& Basics of Light Microscopy Imaging

& Basics of Light Microscopy Imaging
Basics of
Light Microscopy
Imaging
&
SPECIAL EDITION OF
Imaging
&Microscopy
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Introduction
In this wide-ranging review, Olympus microscopy experts
draw on their extensive knowledge and many years of
experience to provide readers with a highly useful and rare
overview of the combined fields of microscopy and imaging.
By discussing both these fields together, this review aims
to help clarify and summarise the key points of note in this
story. Encompassing the creation of highly detailed specimen views through to final image capture and processing,
the story will also be backed up by useful insights, examples, as well as handy hints and tips along the way.
To make it easy to dip in and dip out of this comprehensive
Content
overview, the review is clearly structured into several chap-
Editorials
ters. These take the reader from explanations of the physics
Trust the Colours
of light, colour and resolution, through details of contrast
The Resolving Power
12
and fluorescence techniques, to finish up with discussion
Contrast and Microscopy 22
on 3D imaging. Highlighting and summarising key points
Shining Fluorescence Details
34
made within this review are a series of information boxes.
3D Imaging 42
These also aim to provide useful insight on techniques, as
Contact and Acknowledgement 52
well as expert advice on microscope set up.
Imprint
2–3
4
52
Basics of light Microscopy & Imaging • Dear Reader
“A Huge Number of
Techniques in a Relatively
Short Space”
Although microscopes are becoming more and more easy to use, it still remains important to have an
appreciation of the fundamental principles that determine the resolution and contrast seen in microscope images. This series of articles, by authors from Olympus, aims to provide an overview of the
physical principles involved in microscope image formation and the various commonly used contrast
mechanisms together with much practically oriented advice. Inevitably it is impossible to cover any of
the many aspects in great depth. However, their approach, which is to discuss applications and provide practical advice for many aspects of modern day microscopy, will prove attractive to many.
The articles begin by setting the scene with a discussion of the factors that determine the resolving
power of the conventional microscope. This permits the introduction of important concepts such as
numerical aperture, Airy disc, point spread function, the difference between depth of focus and depth
of field and the concept of parfocality. Common contrast mechanisms such as darkfield and phase
contrast are then introduced, followed by differential interference contrast and polarisation contrast
imaging. Throughout the discussion, the importance of digital image processing is emphasised and
simple examples such as histogram equalisation and the use of various filters are discussed.
Tony Wilson Ph.D
Professor of Engineering Science
University of Oxford
United Kingdom
The contrast mechanisms above take advantage of the fact that both the amplitude and phase of the
light is altered as it passes through the specimen. Spatial variations in the amplitude (attenuation)
and/or the phase are used to provide the image contrast. Another extremely important source of image contrast is fluorescence, whether it arise naturally or as a result of specific fluorescent labels having been deliberately introduced into the specimen. The elements of fluorescence microscopy and
techniques of spectral unimixing are discussed and brief mention is made of more advanced techniques where spatial variations in, for example, fluorescence lifetime are used to provide image contrast. Finally, techniques to provide three-dimensional views of an object such as those afforded by
stereo microscopes are discussed together with a very brief mention of the confocal microscope.
The authors have attempted the very difficult task of trying to cover a huge number of techniques in a
relatively short space. I hope you enjoy reading these articles.
“Unique presentation of technical aspects
in connection with image processing”
As a regular reader of “Imaging & Microscopy,” the title of this issue, “Basics of Light Microscopy &
Imaging,” must certainly seem familiar to you. From March 2003 to November 2005, we had a series
of eleven articles with the same name and content focus.
In collaboration with the authors, Dr Manfred Kässens, Dr Rainer Wegerhoff, and Dr Olaf Weidlich of
Olympus, a lasting “basic principles” series was created that describes the different terms and techniques of modern light microscopy and the associated image processing and analysis. The presentation of technical aspects and applications in connection with image processing and analysis was
unique at that time and became the special motivation and objective of the authors.
For us, the positive feedback from the readers on the individual contributions time and again confirmed the high quality of the series. This was then also the inducement to integrate all eleven articles
into a comprehensive compendium in a special issue. For this purpose, the texts and figures have been
once more amended or supplemented and the layout redesigned. The work now before you is a guide
that addresses both the novice and the experienced user of light microscopic imaging. It serves as a
reference work, as well as introductory reading material.
Dr. Martin Friedrich
Head Imaging & Microscopy
GIT Verlag, A Wiley Company
Germany
A total of 20,000 copies of “Basics of Light Microscopy & Imaging” were printed, more than double
the normal print run of ”Imaging & Microscopy.” With this, we would like to underline that we are just
as interested in communicating fundamental information as in the publication of new methods, technologies and applications.
Enjoy reading this special issue.
• Basics of light Microscopy & Imaging
“You Can Get
Keywords for Expanding
Your Knowledge”
Alberto Diaspro Ph.D
Professor of Applied Physics
University of Genoa
Italy
There is a great merit for publishing “Basics of LIGHT MICROSCOPY and IMAGING: From Colour to
Resolution, from Fluorescence to 3D imaging”. It is a great merit since it is very important to define, to
clarify, and to introduce pivotal elements for the comprehension and successive utilisation of optical
concepts useful for microscopical techniques and imaging. The niche still occupied by optical microscopy, within a scenario where resolution “obsession” plays an important role, is mainly due to the
unique ability of light-matter interactions to allow temporal three-dimensional studies of specimens.
This is of key relevance since the understanding of the complicate and delicate relationship existing
between structure and function can be better realised in a 4D (x-y-z-t) situation. In the last years, several improvements have pushed optical microscopy, from confocal schemes [1-6] to multiphoton architectures, from 7-9 folds enhancements in resolution to single molecule tracking and imaging allowing to coin the term optical nanoscopy [7]. Advances in biological labelling as the ones promoted by
the utilisation of visible fluorescent proteins [8-12] and of overwhelming strategies like the “F” techniques (FRAP – fluorescence recovery after photobleaching, FRET – fluorescence resonance energy
transfer, FCS – fluorescence correlation spectroscopy, FLIM – fluorescence lifetime imaging microscopy) [4, 13-15] collocate optical microscopy in a predominant position over other microscopical techniques. So it becomes mandatory for primers, and more in general for all those researchers looking for
answers about their biological problems that can be satisfied by using the optical microscope, to have
a good starting point for finding the optimal microscopical technique and for understanding what can
be done and what cannot be done. This long note on Basics can be a good point for starting. It brings
the reader through different concepts and techniques. The reader can get the keywords for expanding
her/his knowledge. There are some important concepts like the concept of resolution and the related
sampling problems, spectral unmixing and photon counting that are introduced for further readings.
This article reports some interesting examples and a good link between the different mechanisms of
contrast, from DIC to phase contrast until fluorescence methods. Treatment is not rigorous, but it keeps
the audience interested and is sufficiently clear. I read it with interest even if I would prefer to have
more bibliographic modern references to amplify the achievable knowledge.
References
[1] Amos B. (2000): Lessons from the history of light microscopy. Nat Cell Biol. 2(8), E151-2.
[2] Wilson T., Sheppard C. (1984): Theory and practice of scanning optical microscopy. Academic Press, London.
[3] Diaspro A. (2001): Confocal and two-photon microscopy : foundations, applications, and advances. Wiley-Liss, New York.
[4] Pawley JB. (2006): Handbook of Biological Confocal Microscopy, 3rd edition, Plenum-Springer, New York.
[5] Hawkes PW., Spence JCH. (2006): Science of Microscopy, Springer, New York.
[6] Masters BR. (2006): Confocal Microscopy And Multiphoton Excitation Microscopy: The Genesis of Live Cell Imaging, SPIE Press Monograph Vol. PM161, USA.
[7] Hell SW. (2003): Toward fluorescence nanoscopy. Nat Biotechnol. 21, 1347.
[8] Jares-Erijman EA., Jovin TM. (2003): Nat Biotechnol. 21(11), 1387.
[9] Tsien RY. (2006): Breeding molecules to spy on cells. Harvey Lect. 2003-2004, 99, 77.
[10] Tsien RY. (1998): The green fluorescent protein. Annu Rev Biochem. 67, 509.
[11] Pozzan T. (1997): Protein-protein interactions. Calcium turns turquoise into gold. Nature. 388, 834
[12] Diaspro A. (2006): Shine on ... proteins. Microsc Res Tech. 69(3),149
[13] Periasamy A. (2000): Methods in Cellular Imaging, Oxford University Press, New York.
[14] Becker W. (2005): Advanced Time-Correlated Single Photon Counting Techniques, Springer, Berlin.
[15] Periasamy A., Day RN. (2006): Molecular Imaging : FRET Microscopy and Spectroscopy, An American Physiological Society Book, USA.
Imprint
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Basics of light Microscopy & Imaging • Trust the Colours
• Basics of light Microscopy & ImagingTrust the Colours
Light
How do we describe light that reaches
our eyes? We would probably use two
key words: brightness and colour. Let us
compare these expressions to the way
light is characterised in natural sciences:
It is looked upon as an electromagnetic
wave having specific amplitude and distinct wavelength. Both, the workaday
and the scientist’s perspective essentially
mean the same. The wave’s amplitude
gives the brightness of the light, whereas
the wavelength determines its colour.
Figure 1 shows how colour and wavelength are related.
by different colour models, i.e. in different colour spaces. These are what the tokens HSI, RGB, and CYMK stand for. Each
of these models gives a different perspective and is applied in fields where it can
be used more easily than the other models. But nevertheless each model describes the same physical reality.
HSI
Let us stick to the grass and assume it is
fresh spring grass. How would we describe it in everyday language? We would
characterise it as green – and not as blue
or orange. This is the basic hue of a col-
Colour
Colour always arises from light. There is
no other source. The human eye can perceive light in the colours of a wavelength
range between 400 and 700 nm. But instead of having just one distinct colour,
light is usually composed of many fractions with different colours. This is what
a spectrum of a light implies. For example look at the spectrum of our life
source, the central sun, which emits light
of all different colours (fig. 2).
Fig. 1: Visible light spectrum.
White light
All colours of a light source superimpose.
