reticle. See
stereoscopic contact.
Stereoscopic vision - Vision in depth or three
dimensions due to the spacing of the eyes. This spacing
of the eyes permits them to see objects from slightly
different angles. An impression of the shape, depth, and
position of an object with relation to other objects is
received when the brain fuses the two separate pictures
seen by both eyes into a single image (fig 3-7).
Strabismus - Affection of the eye in which the lines of
sight cannot be directed to the same object at once
because of abnormal contraction, relaxation of, or injury
to one or more of the muscles controlling the movement
of the eyeball. A condition of the eyes in which the lines
of sight tend to turn inwardly is known as convergent
strabismus, esophoria, crosseye, or squint . A condition
of the eyes in which the lines of sight tend to turn
outward is known as divergent strabismus, exophoria, or
wall eye.
Strain - Distortion or fault in optical glass caused by
internal stress or tension and brought
about by improper cooling or annealing during
manufacture of the glass.
Straie - Localized variations in the index of refraction
of a piece of optical glass, usually occurring in streaks. It
is caused by improper mixing of the ingredients during
manufacture.
Sunshade - A tubular projection provided to protect
the objective from the direct rays of the sun (fig 4-37).
Surface plate - A plate having a very accurate plane
surface used for testing other surfaces or to provide a
true surface for accurately locating testing fixtures. It is
usually mounted on three adjustable legs so that it can
be accurately leveled in a horizontal plane.
Tangent - A straight line which touches a curve or a
curved surface but does not cut in.
Telescope - An optical instrument containing a
system of lenses, usually with magnifying power, which
renders distant objects more clearly visible by enlarging
their images on the retina of the eye, or which
superimposes a reticle pattern on the image of the object
to assist in aiming or in the gaging of distance (figs 5-6,
6-8, 5-9, and 5-13).
Thermoplastic cement - Cement capable of being
repeatedly softened by increase of temperature and
hardened by lowering of temperature, such as resin
derived from Canada balsam. It is used because it has
approximately the same index of refraction as glass.
See Canada balsam.
Thermosetting cement - A clear, colorless cement
used in cementing optical elements together, particularly
the components of a compound lens. It is used because
it has approximately the same index of refraction as
glass. It changes into a substantially infusible product
when cured under application of heat.
Toroidal (toric) lens - A lens having for one of its
surfaces a segment of a tore (the surface described by a
circle or a curve rotating about a straight line in its own
plane). Used in spectacles to correct astigmatism of the
eyes. See astigmatism.
Total internal reflection - Reflection which takes
place within a substance because the angle of indidence
of light striking the boundary surface is in excess of the
critical angle. See critical angle.
Tourmaline - A mineral that has the faculty of
polarizing light.
Trajectory - The curved path through the air
described by a projectile in its flight (fig B-3).
Traverse - Movement in azimuth; rotation in a
horizontal plane. See azimuth.
Triple mirror prism - A prism with four triangular
surfaces arranged like two triangular roofs placed at right
angles to one another. Three of the four surfaces are
used as mirrors in reflecting light. This prism has the
property of deviating,
B-29
TM 9-258
through an angle of 1800, any ray of light entering it (fig
4-15).
Triplet - A three-element compound lens with inner
surfaces curved identically and cemented together
(fig 4-3).
True field of view - See field of view.
True North - Geographic north; the direction along the
geographical meridian of the geographic north pole from
a point on the earth’s surface.
Unit of correction - An arbitrary scale graduation as
laid off on the correction wedge scale of a range finder.
Unit of error (UOE) - Unit of measurement
corresponding to 12 seconds of arc. When checking the
accuracy of an observer’s readings of range, as
determined by a range finder, it is necessary to have
some unit of measure. It has been determined by test
that the mean error of the normal, well-trained observer
under good conditions of observation should not exceed
12 seconds of arc for a series of readings, regardless of
the range involved. This mean error is taken as the
basis for determining the accuracy of range finder
readings. The unit of error (UOPE) is the unit of
measure arbitrarily selected and all errors in range
readings are converted to this unit during analysis.
Velocity of light - The speed of light. It is numerically
equal to the frequency multiplied by the wavelength. The
speed of light in vacuum equals 186,330 miles per
second.