So light sources have the colour dependent on the predominant wavelength palette. A candle for example may look
­yellowish, because the light mainly comprises wavelengths in the 560–600 nm
range. Light from sources emitting in the
whole spectral range with somewhat comparable amounts appears as white light.
Green grass
What makes us see objects, which are not
light sources themselves? There are different processes, which happen when
light meets physical matter. They are
called reflection, refraction, diffraction
and absorption. They all happen together,
but usually one process dominates. See
for example grass or other dense physical
matters: What we see when observing
grass is mainly reflected light. But why
does grass appear to be green? The reason is that grass reflects only portions of
the white daylight. At the same time it absorbs the red and blue portions of the
daylight. Thus the green light remains.
Colour models
Much that we know and observe related
to colour can be mathematically described
Trust the Colours Fig. 2: Spectrum of the sun, and spectra of common sources of visible light.
our. We would probably characterise it
additionally as “full” or “deep” – and not
as pale. This is the saturation of a colour.
Then one would describe it as bright –
and not as dark. This is the brightness or
intensity of a colour. Hue, Saturation,
and Intensity form the HSI colour model.
The Munsell Colour Tree (fig. 3) gives a
three-dimensional geometrical representation of this model. Here, the hue value
is represented by an angle, the saturation by a radius from the central axis,
and intensity or brightness by a vertical
position on the cylinder.
RGB
The HSI model is suitable to describe and
discern a colour. But there is another colour model, which better mirrors how our
human perception mechanism works.
The human eye uses three types of cone
Basics of light Microscopy & Imaging • tical application of the RGB model is
many-fold: For example, besides human
perception, digital cameras, monitors,
and image file formats also function in a
way which can be described by the addition of the three primary colours.
CYM
The overlap of the three primary additive
colours red, green, and blue creates the
colours cyan (C), yellow (Y), and magenta
(M). These are called complementary or
primary subtractive colours, because
they are formed by subtraction of red,
green, and blue from white. In this way,
yellow light is observed, when blue light
is removed from white light. Here, all the
other colours can be produced by subtracting various amounts of cyan, yellow,
and magenta from white. Subtraction of
all three in equal amount generates
black, i.e. the absence of light. White
cannot be generated by the complementary colours (fig. 4). The CYM model and
its more workable extension, the CYMK
model, find their applications in the technology of optical components such as filters as well as for printers, for example.
Colour shift
cell photo receptors which are sensitive
to light, respectively in the red V(R),
green V(G), and blue V(B) spectral range.
These colours are known as primary colours. The clue is that all of the existing
colours can be produced by adding various combinations of these primary colours. For example, the human eye per-
Fig. 3: Munsell Colour Tree.
ceives an equal amount of all the three
colours as white light. The addition of an
equal amount of red and blue light yields
magenta light, blue and green yields
cyan, and green and red yields yellow
(fig. 4). All the other colours are generated by stimulation of all three types of
cone cells to a varying degree. The prac-
Fig. 4: Primary colours.
Let us now examine a colour phenomenon which astonishes us in daily life: the
colour shift. This also gives us the chance
to take the first step into light microscopy
and look closer into its typical light
sources halogen bulb, xenon arc lamp
and mercury arc lamp.
It is a common experience to buy a
pair of black trousers that is obviously
dark blue when you look at them back
home. This inconvenient phenomenon of
colour shift is not restricted to the blue
trousers. The so-called “white light” that
is generated by a halogen bulb at a microscope, differs a lot from light from a
xenon burner. At first glance it is the intensity that is obviously different, but
even if you reduce the light intensity of a
xenon burner, the halogen light will give
you a more yellowish impression when
projected onto a white surface. Furthermore, dimming the halogen bulb light
can make the colour even more red-like.
This can be easily observed at the microscope if you focus on a white specimen
area and increase the light power slowly.
The image observed will change from a
yellow-red to a more bluish and very
bright image. This means that with increasing power, the intensity or availability of different wavelengths (colours)
has been changed. An additional aspect
to consider here are the subsequent light
perception and interpretation. They are
• Basics of light Microscopy & ImagingTrust the Colours
Fig. 5a-c is showing the emission spectra of three typically used light sources at a microscope: (a) the tungsten halogen bulb, TF = colour temperature at
different power settings, (b) the mercury burner, (c) the xenon burner.
done by eye and brain and result in the
eventually recognised colour.
But let us come back to the light generation. The overall spectrum of a light
source at a defined power setting is described with the term colour temperature. The term colour temperature is a
help to describe the spectrum of light
sources as if a black piece of ideal metal
is heated up. If a temperature of about
3500 K is reached, this metal will have a
yellowish colour. This colour will change
into a bluish white when it reaches
6000K.
For a 12V / 100W tungsten halogen
lamp at +9 Volt the colour temperature is
approximately 3200K (fig. 5a) whereas
for a 75 W xenon burner it is 6000K. So
the colour temperature gives us a good
hint about the overall colour shift. Yet it
will not give an idea of the individual intensities at defined wavelengths. This
knowledge is of high importance if we
have to use a light source for example for
fluorescence microscopy. In this case
the light source has to produce a sufficient intensity of light in a range of
­wavelengths that match the excitation
range of the fluorochrome under observation.
Microscope Light Sources
Tungsten – halogen bulbs
Nearly all light microscopes are equipped
with a halogen lamp (10W–100W) either
for general use, or as an addition to another light source. A wide range of optical contrast methods can be driven with
this type of light source, covering all
wavelengths within the visible range but
with an increase in intensity from blue to
red. Additionally, the spectral curve alters with the used power (fig. 5a). To
achieve similar looking colours in the
prevalent brightfield microscopy the
Trust the Colours power setting should be kept at one level,
e.g. 9V (TF= 3200 K; colour temperature
at +9V). This light intensity level is often
marked at the microscope frame by a
photo pictogram.
But here a problem arises: If the light
intensity is to be kept fixed in the light
microscope the light might be too bright
for observation. In daily life if sunlight is
too bright sunglasses help – but they
Box 1: Centring of a mercury burner
Depending on the type (inverted or upright) and manufacturer of the microscope there will be some individual differences but the strategy remains the
same.
Please also see the instruction manual of the microscope.
1. Start the power supply for the burner, use a UV-protection shield and
ensure that a mirror-cube is in the light path.
2. Locate a white paper card on the stage of the microscope and open the
shutter. If the light is too bright insert optional available neutral density
filters (e.g. ND25 – only 25 % of the light intensity will pass through).
3. Bring the focus to the lowest position.
4. Get a free objective position or on upright microscopes a 20x objective
in the light path.
5. If available, open the aperture stop and close the field stop.
6. Optimise brightness with the burner centring knobs (mostly located at
the side of the lamp house)
7. From the lamp house, an image of the arc of the burner itself and the
mirrored image are projected on the card. To see them in focus, use a
collector focussing screw (Figure A).
8. If only one spot can be located or the second is not the same size, the
mirror (Figure B) has to be re-centred as well. At most lamp houses
there are screws at the back for screwdrivers for this option. Rotate
them until the images have the same size as shown in (Figure C).
9. Locate both images parallel to each other and overlay them by using the burner centring screws
(Figure D).
10. Defocus the images with the collector focusing screw and open the field stop.
11. Remove the white paper card and bring a homogenous fluorescent specimen into focus (e.g. try some
curry on the cover slip).
12. Fine adjustment of the homogenous illumination can only be performed under observation: if necessary
readjust the collector focusing screw so that the total field of view is equally illuminated.
13. If digital acquisition is of prime interest, the fine adjustment can also be performed at the monitor, to
optimise the illumination for the size of the CCD.
Basics of light Microscopy & Imaging • might not only reduce the intensity but
also let us see the world in other colours.
In light microscopy there are light filters
that only reduce the intensity. These filters are called neutral density filters (ND)
or neutral grey filters. They are characterised by the light that they will transmit. Therefore, a ND50 will allow half
the light intensity to pass through
whereas a ND25 will reduce the intensity
to 25 % without changing the colours. If
the spectrum is changed, we will have
colour filters. There is a huge variety of
colour filters available but here we will
only discuss the so-called light balancing
daylight filter (LBD).
This filter is used together with the
halogen light source to compensate for
the over distribution of the long (red)
wavelengths. This enables us to see the
colours of a stained pathology section for
example on a neutral white background
in brightfield microscopy (fig. 11).
Mercury Arc lamp
The mercury burner is characterised by
peaks of intensity at 313, 334, 365, 406,
435, 546 and 578nm and lower intensities at other wavelengths (see fig. 5b).
This feature enables the mercury burner
to be the most used light source for fluorescence applications. Whenever the
peak emissions of the burner match the
excitation needs of the fluorochromes a
good (depending on the specimen) signal
can be achieved. However, these benefits
are reduced by a relative short lifetime of
the burner of about 300 hours and a
small change of the emission spectrum
due to deposits of cathode material to the
Table 1: Comparison of different light sources and their colour temperature performance
Light source
Colour temperature
Vacuum lamp (220 W / 220 V)
Nitraphot (tungsten filament) lamp B (500 W / 220 V)
Photo and cinema lamps as well as colour control lamp (Fischer)
Photo and cinema (e.g., nitraphot (tungsten filament) lamp S)
Yellow flash bulb
Clear flash bulb
Moonlight
Beck arc lamp
White arc lamp as well as blue flash bulb
Electron flash unit
Sun only (morning and afternoon)
Sun only (noontime)
Sun and a cloudless sky
Overcast sky
Fog, very hazy
Blue northern sky at a 45° vertical angle
International standard for average sunlight
2790 K
3000 K
3200 K
3400 K
3400 K
3800 K
4120 K
5000 K
5500 K
5500-6500 K
5200-5400 K
5600 K
6000 K
6700 K
7500-8500 K
11000 K
5500 K
inner glass surface of the burner with
ongoing lifetime.
Xenon Arc lamp (XBO)
Xenon burners are the first choice light
sources when a very bright light is
needed for reflected microscopy, such as
differential interference contrast on dark
objects, or quantitative analysis of fluorescence signals as for example in ion
ratio measurement. They show an even
intensity across the visible spectrum,
brighter than the halogen bulb but they
do not reach the intensity peaks of the
mercury burners. The xenon burner
emission spectrum allows the direct analysis of intensities at different fluorescence
excitation or emission wavelengths. The
lifetime is of about 500–3000 hours depending on use (frequent on/off switching reduces the lifetime), and the type of
burner (75 or 150W). For optimisation of
illumination especially with the mercury
and xenon burners the centring and
alignment is very important.
Coming to the point
To ensure that the light source is able to
emit the required spectrum is one part of
the colour story; to ensure that the objective lenses used can handle this effectively is another. When “white” light is
passing through the lens systems of an
objective refraction occurs. Due to the
physics of light, blue (shorter) wavelengths are refracted to a greater extent
than green or red (longer) wavelengths.
This helps to create the blue sky but is
not the best for good colour reproduction
at a microscope. If objectives did not
compensate this aberration then as outlined in fig. 6 there would be focus points
• Basics of light Microscopy & ImagingTrust the Colours
Fig. 6: Schematic of axial chromatic aberration.
for all colours along the optical axes.
This would create colour fringes surrounding the image.
To reduce this effect achromatic lens
combinations have been used since the
18th century. These types of lenses combine the blue and red wavelengths to one
focus point. Further colour correction at
the visible range can be achieved by adding different sorts of glass systems together to reach the so-called Fluorite objectives (the name originally comes from
the fluorspar, introduced into the glass
formulation). They are also known as Neofluar, Fluotar, or Semi-apochromat. To
correct the complete range from near infrared (IR) to ultraviolet (UV) the finest
class of objectives, apochromate, has
been designed. Beside colour correction
the transmission power of an objective
can also be of prime interest. This is especially the case if high power near UV is
needed at e.g. 340 nm (excitation of the
calcium sensitive dye Fura-2) or IR is
needed for 900 nm IR-DIC.
The look of colour
Let us now go one step further and see
how the light with its colours – after having passed the microscope – is detected
in modern camera systems. Here, digital
cameras have become standard for acquisition of colour or monochrome images in the microscopy field. But how do
these cameras work and how can it be
guaranteed that the colours in the images they provide are accurate? This is
of critical importance because it is a primary prerequisite for all activities such
as image documentation, archiving or
analysis.
Detecting colour
Light detection functions in the same way
for both colour and monochrome cameras. Fig. 7 illustrates how CCD digital
cameras (Charged Coupled Device) function. In colour cameras, mosaic filters
are mounted over the CCD elements
Fig. 8: Bayer colour filter mosaic array and underlying photodiodes.
Trust the Colours Fig. 7: The principle behind how CCD cameras function.
(fig. 8). These filters ensure that only
green, red or blue light is permitted to
come into contact with the light-sensitive
part of the single sensors or pixels (picture element). The proportion of colours
is generally two green pixels to one red
and one blue pixel. The colour image is
actually generated by the computer initially via complex algorithms. This involves assessing and processing the respective signals of the green, red and
blue pixels accordingly. For example, a
single pixel with a bit depth of 8 becomes
a 24-bit colour pixel (fig. 9).
Depending on the imaging method,
the reflected or transmitted light of the
object is focused onto a light-sensitive
CCD sensor. So what is a CCD (Charge
Coupled Device) element? This is semiconductor architecture designed to read
out electric charges from defined storage
areas (figs. 7, 10). Due to the special readout method, the light-sensitive area of a
pixel is restricted to just 20 % of the actual pixel surface. This is why special
Fig. 9: Bit depth and grey levels in digital images.
Basics of light Microscopy & Imaging • lenses above the sensor focus all incoming light onto the light-sensitive area of
the pixel surface. The light generates
electron-hole pairs via the photoelectric
effect. These electrical charges are collected, combined into charge packets and
subsequently transported through the
entire CCD sensor. The charge is converted into a voltage first because
processing voltage is significantly easier
than processing current. This analogue
output signal is then amplified firstly on
the chip itself and then again outside the
chip. An analogue/digital converter converts the voltage (the signal) into a binary format.
There are various bit depths, with the
standard being 8 bit. This means that 256
combinations (intensity values) are available for each pixel (figs. 9, 10). Currently,
many new cameras appear on the market
with bit depths of 12 bit (4096 intensity
levels). There are even cameras offering
bit depths up to 16 bit (65656 intensity
levels). The number of pixels a camera
has depends on the CCD chip used in it
and the technology to create the image.
At the moment cameras are on offer with
a resolution of up to 12.5 million pixels.
Colour temperature and white balance
Let us now continue on the colour temperature subject of the microscope light
sources and review its impact on the colours in digital imaging. As we know light
is essential for both microscopes and
cameras. There are many types of light,
as was explained above for microscope
light sources. The sun may yield different
colours depending on the time of day and
in the microscope there are variations
depending on the source used as well as
on the acquisition environment. The reason for this is what is referred to as colour temperature. As we have seen this
term describes a specific physical effect
– i.e., the spectral compositions of differing light sources are not identical and
furthermore are determined by temperature (see Table 1). This means that colour temperature is of tremendous significance with regard to digital image
acquisition and display as it influences
both colour perception and colour display to such a critical degree. A person’s
eyes automatically correct against this
effect subconsciously by adapting to
changing lighting conditions. This means
that he/she will see those things that are
known to be white as white.
Digital or video cameras are unfortunately not intelligent enough to register
changing light conditions on their own
and to correct against the resulting colour shifts. This is why these kinds of
cameras often yield images whose colours are tinged. Correcting colour tinge(s)
in true-colour images is known as white
balance (see fig. 11). Many cameras tend
to adapt image colours at acquisition,
this kind of colour displacement can be
corrected retroactively. To do so, requires an image with an area where the
user knows there should be no colour. In
fact, this particular image area should be
black, white or grey.
Automated white balance adjustments
Most imaging software programmes offer embedded algorithms to correct for
colour tinges. To do this, the user has to
define a section interactively within the
image area where it is certain that the
pixels should be white, black or grey (but
at present are tinged). First of all, three
correction factors will be calculated
based on the pixels within the section –
one for each of the three colour components Red (R), Green (G), Blue (B). These
correction factors are defined such that
the pixels within the section will be grey
on average – the section will have no colour at all. The whole image is then corrected automatically using these correction factors (fig. 11c, d).
The following is a more detailed description of what takes place. The average intensity I for each pixel (n) within
the section will be calculated: In=
(R+G+B)/3. The colour factor (F) for each
colour component, of each pixel within
the section will then be determined based
on this calculation, e.g., for the red colour factor: Fn (R) = (In/R).
Take a look at this example. A pixel
has the following colour components:
(R,G,B) = (100,245,255) – thus an average
intensity of In =200 and a red colour factor of Fn (R) = (200/100) = 2.0. The three
colour factors will be averaged for all
pixels within the circle, meaning that a
correction factor (<F(R)>, <F(G)> and
<F(B)>) will be determined for each colour component. And now, the colour
components of all the image’s pixels will
be multiplied by the corresponding correction factor(s).
Why do colours not match easily?
It would be ideal to be able to pick any
digital camera, monitor or printer at random and have the resulting colours onscreen and in the printed output be acceptably close to the original. But
unfortunately, getting colours to match is
hard to achieve.
For one thing, colour is subjective.
This means that colour is an intrinsic
feature of an object and colours are
purely subjective – as interpreted by the
visual system and the brain. Another
critical aspect is that lighting affects col-
Fig. 10: Creation of a digital image.
10 • Basics of light Microscopy & ImagingTrust the Colours
Fig. 11: White balance adjustment using microscope and software options on a histological specimen.
a: Image with wrong microscope settings (low power 3V, no LBD filter), note that areas that are not stained (background) show a
yellowish colour.
b: image using optimised microscope settings (9V + LBD filter + Neutral density filer ND6), note that the background becomes
white.
c: Image after digital white balancing of image (a), note that with the help of a software white balancing on the yellowish image
(a) the colours can be recalculated to some extend to a well balanced image (compare with b or d).
d: Image after digital white balancing of image (b), note that an already well balanced image (b) can still be enhanced in quality.
Areas that are used to define (neutral background) regions of interest (ROI), are marked with (x).
our. So a printout will vary depending on
the lighting. It will look different under
incandescent light, fluorescent light and
in daylight. Furthermore, colours affect
other colours. This means your perception of a colour will change depending on
the colours surrounding that colour. In
addition, monitors can display colours
that printers cannot print. Printers can
print colours that monitors cannot display. Cameras can record colours that
neither monitors nor printers can produce. A colour model is at the very least
simply a way of representing colours
mathematically. When different devices
use different colour models they have to
translate colours from one model to another. This often results in error. Given
all the limitations inherent in trying to
match colours, it is important to understand the difference between correctable
colour errors and non-correctable errors. Correctable errors are ones where
you can do something about them via
software – such as with a colour management system or white balance. Noncorrectable errors are the ones that you
can’t do anything about because the information you need to correct them simply does not exist. Better lenses, better
optical coatings and improved CCD arrays can all minimise the non-correctable errors. Correctable errors, however,
Trust the Colours require colour management through
software.
If every program used the same approach to colour management and every
printer, monitor, scanner and camera
were designed to work with that colour
management scheme, colour management would obviously be a snap. If the
hardware devices all used the same
standard colour model, for example, or
came with profiles that would translate
colour information to and from a standard as needed, you’d be able to move colour information around – from program
to program or scanned image to printer
– without ever introducing errors.
Conclusion
The light that reaches our eye consists of
many different colours, and different
light sources produce a different mix of
these. Different objects in the world absorb and reflect different wavelengths –
that is what gives them their colour.
There are many colour models to map
this multi-dimensional world, each describing the same physical reality but
having different applications. Our vision
systems give us only a partial view of it.
For effective imaging, the microscopist
must be aware of the complexity of the
world of light. There is a variety of light
sources, objectives and filters, to enable
the assembly of a system with the appropriate properties for a particular application. The photographer in turn works
with an imaging system which takes partial information and processes it to
achieve colour balance. Printers and
monitors in turn repeat the process – and
remove one further from the original
data.