Vertex - Highest or lowest, the most or least
projecting point on the axis of a convex or concave
mirror or lens.
Vertical travel - Movement of the line of sight of an
optical instrument or gun in a vertical plane.
Virtual base - The actual base or baseline of a range
finder multiplied by the power of magnification of the
instrument.
Virtual image - The image formed by a diverging
lens. It is formed by rays which come from the direction
of the image but do not originate in it.
It cannot be thrown upon a screen. It exits only in the
mind of the observer (fig 2-50). Similar image formed by
a converging lens if the object is inside one focal length
(fig 2-48). Apparent position of reflected point which
appears to be located behind the mirror (figs 2-40 and
2-41).
Vision persistence - See persistence of vision.
Visual acuity - Acuteness or sharpness of sight.
It is measured by the ability to distinguish letters of
specified size at a given distance.
Visual purple - A substance surrounding the lightsensitive visual cells in the retina of the eye and
stimulating the rods for keener sight under poor light
conditions.
Vitreous humor - A transparent, jelly-like substance
with which the rear and greater portion of the eye is filled
and which is part of the refracting mechanism of the eye
(fig 3-2).
Walleye - See strabismus.
Wave - Vibration; a form of movement by which all
radiant energy of the electromagnetic spectrum is
assumed to travel (fig B-29).
Figure B-29. Waves and have fronts .
Wave front - A surface normal (at right angles) to a
bundle of rays as they proceed from a source. The wave
front passes through those parts of the waves which are
in the same phase and go in the same direction. For
parallel rays, the wave front is a
plane; for rays diverging from or converging toward a
point, the wave front is spherical (fig B-29).
Wavelength - Length of a wave measured from any
point on one wave to the corresponding point, in the
same phase on the next wave; usually measured
B-30
TM 9-258
from crest to crest (fig 2-9). Wavelength determines the
nature of the various forms of radiant energy which
comprise the electromagnetic spectrum; it determines
the color of light (fig 2-3).
Wedge - An optical element with plane inclined
surfaces, Usually the faces are inclined toward one
another at very small angles. Wedges divert light toward
their thicker portions. They may be circular, oblong, or
square. See correction wedge and measuring wedge.
White light - Light, such as sunlight and daylight,
which is composed of all the different wavelengths of the
visible spectrum.
Window - A piece of glass with plane parallel
surfaces used to admit light into an optical instrument but
exclude dirt and moisture. Also see correction window.
Worm - A shaft with a gear in the form of a screw
which meshes with a worm-wheel. The axes of the
worm and worm-wheel are generally-perpendicular to
each other.
Wormwheel - A gear designed to mesh with a worm.
X-ray unit - A unit of measure equaling one
ten-millionth part of a micron, or one-ten thousand
millionth part of a millimeter.
Yaw - The angle between the axis of the projectile at
any moment and the tangent to the trajectory.
Y-azimuth - The azimuth rading taken from the YNorth direction. See Y-North.
Yellow spot - See macula.
Y-North - Grid North.
The north direction as
determined by the grid lines on a map. The United
States is laid out in seven grid zones. The central grid
line of each of these zones points to True North. The
other grid lines being parallel to the central grid line will,
due to the spherical shape of the earth’s surface, not lie
in a true northsouth line. The deviation between True
North and grid north at any point due to this curvature,
grid declination will never exceed.39. Arbitrarily selected
North direction on a drawing or a map, different from
True North, usually parallel to or at right angles to some
objects shown, and used for reference in that area only.
Z-rotating prism - See Pechan or Z-rotating prism.
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TM 9-258
INDEX
Paragraph
Diopter movement adjustment ........... 5-14
Dioptometer ........................................ 5-15
Distance
Eye .............................................. 2-34
Interpupillary................................ 3-9f
Distortion ............................................ 2-42
Divergent lens:
Description .................................. 2-156b
Image formation .......................... 2-25
Dove prism ......................................... 4-5f
Dynameter, Ramsden......................... 5-24
E
Einstein’s theory of light...................... 2-1a(7)
Electromagnetic spectrum .................. 2-lb (7)
Electromagnetic theory of light
(Maxwell, Boltzmann, and Hertz). 2-1a(5)
Erecting systems ................................