Several hundred years in the development of optics have managed to hide
some of the complexity from the casual
user. But microscopy is a good deal more
complicated than it might seem. An
awareness of what is really going on – in
our eyes, in our brains, in the optics, on
the CCD and on the screen – enables us
to appreciate just what a wonderful
achievement that final image is, and how
much care is required to ensure the best
results.
Basics of light Microscopy & Imaging • 11
The Resolving Power
12 • Basics of light Microscopy & ImagingThe Resolving Power
Human vision
For most of us, seeing something is normal and we are accustomed to evaluate
things by looking at them. But the seeing
act uses many different and complex
structures that enable us not only to
catch images but to process and interpret them at the same time. Every image
that is projected on the retina of our eye
is transformed into neuronal impulses
creating and influencing behaviour: this
is what we call seeing. Therefore, what
we see is not necessarily what another
person sees.
Compared to these processes within
our brain, the first steps of vision seem to
be much simpler. To create a projection
onto the layer of the retina, where millions of light sensitive sensory cells are
located, the light passes through an optical system that consists of cornea, aqueous humour, iris, pupil, focus lens and
the vitreous humour, see fig. 12. All these
elements together create what the raster
framework of sensory cells can translate
within their capability into neuronal activity.
Eagles and mice
Those two different set ups – the optical
system and the sensor system – restrict
area, spread over the total amount of
sensory cells. More individual sensors
can catch differences and translate them
into a recognised image.
Unfortunately, we can move closer
only up to a certain distance, after which
our flexible optical system is no longer
able to project the image clearly onto the
retina. If the words are typed too small,
like these [“you can not resolve it without
optical help”]
our resolution is
reached.
The wavelength that transports the
information of the image also influences
what we can resolve. Shorter blue wavelengths carry finer details than longer
ones from the same object. Using a microscope it would be easy to read the text
and to document it via the sensors of a
camera. But these tools are also restricted by the resolution of their optical
system and the number and sensitivity of
sensors – the pixels.
“You can not resolve it without optical help”,
What is resolution?
Resolution or resolving power of a microscope can be defined as the smallest distance apart at which two points on a
specimen can still be seen separately. In
the transmitted light microscope, the xyresolution R is determined essentially by
three parameters: the wavelength λ of
Fig. 12: Anatomy of the human eye and cross section of the retina.
the options of human vision at the very
first step. Details can be found at www.
mic-d.com/curriculum/lightandcolor/
humanvision.html. Having eagle eyed optical systems alone will not enable us to
see mice from far away. To resolve a
small object (the mouse) against a large
background needs special physical properties of the optics and also requires a
certain sensitivity and number of sensors
used to translate the information.
If this text is getting too small, you have
.
This means your eyes can get light from
the small letters in a wider angle than
before. You are now focusing on a smaller
to come closer to
The Resolving Power resolve it
the illuminating light, and the numerical
aperture (NA) of the objective (NAobj) as
well as the condenser (NAcond).
R = 1.22* λ / (NAobj+NAcond) (1)
When the aperture of the condenser is
adjusted to that of the objective, i.e. the
aperture of the condenser is essentially
the same as the objective aperture, the
equation (1) simplifies to:
R = 0.61* λ /NAobj (2)
This equation is used for both transmitted light microscopy and for reflected
Basics of light Microscopy & Imaging • 13
light microscopy. Note here that resolution is NOT directly dependent on the
magnification. Furthermore the end
magnification should not be higher than
1000x the NA of the objective, because
then the image will be only enlarged but
no further resolution will be visible. This
is called empty magnification.
What is the numerical aperture of an
objective?
The numerical aperture of a microscope
objective is a measure of its ability to
gather light and thus resolve fine specimen detail at a fixed objective distance.
The numerical aperture is calculated
with the following formula:
NA = n*(sin µ)
(3)
n is the refractive index of the medium
between the front lens of the objective
and the specimen cover glass. It is n =
1.00 for air and n = 1.51 for oil. µ is one
half the angular aperture, as can be seen
in fig. 13. The bigger µ, the higher is the
numerical aperture. Working in air, the
theoretical maximum value of the numerical aperture is NA = 1 (µ = 90°). The
practical limit is NA = 0.95.
Numerical aperture in practice
For all microscope objectives the resolving power is mentioned with the number
for the numerical aperture shown di-
Fig. 13: Angular aperture of an objective.
Table 2: Numerical Apertures (NA) for different types of objectives and magnifications
Magnification
Plan Achromat
Plan Fluorite
Plan Apochromat
4 x
10x
20x
40x
60x
100x
0.1
0.25
0.4
0.65
0.8 (dry)
1.25 (oil) 0.13
0.3
0.5
0.75
0.9 (dry)
1.3 (oil)
0.16
0.4
0.75
0.9
1.42 (oil immersion)
1.4 (oil)
rectly following the index for magnification (fig. 14), e.g. a UPlanFLN 60x/0.9 objective produces a 60x magnification
with a numerical aperture of 0.9. The
Numerical Aperture strongly differs depending on the type of objective or condenser, i.e. the optical aberration correction. Table 2 lists some typical numerical
apertures for different objective magnifications and corrections.
Box 2: How to set Köhler illumination
To align a microscope that can change the distance of the condenser to the area of the specimen for Köhler
illumination you should:
1. Focus with a 10x or higher magnifying objective on a specimen, so that you can see at least something of
interest in focus. (If your condenser has a front lens please swing it in when using more then 10x magnifying objectives.)
2. Close the field stop at the light exit.
3. Move the condenser up or down to visualise the closed field stop. Now only a round central part of your
field of view is illuminated.
4. If the illumination is not in the centre the condenser has to be moved in the XY direction to centre it.
5. Finely adjust the height of the condenser so that the edges of the field stop are in focus and the diffraction
colour at this edge of the field stop is blue green.
6. Open the field stop to such an amount that the total field of view is illuminated (re-centring in xy direction
may be necessary).
7. For best viewing contrast at brightfield close the aperture stop to an amount of 80 % of the objective numerical aperture.
In practice, you can slowly close the aperture stop of your condenser while looking at a specimen. At the moment when the first change in contrast occurs, the NA of the condenser is getting smaller then the NA of the
objective and this setting can be used. For documentation the condenser NA should be set according to that
of the objective. To visualise the aperture stop directly you can remove one eyepiece and see the aperture
stop working as a normal diaphragm.
To achieve higher NA than 0.95 for objectives they have to be used with immersion media between front lens and specimen. Oil or water is mostly used for that
purpose. Because objectives have to be
specially designed for this, the type of immersion media is always mentioned on the
objective like on the UPlanFLN 60x/1.25
Oil Iris objective. This objective needs oil
as an immersion media and can not produce good image quality without it.
For special techniques like the Total
Internal Reflection Fluorescence Microscopy (TIRFM), objectives are produced
with very high NA leading by the Olympus APO100x OHR with a NA of 1.65. The
resolution that can be achieved, particularly for transmitted light microscopy, is
depending on the correct light alignment
of the microscope. Köhler illumination is
recommended to produce equally distributed transmitted light and ensure the
microscope reaches its full resolving potential (see box 2).
What resolution can be reached with a
light microscope?
To make the subject more applicable,
some resolution numbers shall be given
here. Using a middle wavelength of 550
nm, the Plan Achromat 4x provides a
resolution of about 3.3 µm, whereas the
Plan Apochromat reaches about 2.1 µm.
The Plan Achromat 40x provides a resolution of 0.51 µm and the Plan Apochromat of 0.37 µm. The real-world limit for
14 • Basics of light Microscopy & ImagingThe Resolving Power
the resolution which can be reached with
a Plan Apochromat 100x is often not
higher than R = 0.24 µm.
To give a comparison to other microscopes that do not work with visible light,
the resolution reached nowadays in a
scanning electron microscope is about R
= 2.3 nm. In a transmission electron microscope, structures down to a size of 0.2
nm can be resolved. Scanning probe microscopes even open the gates to the
atomic, sub Angstrøm dimensions and
allow the detection of single atoms.
What are airy disks?
Every specimen detail that is illuminated
within a microscope creates a so-called
diffraction pattern or Airy disk pattern.
This is a distribution of a bright central
spot (the Airy disk or primary maximum),
and the so called secondary maxima separated by dark regions (minima or rings
of the Airy disk pattern; see fig. 15, 16)
created by interference (More details:
www.mic-d.com/
curriculum/lightandcolor/diffraction.html). When two details
within the specimen are closely together
we can only see them separated if the
two central spots are not too close to
each other and the Airy disks themselves
are not overlapping. This is what the
Rayleigh criterion describes. Good distinction is still possible when one Airy
disk just falls in the first minimum of the
other (fig. 15).
The smaller the Airy disks, the higher
the resolution in an image. Objectives
which have a higher numerical aperture
produce smaller Airy disks (fig. 16) from
the same specimen detail than low NA
objectives. But the better the resolution
in the xy direction, the less is the specimen layer that is in sharp focus at the
same time (Depth of field), because the
resolution also gets better in the z direction. Like higher magnification, higher
resolution always creates less depth of
field. Also in most cases the increase of
NA means that the objective gets closer
to the specimen (less working distance)
compared to an objective of lower NA but
with same magnification. Therefore,
choosing the best objective for your application may not only depend on the resolving power.
Resolution in digital images – is it
important?
The next step is to go from the optical
image to the digital image. What happens here? The “real” world conversion
from an optical to a digital image works
via the light sensitive elements of the
The Resolving Power Table 3: Optical resolution and number of needed pixels. The number of pixels a 1/2 inch chip should
have to meet the Nyquist criterion (2 pixels per feature) and the optimum resolution (3 pixels per
feature). The number for LP/mm is given for the projection of the image on the CCD.
Objective Magnification NA Resolution Lp/mm CCD resolution 1/2´´ CCD resolution 1/2´´
(µm)
(on CCD) Nyquist limit
Necessary resolution
2 pixel/lp
3 pixel/lp
PlanApoN
UPlanSApo
UPlanSApo
UPlanSApo
UPlanSApo
UPlanSApo
2
4
10
20
40
100
0,08
0,16
0,4
0,75
0,9
1,4
4,19
119
2,10
119
0,84
119
0,45
111
0,37 67
0,24 42
CCD chips in digital cameras, for example. Or, a video camera sensor may provide voltage signals that are read out and
digitised in special frame grabber cards.