4-7
Erfle eyepiece.....................................
4-4j
Exit pupil:
Definition ..................................... 2-34
Measurement ..............................
9-3
Eye, human:
Comparison with camera.............
3-2
Defects ........................................
3-5
Fatigue ........................................ 3-14
Introduction .................................
3-1
Optical instruments and the eyes:
Accommodation ..................... 3-13
General .................................. 3-12
Refracting mechanism ................
3-5
Response mechanism .................
3-6
Structure......................................
3-3
Tension or fatigue........................ 3-14
Three coats or tunics ...................
3-4
Eye distance or relief:
Definition ..................................... 2-34b
Measurement ..............................
3
Eyepieces ...........................................
4-4
Eyeshields .......................................... 4-19
Field of view:
F
Human eye .........................................
3-8
Measurement......................................
9-5
Optical systems .................................. 2-30,2-31
Field stops .......................................... 4-10
Filters:
Colored........................................ 4-11b
General........................................ 4-11a
Polarizing..................................... 4-11c
Five-sided prism ................................. 4-5h
Fizeau’s method of measuring
speed of light ............................... 2-2d
Focal length and plane ..................... 2
-22
Focusing devices................................ 4-21
Focusing of optical instruments .......... 2-36
Fresnel’s theory of light ...................... 2-1a(4)
G
Galilean telescope .............................. 5-11
Paragraph
Page
A
Abbe prism system ............................. 4-5j,4-7d
4-19,4-22
Aberrations, elimination ...................... 2-37,2-41
2-42,2-48
Absorption, light (See Light)
Adapter ............................................... 4-17
4-35
Adjustments:
Collimation ................................... 5-16
5-11
Diopter movement ....................... 5-14
5-10
Dioptometer ................................. 5-15
5-11
Aiming circle ....................................... 8-26
8-28
Amici prism ......................................... 4-5d,4-7d
4-12, 4-22
Astigmatism in lenses ......................... 2-40
2-46
Aqueous humor (human eye .............) 3-4a
3-3
B
Barrel distortion................................. 2 -42h
2-53
Bars, optical ........................................ 4-22
4-37
Binocular vision...................................
3-7
3-7
Binoculars: .
Battery commander’s periscope .. 8-20
8-18
General........................................ 8-18
8-16
General use ................................. 8-19
8-18
C
Caps, objective ................................... 4-20
4-36
Cells, lens ...........................................
-17
4-35
Centering devices ............................... 4-18
4-35
Choriod (human eye) ..........................
3-4
3-3
Chromatic aberration .......................... 2-39
2-44
Coated optics ...................................... 4-13
4-31
Collimation adjustment ....................... 5-16
5-11
Collimator sight ................................... 8-24
8-26
Coma .................................................. 2-41
2-48
Compound microscope .......................
5-3
5-1
Compton’s theory of light .................... 2.1a(8)
2-2
Convergent lens:
Description................................... 2-15
2-20
Image formation........................... 2-24
2-32
Magnification ............................... 2-24
2-32
Cornea (human eye) ........................... 3-4a
3-3
Critical angle
Of various substances ................. 2-17
2-25
Total internal reflection .............. 2
-16
2-23
Crystalline lens (human eye) .............. 3-3b
3-2
Correction .................................... 2-42i
2-53
Curvature:
Field............................................. 2-42d
2-50
Image, formation.......................... 2-42e
2-51
D
Definition of optical terms(See Glossary.)
Devices:
Centering ..................................... 4-18
4-35
Focusing ...................................... 4-21
4-36
Diagrams, explanation of symbols .2-4
2-8
Diaphragms ........................................ 4-10
4-28
Diffraction............................................ 2-436
2-53
Diopter: ...............................................
Lens............................................. 10-26
10-1
Prism ........................................... 10-2b
10-1
I-1
Page
5-10
5-11
2-41
3-11
2-49
2-23
2-35
4-13
5-14
2-1
2-1
2-1
4-21
4-9
2-41
9-1
3-1
3-3
3-17
3-1
3-17
3-17
3-3
3-5
3-2
3-17
3-3
2-41
9-1
4-4
4-36
3-8
9-1
2-39
4-28
4-29
4-29
4-29
4-16
2-7
2730
4-36
2-42
2-1
5-6
TM 9-258
Paragraph
Page
Frequency ...................................