But what is of more interest here is the
principle which lies behind the actual realisations. The optical image is continuous-tone, i.e. it has continuously varying
areas of shades and colour tones. The
continuous image has to be digitised and
quantified; otherwise it cannot be dealt
with in a computer. To do so, the original
image is first divided into small separate
blocks, usually square shaped, which are
called pixels. Next, each pixel is assigned
a discrete brightness value. The first step
is called digital sampling, the second
pixel quantisation. Both convert the continuous-tone optical image into a two-dimensional pixel array: a digital image.
Digital resolution – what is it for?
The pixel quantisation of the
image intensities depends
on the bit depth or dynamic range of the
converting system. The bit
depth defines
the number of
grey levels or
1526 x 1145
1526 x 1145
1526 x 1145
1420 x 1065
858 x 644
534 x 401
2289 x 1717
2289 x 1717
2289 x 1717
2131 x 1598
1288 x 966
801 x 601
the range of colour values a pixel can
have, and thus determines a kind of a
brightness or colour resolution. Yet it is
the digital sampling which defines the
spatial resolution in a digital image. Both
spatial and brightness resolutions give
the image the capability to reproduce
fine details that were present in the original image. The spatial resolution depends on the number of pixels in the digital image. At first glance, the following
rule makes sense: the higher the number
of pixels within the same physical dimensions, the higher becomes the spatial resolution. See the effect of different numbers of pixels on the actual specimen
structure in fig. 17. The first image (175
x 175) provides the image information as
reasonably expected, whereas specimen
details will be lost with fewer pixels (44 x
44). With even fewer, the specimen features are masked and not visible any
more. This effect is called pixel
blocking.
Fig. 14: What is what on an objective?
Olympus:
Manufacturer
PlanApo: Plan: Flat field correction; Apo:
Apochromatic;
60x:
Linear magnification
1,42 Oil: Numerical Aperture (needs oil immersion)
∞:
Infinity corrected optic (can not be
mixed with finite optics that belongs
to the 160 mm optics)
0.17:
Cover slip correction. Needs a cover
slip of 0.17mm thickness.
Note: Take care of this parameter. There can
also be a “0“ for no coverslip, a “1“
for 1mm thickness or a range of mm.
Wrong usage of objectives will create
“foggy” images (spherical aberration).
FN 26.5: Field number 26.5 (When using an
ocular and tube that can provide a FN
of 26.5 you may divide this number
with the magnification to achieve the
diameter in your field of view in mm.
26.5/60 = 0,44 mm).
Basics of light Microscopy & Imaging • 15
Box 3:
Multiple Image Alignment (mia)
Multiple image alignment is a software approach
to combine several images into one panorama
view having high resolution at the same time.
Here, the software takes control of the microscope, camera, motor stage, etc. All parameters
are transferred to the imaging system through a
remote interface. Using this data, the entire microscope and camera setups can be controlled
and calibrated by the software. After defining the
required image size and resolution, the user can
execute the following steps automatically with a
single mouse click:
1. Calculation of the required number of image
sections and their relative positions
2. Acquisition of the image sections including
stage movement, image acquisition and computing the optimum overlap
3. Seamless “stitching” of the image sections with
sub-pixel accuracy by intelligent pattern recognition within the overlap areas.
Is there an optimum digital resolution?
Thus, the number of pixels per optical
image area must not be too small. But
what exactly is the limit? There shouldn’t
be any information loss during the conversion from optical to digital. To guarantee this, the digital spatial resolution
should be equal or higher than the optical resolution, i.e. the resolving power of
the microscope. This requirement is formulated in the Nyquist theorem: The
sampling interval (i.e. the number of pixels) must be equal to twice the highest
spatial frequency present in the optical
image. To say it in different words: To
capture the smallest degree of detail, two
pixels are collected for each feature. For
high resolution images, the Nyquist criterion is extended to 3 pixels per feature.
To understand what the Nyquist criterion states, look at the representations in
fig. 18 and 19. The most critical feature
to reproduce is the ideal periodic pattern
of a pair of black and white lines (lower
figures). With a sampling interval of two
pixels (fig. 18), the digital image (upper
figure) might or might not be able to resolve the line pair pattern, depending on
the geometric alignment of specimen and
camera. Yet a sampling interval with
three pixels (fig. 19) resolves the line pair
pattern under any given geometric alignment. The digital image (upper figure) is
always able to display the line pair structure.
With real specimens, 2 pixels per feature should be sufficient to resolve most
details. So now, we can answer some of
the questions above. Yes, there is an optimum spatial digital resolution of two or
three pixels per specimen feature. The
resolution should definitely not be
smaller than this, otherwise information
will be lost.
Calculating an example
A practical example will illustrate which
digital resolution is desirable under which
circumstances. The Nyquist criterion is
expressed in the following equation:
R * M = 2 * pixel size (4)
R is the optical resolution of the objective; M is the resulting magnification at
the camera sensor. It is calculated by the
objective magnification multiplied by the
magnification of the camera adapter.
Assuming we work with a 10x Plan
Apochromat having a numerical aperture (NA) = 0.4. The central wavelength
of the illuminating light is l = 550 nm. So
the optical resolution of the objective is R
= 0.61* l/NA = 0.839 µm. Assuming further that the camera adapter magnification is 1x, so the resulting magnification
of objective and camera adaptor is M =
10x. Now, the resolution of the objective
has to be multiplied by a factor of 10 to
calculate the resolution at the camera:
R * M = 0.839 µm * 10 = 8.39 µm.
Thus, in this setup, we have a minimum
distance of 8.39 µm at which the line
pairs can still be resolved. These are 1 /
8.39 = 119 line pairs per millimetre.
The pixel size is the size of the CCD
chip divided by the number of pixels.
A 1/2 inch chip has a size of 6.4 mm *
4.8 mm. So the number of pixels a 1/2
inch chip needs to meet the Nyquist cri-
4. The overlap areas are adjusted automatically
for differences in intensity
5. Visualisation of the full view image.
Fig. 15: Intensity profiles of the Airy disk patterns of one specimen detail and of two details at different distances.
16 • Basics of light Microscopy & ImagingThe Resolving Power
Fig. 16: Airy disk patterns of different size as an example of the
resolving power for low NA (left) and high NA (right) objectives.
terion with 2 pixels per feature, is 1/(R
* M) * chip size * 2 = 119 line pairs / mm
* 6.4 mm *2 = 1526 pixels in horizontal
direction. If you want 3 pixels per line
pair, the result is 2289 pixels. This calculation can be followed through for different kind of objectives. Please check out,
which numbers of pixels we need for a
1/2 inch chip in table 3.
What might be astonishing here is the
fact, that the higher the magnification,
the fewer pixels the chip of a CCD camera needs! Working with a 100x Plan
Apochromat objective combined with an
1/2 inch chip, we need just 800 x 600 pixels to resolve digitally even the finest optically distinguished structure. The
higher number of pixels of about 2300 x
1700 is necessary only at lower magnifications up to 10.
Which camera to buy?
The resolution of a CCD camera is clearly
one important criterion for its selection.
The resolution should be optimally adjusted to your main imaging objective.
Optimum resolution depends on the objective and the microscope magnification
you usually work with. It should have a
minimum number of pixels to not lose
any optically achieved resolution as described above. But the number of pixels
also should not be much higher, because
the number of pixels is directly correlated with the image acquisition time.
The gain in resolution is paid for by a
slow acquisition process. The fastest
frame rates of digital cameras working
at high resolution can go up to the 100
milliseconds or even reach the second
range, which can become a practical disadvantage in daily work. Furthermore,
unnecessary pixels obviously need the
same amount of storage capacity as necessary pixels. For example, a 24 bit true
colour image consisting of 2289 x 1717
pixels has a file size of almost 12 MB, if
The Resolving Power Fig. 17: Four representations of the same image, with different numbers of pixels used. The numbers of pixels is written below each image.
there is no compression method applied.
The slow frame rate and the file size are
just two aspects which demonstrate, that
the handling of high resolution images
becomes increasingly elaborate.
High resolution over a wide field
of view
The xy-resolution desired is one aspect
which makes an image ‘high quality’. Another partially contradicting aspect is
that we usually want to see the largest
possible fraction of the sample – in best
case the whole object under investiga-
tion. Here, the microscope’s field of view
becomes important. The field of view is
given by a number – the so called field
number – e.g. if the microscope is
equipped for the field number 22 and a
10x magnifying objective is in use, the diagonal of the field of view (via eyepieces
and tube that supports the given FN) is
22/10 = 2.2 mm. Using lower magnifying
objectives will enable a larger field of
view and using large field tubes and oculars can enlarge the field number to 26.5
(e.g. field number 26.5 and 4x objective
allows a diagonal of 26.5/4 = 6.624 mm),
but unfortunately low magnifying objec-
Box 4: Using hardware to increase the resolution in fluorescence microscopy
Several hardware components are available, to enable the acquisition of images that are not disturbed by out
of focus blur. For example, grid projection, confocal pinhole detection and TIRFM.
Since out of focus parts of a specimen produce blur in an image, there is the simple option of eliminating this
stray light from the image. With most confocal microscopes, a small pinhole is located in front of the sensor
and only those light beams that are originally from the focus area can pass through, others are simply absorbed. The resulting point image only contains information from the focus area. To create an image of more
then just one point of the focus area, a scanning process is needed.
This scanning process can be performed with the help of spinning disks for an ordinary fluorescence microscope or by a scanning process with a laser beam in a confocal laser scanning microscope (cLSM) set-up.
Each system produces different levels of improvement to the resolution. However, all of them need a professional digital sensor system to display the images.
TIRFM (Total Internal Reflection Fluorescent Microscopy) uses
a completely different concept. With this method, a very thin
layer of the specimen (around 200 nm) is used to create the
image. Therefore, TIRFM is ideal to analyse e.g. single molecule
interactions or membrane processes. To achieve this target, a
light beam is directed within a critical angle towards the cover
slip.
Because of the higher refractive index of the cover slip compared to the specimen, total internal reflection occurs. This
means that almost no direct light enters the specimen – but
due to the physics of light a so called evanescent wave travels in the specimen direction. This wave is only
strong enough to excite fluorochromes within the first few hundred nanometres close to the cover slip. The
fluorescent image is restricted to this small depth and cannot be driven into deeper areas of the specimen,
and also does not contain out of focus blur from deeper areas. (More information about TIRF can be found at
www.olympusmicro.com/primer/techniques/fluorescence/tirf/tirfhome.html).
Basics of light Microscopy & Imaging • 17
Box 5: Changing objectives but keeping the digital live image in focus –
Fig. 18: Line pair
pattern achieved
with 2 pixels per
line pair. Please
refer to the text
for the description.
Parfocal alignment.
High class objectives are designed to be parfocal. That means even when you change from a 4x magnifying objective to a 40x objective – the structure under observation remains in focus. This is especially a need for imaging with automated microscopy. To use this feature at the microscope the following short guideline will help:
We assume that the microscope in use is in proper Köhler illumination setting, and the digital camera is connected via a camera adapter (c-mount) that allows focus alignment.
(Some adapters are designed with a focusing screw; some can be fixed with screws in different distance positions.)
Fig. 19: Line pair
pattern resolved
with 3 pixels per
line pair. Please
refer to the text
for the description.
tives can not reach the same maximum
NA as high magnifying objectives. Even
when we use the best resolving lens for a
4x objective, the N.A of this Apochromat
(0.16) is still lower then the lowest resolving 20x Achromat with a N.A. of 0.35.
Having a lower NA produces a lower resolution. In a number of applications, a
field of view of several millimetres and a
resolution on the micrometer or nanometre scale is required simultaneously
(fig. 20). How can we overcome this problem, especially when we recognise that
the CCD sensor of the camera reduces
1. Use a high magnifying objective (40x or more) to get well recognisable detail of a specimen into focus
(camera live image), with the help of the course and fine focus of the microscope frame.
2. Change to a low magnifying objective (e.g. 4x), and do not change the course or fine focus at the microscope, but, align the focus of the camera adapter until the camera live image shows a clear in focus image.
3. Changing back to high magnification – the image is still in focus – you do not believe? Have a try.
the field of view (monitor image) once
more? Some image processing can offer
an alternative. In the first step, these systems automatically acquire individual
images at the predefined high resolution.
In the next step the software performs
intelligent pattern recognition together
with a plausibility check on the overlapping parts of the individual image sections to align them all in one image with
excellent accuracy (better than a pixel).
The computed result shows one combined image, maintaining the original
resolution. So you get an image which
has both high resolution and a large field
of view (fig. 21). Please check box 3 for
the description of the image processing
procedure. Additionally, box 7 describes
its sophisticated and extended follower
called Digital Virtual Microscopy.
Physical limits and methods to
­overcome them
As described before, the resolution of a
microscope objective is defined as the
smallest distance between two points on
a specimen that can still be distinguished
as two separate entities. But several limitations influence the resolution. In the
following we cover influence of image
blurring and resolution depth capacity.
Convolution and deconvolution
© guidolüönd
Stray light from out of focus areas above
or below the focal plane (e.g. in fluorescence microscopy) causes glare, distortion and blurriness within the acquisition
(fig. 22.a). These image artefacts are
known as convolution and they limit
one’s ability to assess images quickly, as
well as make more extensive evaluations.
There are several sometimes sophisticated hardware approaches in use which
allow reducing or even avoiding out of
focus blur at the first place when acquiring the images. Please see the description in box 4. A different approach is the
so-called deconvolution which is a recognised mathematical method for eliminating these image artefacts after image ac-
Fig 20: Mr. Guido Lüönd, Rieter AG, Department
Werkstoff-Technik DTTAM, Winterthur, Switzerland, works on fibres using a microscope. For
their investigations they often have to glue the
single images into an overview image. Previously this was a considerable challenge. Now a
piece of software takes over his job.
18 • Basics of light Microscopy & ImagingThe Resolving Power
quisition. It is called deconvolution. If the
point spread function (PSF) is known, it
is possible to deconvolute the image. This
means the convolution is mathematically
reversed, resulting in a reconstruction of
the original object. The resulting image
is much sharper, with less noise and at
higher resolution (fig. 22.b).
What exactly is the point spread
­function?
The point spread function is the image of
a point source of light from the specimen
projected by the microscope objective
onto the intermediate image plane, i.e.
the point spread function is represented
by the Airy disk pattern (fig. 15, 16).
Mathematically, the point spread function is the Fourier transform of the optical transfer function (OTF), which is in
general a measurement of the microscope’s ability to transfer contrast from
the specimen to the intermediate image
plane at a specific resolution. PSF (or
OTF) of an individual objective or a lens
system depends on numerical aperture,
objective design, illumination wavelength, and the contrast mode (e.g.
brightfield, phase, DIC).
The three-dimensional point spread function
The microscope imaging system spreads
the image of a point source of light from
the specimen not only in two dimensions,
but the point appears widened into a
three-dimensional contour. Thus, more
generally speaking, the PSF of a system
is the three dimensional diffraction pattern generated by an ideal point source
of light. The three-dimensional shapes of
Fig. 21: The analysis of non-metallic inclusions
according to DIN, ASTM and JIS requires the
processing of areas up to 1000 mm2 while the
sulphide and oxide inclusions to be analysed are
themselves are less than micrometres wide. No
camera is available offering such a large CCD
sensor. The image processing solution stitches
single overlapped images to give one high
resolution image together.
The Resolving Power a)
b)
Fig. 22: Via deconvolution artefacts can be computed out of fluorescence images. a) These artefacts
are caused by the stray light from non-focused areas above and below the focus level. These phenomena, referred to as convolution, result in glare, distortion and blurriness. b) Deconvolution is a recognised mathematical procedure for eliminating such artefacts. The resulting image displayed is sharper
with less noise and thus at higher resolution. This is also advantageous for more extensive analyses.
the PSF create the so-called out-of-focus
blur, which reduces the resolution and
contrast in images, e.g. in fluorescence
microscopy. This blurring or haze comes
from sections within the specimen which
are outside of the focal plane of the actual image. So, an image from any focal
plane of the specimen contains blurred
light from points located in that plane
mixed together with blurred light from
points originating in other focal planes
(fig. 23).
What is deconvolution used for?
When the PSF of a system is known, it
can be used to remove the blurring
present in the images. This is what the
so-called deconvolution does: It is an image processing technique for removing
out-of-focus blur from images. The deconvolution algorithm works on a stack
of images, which are optical sections
through the specimen and are recorded
along the z-axis of the microscope. The
algorithm calculates the three-dimensional PSF of the system and reverses the
blurring present in the image. In this respect, deconvolution attempts to reconstruct the specimen from the blurred image (fig. 23).
Depth of focus versus depth of field
Two terms – depth of focus and depth of
field – are often used to describe the
same optical performance, the amount of
specimen depth structures that can be
seen in focus at the same time. However,
Basics of light Microscopy & Imaging • 19
Fig. 23: Stray light originating from areas above and below the focal plane results in glare, distortion
and blurriness (convolution) especially in fluorescence microscopy and histology. Deconvolution is a
recognised mathematical method for correcting these artefacts. The degree to which an image is distorted is described by what is known as the point spread function (PSF). Once this is known, it becomes
possible to “deconvolute” the image. This means that the convolution of the image is mathematically
reversed and the original contours of the specimen are reconstructed. The greater the precision with
which the degree of distortion – i.e., PSF – is known, the better the result. What is this result? A
sharper and noise-free version of the image at higher resolution with enhanced image quality.
only the term “Depth of Field” should be
used for this feature. As we will point out
later the term “Depth of Focus” is needed
for a different optical feature.
An optical system, such as the eye,
which focuses light, will generally produce a clear image at a particular distance from the optical components. In a
camera, the ideal situation is where a
clear image is formed on the chip, and in
the eye it is where a clear image is
formed on the retina. For the eye, this
happens when the length of the eye
matches its optical power, and if a distant object is in focus when the eye is relaxed. If there is a difference between
the power and length in such a situation,
then the image that is formed on the retina will be very slightly out of focus. However, such a discrepancy may be small
enough that it is not noticed, and thus
there is a small amount of “slop” in the
system such that a range of focus is considered to be acceptable. This range is
termed the “depth of focus” of the eye.
Looking at it the other way round, the
eye might be precisely in focus for a particular distance – for example an object
one metre away. However, because of the
slop in the system, other objects 90 cm
and 110 cm away may also be seen
clearly. In front of, and behind, the precise focal distance there is a range where
vision is clear, and this is termed the
“depth of field”.
Now look at the physical understanding of depth of focus in microscopy and
the depth of field, respectively. Depth of
field Δfi in a microscope is the area in
front of and behind the specimen that
will be in acceptable focus. It can be defined by the distance from the nearest
object plane in focus to that of the farthest plane also simultaneously in focus.
This value describes the range of distance along the optical axis in which the
specimen can move without the image
appearing to lose sharpness. Mathematically Δfi is directly proportional to
Δfi ~ λ/2*NA2 (5)
Δfi = depth of field
λ = wavelength of light (emission)
NA = numerical aperture
Δfi obviously depends on the resolution
of the microscope. Large lenses with
short focal length and high magnifications will have a very short depth of field.
Small lenses with long focal length and
low magnifications will be much better.
Depth of focus Δfo in a microscope is the
distance above and below the image
plane over which the image appears in
focus. It’s the extent of the region around
the image plane in which the image will
appear to be sharp. Δfo is directly proportional to
Δfo ~ M2/NA
Δfo = depth of focus
M
= magnification
NA = numerical aperture
(6)
Δfo refers to the image space and depends strongly on the magnification M,
but also on changes in numerical aperture NA.
As a take home message – high resolution will create relative low depth of
field, high magnifications will create a
higher depth of focus – this is also why
the procedure for parfocal alignment will
work.