2-3
2-7
Image formation .......................... 2-24
2-32
Known facts................................. 2-1b
2-1
Loss............................................. 2-45
2-55
Principle of dispersion ................. 2-27
2-37
Selective reflection and absorption 2-28
2-39
Separation of "white" light ........... 2-27
2-37
Significance of color .................... 2-1b(2)
2-3
Speed ..........................................
2-2
2-6
Theories ......................................
2-1
2-1
Transmission of (energy)............. 2-1b(1)
2-2
Wavelength ............................ 2-3
2-7
Light paths, principle of reversibility 2-lb(5)2-6
Light rays:
Definition-reflected, emergent,
incident, and refracted ........... 2-6,2-11
2-11,2-16
From distant sources ................... 2-1b(4)
2-5
2-5
General........................................ 2-1b(3)
2-2
Properties .................................... 2-1 b
Used in diagrams ........................
2-4
2-8
Lighting ............................................... 4-12
4-30
M
Magnification:
Definition ..................................... 2-23
2-32
Telescope .................................. 5-21
5-13
Magnification, eyepiece with variable . 4-4n
4-11
Magnifier, simple ................................
5-2
5-1
Maxwell’s theory of light...................... 2-1a(5)
2-1
Microscope, compound ......................
5-3
5-1
Military telescopes .............................. 2-30
2-39
Millikan’s theory of light ...................... 2-1a(8)
2-2
Mirrors, front surface ..........................
4-8
4-25
Mirror, plane .......................................
2-5
2-9
N
Newton:
Rings ........................................... 2-44
2-54
Theory of light.............................. 2-1a(2)
2-1
Objective caps .................................... 4-20
4-36
Optical bars ........................................ 4-22
4-37
Optical defects:
Aberrations:
Chromatic .................................... 2-39
2-44
General .................................. 2-37
2-42
Spherical ................................ 2-38
2-42
Optical glass .......................................
4-1
4-1
Optical instruments:............................
Accommodation .......................... 3-13
3-17
Focusing 2................................... 2-36
2-42
General........................................ 3-12
3-17
History .........................................
5-1
5-1
Specialized, military:
Collimator sight ...................... 8-24
8-26
Reflector (reflex) sight ............ 8-25
8-27
Optical properties of instruments, testing:
Determination:
Curvature of image ................
9-7
9-2
Spherical aberration ...............
9-6
9-1
General...............................................
9-1
9-1
Magnifying power ...............................
9-4
9-1
Measurement:
Field of view ................................
9-5
9-1
Objective aperture ..............................
9-2
9-1
Paragraph
Page
Galilean-type rifle sight ....................... 5-20
5-12
Glass, optical ......................................
4-1
4-1
Glossary..............................................
B-2
B-1
Greek theory of light ........................... 2-1a(1)
2-1
H
Helmet directed subsystem ................ 8-28
8-3
Hourglass distortion ............................ 2-42g
2-52
Huygenian eyepiece ........................... 4-4h
4-7
Huygen’s theory of light ...................... 2-1a(3)
2-1
Images:
Brightness.................................... 2-33
2-40
Classification ............................... 2-19
2-26
Formation by convergent
lens, magnification ................. 2-24
2-32
General........................................ 2-18
2-26
Reflection by plane mirror ............ 2-20
2-27
Transmission by mirror and prism2-212-29
Infrared:
Detectors .....................................
7-4
7-1
General........................................
7-1
7-1
Radiation .....................................
7-2
7-1
Thermal .......................................
7-3
7-1
Instrument lights ................................. 4-12
4-30
Intelpupillary distance ......................... 2-35
2-41
Iris (human eye) .................................. 3-5b
3-3
K
Kellner eyepiece ................................. 4-4f
4-5
L
Laser rangefinder................................ 8-27
8-29
Lasers: ................................................
Gas ..............................................
6-3
6-1
General........................................
6-1
1
Optics ..........................................
6-5
6-3
Solid state ....................................
6-4
6-2
Types...........................................