Parfocality
Fig. 24: The software extracts the focused areas from the component images of an image series and
reassembles them into one infinitely sharp image. The example shown here is the resulting image
computed automatically using 25 images of a Bembidion tetracolum sample.
There is a well-known difficulty in conventional light microscopy which refers
to the limited depth of focus: If you focus
20 • Basics of light Microscopy & ImagingThe Resolving Power
Box 6: Extended Focal Imaging (efi)
Box 7: Virtual microscopy
A lack of depth of field in microscope images is an old and familiar problem. The microscope’s own depth of
field is only capable of focusing a limited height range at the same time. The remaining parts of the image are
then blurry. Electronic image processing points the way out of this dead-end street.
For a motorised microscope equipped with a motorised stage the whole process can be automated. The software takes control of the microscope, camera, motor stage, etc. All parameters are transferred to the imaging
system through a remote interface. Using these data, the entire microscope and camera set-up can be controlled
and calibrated by the software. After having defined the total number of images, and the maximum and minimum height of the stage, the user can execute the following steps automatically with a single mouse click.
1. In the first step the user defines the number of images in the focus series. Using a microscope with a motor stage the user has to define the maximum and the minimum lift of the microscope stage as well as the
total number of individual images he intends to acquire.
2. The defined image series at varying focus levels is acquired.
3. Next the composite image of pixel-by-pixel precision will be generated from these images. This is done by
extracting the respective focused image areas of each separate image and assembling these into a focused composite image. Most of these solutions take into consideration the typical (due to construction)
shift of the optical axis that occurs when focusing stereoscopic microscopes.
4. The “Extended focal image” with practically limitless depth of field is computed.
5. Now the user is able to generate a height map which permits users to reconstruct three-dimensional
views e.g. to measure height differences.
an image with one objective, e.g. 4x, it
might be out of focus when you change to
a another magnification, e.g. 40x. The
term parfocality describes the situation
when the structures remain in focus.
Parfocality is a characteristic of objectives. Please read box 5 how to assure
parfocality with high class objectives.
Fig. 25: This image shows how far money can go:
This Singaporean coin detail was captured using a
ColorView digital CCD camera and processed
using the realignment, extended focus and 3-D
imaging software modules. Capture and automatically align multiple component images into a
high-resolution composite using the MIA module.
Extract the sharpest details within the component
images and reassemble into one single image
having infinite depth of focus with the module
EFI. Finally, create incredibly realistic views using
height and texture information attained through
the perspective functions of the 3D module.
The Resolving Power Automated sharp images
Let us come back to restrictions arising
from the limited depth of field. The better
the lateral resolution, the smaller your
depth of field will be. The problem is in
fact physical, and it cannot be circumvented by any adjustments to the optical
system. Today’s standard light microscopes allow objects to be viewed with a
maximum magnification of about 1000x.
The depth of field is then reduced to
about 1 µm. Only in this area the specimen is perfectly imaged. This physical
limitation of inadequate depth of field is a
familiar problem in microscope acquisitions. In the metal-processing industry,
when analysing or evaluating two-dimensional metallographical objects such as,
e.g., a section sample, a section press is
generally used in order to obtain an exact
orthogonal alignment in relation to the
optical axis of the microscope. The sample to be analysed can then be acquired
totally focused in a single acquisition.
However, objects which have a distinctly three-dimensional structure or for
investigations of, e.g., 3-D wear, at
greater magnifications no satisfactory
overview image is obtainable via stereo
or reflected-light microscope. The microscope can only be focused onto limited
areas of the object. Due to the limited
depth of field, it is actually impossible to
obtain a sharp acquisition of the entire
image field. These physical restrictions
can only be transcended via digital image analysis. The normally wholly-binding laws of physics are “side-stepped”.
Light microscopy is one of the classic imaging
techniques used in medical education and for routine procedures of pathology, histology, physiology and embryology. Pathology makes use of microscopes for diagnostic investigation of tissue
samples to determine abnormal changes. Digitisation has resulted in significant progress in the
field of microscopy. However, digital technology
up until now has presented some decisive limitations. One problem is that the field of view of any
camera is limited for any given magnification. It is
usually not possible to have a complete overview
of the tissue specimen with just one image at a
resolution that allows further analysis. Digital virtual microscopy moves beyond this barrier.
Virtual microscopy is the digital equivalent to conventional microscopy. Instead of viewing a specimen through the eyepiece of the microscope and
evaluating it, a virtual image of the entire slide (a
’virtual slide‘) with perfect image quality is displayed on the monitor. The individual system components (microscope, motor stage, PC, software)
are all optimally inter-coordinated and offer speed,
precision and reliability of use. The Olympus solution .slide scans the entire slide at the resolution
required. Integrated focus routines make sure the
image is always in sharp focus. The single images
acquired are automatically stitched together into
a large montage (the ‘virtual slide‘). The entire
’virtual slide’ can be viewed onscreen. Detail image segments can be selected and zoomed in or
out, the equivalent to working with an actual
glass slide under the microscope with the same
efficiency. With an internet connection, this procedure can be done from anywhere in the world.
In addition, users have all the advantages of digital image processing at their fingertips, including
structured web archiving of images, analysis results and documentation.
What happens is that an image series is
acquired at varying focus levels. Then
special software algorithms are applied
to all the images of the image series and
distinguish between sharply-focused and
unfocused image segments in each image. The sharp image segments of the
images are then pieced back together to
form a single totally-focused image of the
entire sample (fig. 24). Furthermore,
measuring height differences and generating three-dimensional images becomes
feasible (fig. 25). For further detail please
see box 6.
Basics of light Microscopy & Imaging • 21
Contrast and Microscopy
22 • Basics of light Microscopy & ImagingContrast and Microscopy
All cats are grey in the dark
Why are cats grey in the dark? Here, the
term contrast comes into play. Contrast
refers to the difference of intensities or
colours within an image. Details within
an image need intensity or colour differences to be recognised from the adjacent
surroundings and overall background.
Let us first restrict our view to a greyscale level image, like image fig. 26. While
watching the image try to estimate the
number of grey scales that you can distinguish – and keep that number in
mind.
Let us now compare this number with
another example – you enter an office
shop in order to buy a selection of card
samples in different shades of grey. All of
the cards fall down by accident and are
scattered over the floor. Now you have to
replace them in the correct grey level order – how much can you differentiate
now?
Surprisingly, we are only able to differentiate approximately 50–60 grey levels – that means that already the 8 bit
image on your monitor with 256 grey
scales offers greater possible differentiation than we can discriminate with our
own eyes. The card samples in various
shades of grey need to a have an approximate difference in contrast level of about
2 % in order for us to recognise them as
different. However, if we look at the
number of image areas (pixels) that represent a discrete intensity (grey level or
pixel intensity) within an intensity distribution of the image we can understand
and handle contrast and brightness variations more easily. Intensity or grey level
distributions are referred to as histograms, see box 8 for further explanation.
These histograms allow us to optimise
camera and microscope settings so that
all the intensity values that are available
within the specimen are acquired
(fig. 27). If we do not study the available
intensity values at the initial stage before
image acquisition, they are archived and
can not be visualised in additional
processing steps.
The familiar view – brightfield contrast
In brightfield transmitted microscopy the
contrast of the specimen is mainly produced by the different absorption levels
of light, either due to staining or by pigments that are specimen inherent (amplitude objects). With a histological specimen for example, the staining procedure
itself can vary the contrast levels that are
available for imaging (fig. 27). Nevertheless, the choice of appropriate optical
equipment and correct illumination settings is vital for the best contrast.
Fig. 26: Vitamin C crystals observed in polarised
light.
During our explanation about Köhler
alignment (see box 2) we described that
at the end of this procedure the aperture
stop should be closed to approximately
80 % of the numerical aperture (NA) of
the objective. This setting is to achieve
the best contrast setting for our eyes.
Further reduction of the aperture stop
will introduce an artificial appearance
and low resolution to the image.
For documentation purposes however,
the aperture stop can be set to the same
level as the NA of the objective – because
the camera sensors in use are capable to
handle much more contrast levels than
our eyes. For specimens lacking natural
differences in internal absorption of light,
like living cells (phase objects; fig. 28) or
Fig. 27: Histological staining. Cells obtained after a transbronchial needle application. Image and
histogram show that the overall intensity and contrast have been optimised.
Contrast and Microscopy Basics of light Microscopy & Imaging • 23
Box 8: Histogram optimisation during acquisition
An intensity histogram depicts the intensity distribution of the pixels in an image. The intensity values are
plotted along the x axis and range from 0–255 in an 8-bit greyscale image or 24-bit (3x8bit) colour image.
The number of pixels per intensity value is displayed along the y axis.
The intensity histogram provides the means to monitor general characteristics of a digital image like its overall intensity, its contrast, the dynamic range used, possible saturation, the sample’s phases etc. In this respect
the histogram gives a more objective criterion to estimate the quality of an image than just viewing it (which
is somewhat subjective). When displayed in the live mode during image acquisition, the histogram allows optimising and fine tuning of the acquisition modes and parameters on the microscope as well as the camera.
These include microscope alignment, contrast method, microscope settings, or current camera exposure time.
So the histogram helps to acquire a better image containing more image information.
Epithel cells viewed with phase contrast: left side with low contrast setting, right sight with
contrast optimisation of microscope and camera setting.
Left figure is obviously lit correctly but has poor contrast. The corresponding histogram mirrors this: the peak
is roughly located in the middle of the intensity range (x-axis), so the camera’s exposure time is correctly set.
But all pixels are squashed in the middle range between about 75 and 200. Thus, only about half of the dynamic range of the camera system is used. Aligning the microscope better (e.g., light intensity) and adapting
the camera exposure time increases the image contrast (right figure). It also spreads the intensity distribution
over the whole dynamic range without reducing information by saturation (see the corresponding histogram).
Here, more detailed image structures become visible.
General rule
The overall contrast is usually best when the intensity histogram covers the whole dynamic range of the system; but one should usually avoid creating overflow or saturation at the right side of the histogram, i.e. white
pixels.