6-2
6-1
Length, focal ....................................... 2-22
2-30
Lens cells............................................ 4-17
4-35
Lens, divergent:
Description................................... 2-25
2-35
Image formation........................... 2-25
2-35
Lens erecting system .......................... 4-7c
4-21
Lenses:
Astigmatism ................................. 2-40
2-46
Chromatic aberration, correction . 2-39b
2-45
Construction:
Compound.............................. 4-2b
4-2
Single ..................................... 4-2a
4-1
Functions of lenses:
Collective ..................................... 5-12
5-8
Eyepiece ...................................... 5-13
5-9
Field............................................. 5-12
5-8
Ocular .......................................... 5-13
5-9
Magnification, telescope optical system 2-31
2-39
Objectives ...........................................
4-3
4-3
Resolving power ................................. 2-43
2-53
(See also Convergent Lens and
Divergent lens)
Lens retaining rings ............................ 4-17
4-35
Light:
Color ............................................ 2-lb(6),2-26 2-6,2-37
Critical angle ................................ 2-16,2-17
2-23,2-25
l-2
TM 9-258
Paragraph
Page
Optical systems:
Field of view................................. 2-32
2-39
General........................................ 2-29
2-39
Magnification ............................... 2-31
2-39
Military telescopes ....................... 2-30
2-39
Wedges .......................................
4-6
4-19
Optics coated
Identification ................................ 4-15
4-34
Introduction, advantages ............. 4-13
4-31
Serviceability ............................... 4-16
4-34
Theory ......................................... 4-14
4-32
Optics, measurement systems employed:
Candlepower and foot candles .... 10-1
10-1
Degree system ............................ 10-3
10-1
Diopters ....................................... 10-2
10-1
Metric system .............................. 10-5
10-4
Mil system....................................
4
10-1
Orthoscopic eyepiece ......................... 4-4k
4-9
P
Parallax............................................... 2-46
2-55
Pechan prism ...................................... 4-5 g
4-16
Pencils of light .................................... 2-42 f
2-52
Pentaprism.......................................... 4-5h
4-16
Periscopes:
Battery commander’s................... 8-20
8-18
Description...................................
5-4
5-2
General........................................ 8-13
8-10
For self-propelled vehicle ............
8-1
8-13
For tank observation .................... 814
8-12
For tanks...................................... 8-16
8-13
Infrared ........................................ 8-17
8-15
Plack’s theory of light .......................... 2-1a(6)
2-1
Plane mirror, reflection from ...............
2-5
2-9
Plane, focal ......................................... 2-22
2-30
Plossl eyepiece ................................... 4-4m
4-11
Porro erecting system ......................... 4-7d
4-22
Porro prism system ............................. 4-5c
4-12
Principles of sterovision ......................
3-9
3-8
Prisms.................................................
4-5
4-11
Prisms, chromatic aberration, correction 2-39e
2-46
Purpose of manual..............................
1-1
1-1
R
Ramsden dynameter .......................... 5-24
5-14
Rangefinders:
General........................................ 8-21
8-19
Tabulated data .............................. 8-23
8-26
Tank (typical coincidence) ............. 8-22
8-21
Rays.................................................... 2-4,2-42f
2-8,2-52
Reduction............................................ 2-23
2-32
Reflection:
Concave mirror ............................
2-9
2-13
Convex mirror ..............................
2-8
2-13
Law ..............................................
2-6
2-11
Plane mirror .................................
2-5
2-9
Total internal ................................ 2-16
2-23
Types...........................................
2-7
2-11
Reflector (reflex) sight......................... 8-25
8-27
Atmospheric................................. 2-13
2-18
Refraction:
Index............................................ 2-12
2-18
Law .............................................. 2-11
2-16
Through a glass plate .................. 2-10
2-15
Through a triangular glass prism . 2-14
2-20
Through lenses ............................ 2-15
2-20
Paragraph
Page
Through wedges..........................
4-6
4-19
Retaining rings ................................... 4-17
4-35
Descprition ..................................
4-9
4-26
Reticles:
Function ...................................... 5-19
5-12
Retina (human eye) ............................
3-3
3-2
4-13
Rhombiodal prism .............................. 4-5e
Rifle sight, Galilean-type .................... 5-20
5-12
Rings:
Newton’s...................................... 2-44
2-54
Retaining ..................................... 4-17
4-35
Roof-angle prism ................................ 4-5d,4-7d(3) 4-12,4-23
S
Scope of manual ................................
1-2
1-3
Separators .......................................... 4-17
4-35
Sight, collimator .................................. 8-24
8-26
Spaces ............................................... 4-17
4-35
Spectrum, electromagnetic ................. 2-1b(2)
2-3
Spherical aberration:
Elimination................................... 2-38e, f
2-43,2-44
General........................................ 2-38
2-42
Stereovision, principles ......................
3-9
3-8
Stops .................................................. 4-10
4-28
Sunshades ......................................... 4-20
4-36
Symbols..............................................
2-4
2-8
Symmetrical eyepiece ........................ 4-4I
4-9
T
Tabulated data ................................... 8-23
8-26
Telescope design, optical factors:
Factors determining:
Eye relief.................................. 5-27
5-16
Field of view............................. 5-26
5-15
Light transmission or illumination of image 5-28
5-17
Magnification or power of telescope 5-21
5-13
Night glasses ...................................... 5-29
5-17
Optical glass used in design
of military instruments ................. 5-31
5-18
Pupil:
Entrance ...................................... 5-22
5-13
Exit .............................................. 5-23
5-1
Ramsden dynameter .......................... 5-24
5-14
Resolving power of optical systems ... 5-30
5-17
True and apparent fields of telescope 6-25
5-14
Telescopes:
Articulated
For self-propelled vehicle.........
8-7
8-4
For tank....................................
8-6
8-4
General ....................................
8-5
8-3
Astronomical:
Magnification ...............................
5-7
5-4
Reflecting ....................................
5-6
5-3
Refracting ....................................
5-5
5-2
Elbow:
For howitzer (towed &
self-propelled) .......................... 8-9a,8-9c,8-9d 8-5,8-7
For sight units.............................. 8-9b
8-6
General........................................
8-8
8-5
Typical .........................................
8-9
8-5
General...............................................
8-1
8-1
Military ................................................ 2-30
2-39
Observation ........................................
8-4
8-2
Panoramic:
For self-propelled howitzer .......... 8-12
8-9
For towed howitzer ...................... 8-11
8-9
General........................................ 8-10
8-8
1-3
TM 9-258
Paragraph ...................................Page
Rifle .............................................
8-2
Sniper’s sighting device ...............
8-3
Observation .................................
8-4
Terrestrisail:
Galilean ....................................... 5-11
General........................................
5-8
Lens erecting systems .................
5-9
Prism erecting systems ............... 5-10
Telescopic sights, functions:
Advantages.................................. 5-18
Definition...................................... 5-17
Functions of reticles..................... 5-19
Galilean-type rifle sight ................ 5-20
Testing optical properties of instruments 9-1 thru
9-7
Triple mirror prism............................... 4-5i
8-1
8-1
8-2
5-6
5-4
5-4
5-6
5-12
5-12
5-12
5-12
9-1,9-2
3-8
3-7
3-14
3-14
3-8
3-15
W
Wave energy transmission .................
Wavelength and frequency .................
Wavelength, units of measurement ....
Wedges, optical ..................................
2-1b
2-3
2-1b
4-6
2-2
2-7
2-2
4-19
Young’s theory of light ........................ 2-1a(4)
2-1
Y
4-18
Z
V
View, field ...........................................
Paragraph................................... Page
Vision:
Binocular .....................................
3-7
Defects of (human eye) ............... 3-10
Steroacuity .................................. 3-9k
Stereovision.................................
3-9
Visual limitations................................. 3-11
Z prism ...............................................
3-8
I-4
4-5g
4-16
TM 9-258
By Order of the Secretary of the Army:
BERNARD W. ROGERS
General, United States Army
Chief of Staff
Official:
J. C. PENNINGTON
Brigadier General, United States Army
The Adjutant General
DISTRIBUTION:
To be distributed in accordance with DA Form 12-41, Direct/General Support maintenance for Rangefinder, Binocular,
Periscope, and Telescope.
* U.S. GOVERNMENT PRINTING OFFICE : 1990 0 - 261-888 (22709)
PIN: 026334-000
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