It is also possible to use the histogram to improve the image contrast afterwards. However, you can not increase the image content; you can only improve the image display.
reflected microscopy specimens without
significant three dimensional relief structures, the acquired image has flat contrast. To better visualise the existing image features (fig. 28, original image),
subsequent digital contrast optimisation
procedures may be applied (fig. 28, improved image). But to visualise more de-
tails in those specimens optical contrast
methods must be used.
Like stars in the sky – Darkfield Contrast
Dust in the air is easily visible when a
light beam is travelling through the air in
a darkened room. The visibility is only
achieved because the dust particles diffract and/or reflect the light and this light
is now travelling in all directions.
Therefore we can see light originating
from the particle in front of a dark background even when the particle itself is
too small to be resolved or does not show
an appropriate contrast under daylight
conditions. This phenomenon is also used
in darkfield (or dark ground) microscopy.
Light is directed to the specimen in a way
that no direct light enters the objective. If
there is no light scattering particle the
image is dark, if there is something that
diffracts or reflects the light, those scattered beams can enter the objective and
are visible as bright white structures on
a black background (fig. 29).
(See also: http://www.olympusmicro.com/
primer/techniques/darkfieldindex.html)
Transmitted darkfield
For transmitted microscopy including
stereo microscopy, this contrast method
is especially used to visualise scattering
objects like small fresh water micro-organisms or diatoms and fibres (fig. 30).
Almost all upright microscopes can be
easily equipped for darkfield illumination.
The easiest way to achieve simple
darkfield is a central light stop insert for
the condenser. For better illumination or
even high resolving darkfield, special
darkfield condensers are required
(fig. 29).
To ensure that no direct light is entering the objective, the numerical aperture
(NA) of the condenser has to be about
15 % higher than the NA of the objective.
This is in contradiction to all other contrast methods where the objective has a
higher or equal NA than the condenser.
Remember the NA is a number that describes the angle of the light cone a condenser or objective is using. An objective
with high NA like the apochromate is
characterised by a high angle of its light
cone and therefore it may be possible that
some of the direct illumination will also
enter the objective. This would destroy
the darkfield contrast immediately. For
that reason objectives with high NA are
Fig. 28: Brightfield image of
living mouth epithelial cells on a
slide before and after digital
contrast optimisation of the
archived image and corresponding histograms. See the section
“Making it look better“, for
further explanation.
24 • Basics of light Microscopy & ImagingContrast and Microscopy
Fig. 29: Light path for darkfield compared to brightfield set up in transmitted and reflected illumination.
available, designed with an internal iris
diaphragm to reduce the NA to the appropriate amount for darkfield observation.
Reflected darkfield
Within the reflected microscopy applications the darkfield illumination is a very
common contrast technique. It allows the
visualisation of smallest scratches and
changes in height because of the circular
oblique illumination (fig. 31). To achieve
a reflected darkfield, the amount of special adaptations at the microscope is
more sophisticated. The central light stop
is located within a cube of the reflected
light attachment and the special bright-
Box 9: Alignment of a transmitted
darkfield condenser
1. Engage the 10x objective and bring the specimen into focus.
2. While looking through the eyepieces and using
the condenser height adjustment knob, carefully adjust the height of the condenser until a
dark circular spot becomes visible (Figure A).
3. Turn the condenser centring screws to move
the dark spot to the centre of field of view
­(Figure B). This completes the centration.
field/darkfield objective guides the illumination light in an outer ring to the
specimen. Only the scattered light from
the specimen runs in the normal central
part of the objective as image forming
light rays (fig. 29). Those objectives can
also be used for normal brightfield observation or other techniques like differential interference contrast (DIC) and/or
polarisation.
Creating destructive interference –
Phase Contrast
Light that is travelling through part of a
specimen and is not absorbed by amplitude objects will not produce a clearly
visible image. The intensity remains the
same, but the phase is changed compared to the light just travelling in the
surrounding areas. This phase shift of
about a quarter wavelength for a cultured cell is not visible to our eyes. Therefore, additional optical elements are
needed to convert this difference into an
intensity shift. These optical elements
create a contrast where un-deviated and
deviated light are ½ wavelength out of
phase, which results in destructive interference. This means that details of the
cell appear dark against a lighter background in positive phase contrast (see
figures in box 8). (See also: www.olympusmicro.com/primer/techniques/phasecontrast/phaseindex.html)
For phase contrast microscopy two elements are needed. One is a ring slit insert for the condenser, the other is special objectives that contain a phase plate.
Objectives for phase contrast are characterised by green lettering and an indication of the size of the phase ring like Ph1,
Ph2, Ph3 or PhC, PhL, PhP. Corresponding to these objective phase rings the appropriate inserts for the condenser have
to be used.
Both elements are located at the so
called back focal planes. They are visible
Box 10: Alignment of phase contrast
1. A microscope condenser with Köhler illumination has to be in Köhler positioning.
2. Remove one of the eyepieces and look into the empty eyepiece sleeve. When using a centring telescope at
the eyepiece sleeve (highly recommended and in some inverted microscopes already build in at the tube
e.g. Olympus U-BI90CT – binocular tube), bring the bright ring (condenser ring slit) and dark ring (objective phase plate) into focus by turning the eye lens focus.
3. Use the centring screws for the condenser inserts to centre the phase contrast ring, so that
the bright ring overlaps the dark ring within
the field of view (see figure).
4. Engage the desired objective. Using the condenser height adjustment knob, adjust until
the darkfield spot is eliminated and a good
darkfield image is obtained.
Contrast and Microscopy 4. Repeat these steps for each phase and contrast ring set.
5. Remove the centring telescope and replace it
with the eyepiece.
6. Widen the field iris diaphragm opening until the diaphragm image circumscribes the field of view.
Basics of light Microscopy & Imaging • 25
Fig. 30: Epithelial cells with transmitted darkfield contrast.
when an eyepiece is removed and can
then be aligned under optical control (for
better viewing a centring telescope
should be used).
Due to the optical principles of phase
contrast it allows good contrast in transmitted light when the living specimens
are unstained and thin. A specimen
should not be more than 10 µm thick. For
these specimens the dark contrast is
valid, whereas thicker details and overlaying structures produce a bright halo
ring. This halo-effect can be so strong
and superimposed that a clear analysis of
the underlying morphology becomes critical. Nevertheless, the artificial looking
image of thicker structures can be used
by expert eyes to determine how many
cells within an adherent cell culture are
undergoing mitosis or have entered cell
death pathways. Fig. 32 shows bright
Fig. 32: Astrocytes with phase contrast, note: cell
bodies are surrounded by halo rings.
Fig. 31: Wafer at reflected darkfield contrast.
halo rings, visible round the relatively
thick astrocyte cell bodies, whereas the
fine details show dark phase contrast.
When observing cells in a chamber
e.g. those of a 24 well plate, the cells
which are best for imaging may lie at the
border of the well. It is possible to see the
cells, but because the light path is
changed by the border of the well and
the adhesion effect of the medium, the
phase contrast is totally misaligned at
this position. We can realign the phase
rings for this specific specimen position
as described below, but will have to realign them again when the stage is moved
to other positions. Furthermore, the use
of the special Olympus PHC phase contrast inserts instead of the PH1 helps to
ensure better contrast in multi well
plates where meniscus problems are apparent.
Making it look better – for vision only
It is essential to select the optical contrast method appropriate to the sample
investigated and depending on the features which are to be made visible. You
can only build upon something which is
already there.
Having done the best possible job here
you can try to better visualise these features. It often makes sense to adjust the
image contrast digitally. Here you can either elevate the overall image contrast or
you can emphasise special structures.
Whatever you want to do, you should
again utilise the histogram to define exactly the steps for image improvement.
Fig. 28 shows a drastic example (left
side). The optical contrast is so poor that
the cells’ relief is hardly visible. The corresponding histogram shows that the
Fig. 33: Epithelial cells scratched with a spoon from the tongue and transferred
to a cover slip. A,B: DIC illumination at different focus plane; C: Oblique contrast;
D: Olympus Relief Contrast imaged with lower numerical aperture.
26 • Basics of light Microscopy & ImagingContrast and Microscopy
pixels are grouped in the centre of the
histogram indicating that only a small
fraction of the camera’s dynamic range
has been used (intensity values 165–186
out of 256). The image has only 21 different intensity values. The present features
can be made visible by stretching the histogram over the whole dynamic range
from 0–255. This operation does not
change the information content of the
image but lets us at least see what is
there (right side).
This stretching comes to its limits as
soon as there is a single black and a single white pixel in the image. In this case
a moderate cut of up to 3 % on the left
side (dark pixels) and on the right side
(bright pixels) doesn’t cause much information loss but increases the contrast of
the image’s main features in the central
intensity range. To selectively accentuate
features of a distinct intensity range (socalled phase), an individual transfer
function has to be defined which increases the phase’s contrast to the background or other phases. The intensity
and contrast image operations are usually performed after image acquisition.
Often, the camera control of the image
analysis software makes it possible to
have the contrast stretched image calculated and displayed in the live mode.
­Alternatively, the contrast can be set
manually on the live histogram while the
direct results can be monitored in the
live image. Whatever way the digital
­histogram is used, it is a powerful tool
to maintain image fidelity as well as
to create a clear picture of physical nature.
Light alone is not enough – Differential Interference Contrast (DIC)
If you have ever been skiing on a foggy
day, you will know that diffuse light
strongly reduces the ability to see differences in the height of the snow – and that
can cause major problems. An equally
distributed light source does not produce
clear shadows and this causes reduced
visibility of three dimensional structures.
Our human vision is triggered to see
three dimensions and is well trained to
interpret structures if they are illuminated more or less from one point. The
resulting dark and bright areas at the
surface of a structure allow us to easily
recognise and identify them. Using our
experience, we get information of height
and distance. Therefore, a contrast
method that displays differences in a
structure as a pattern of bright and dark
areas is something that looks very familiar to us and seems to be easy to interContrast and Microscopy Box 11: How to set up the transmitted DIC microscope (e.g. Olympus type)
J Use proper Köhler positioning of the microscope with a specimen in focus (best with 10x objective).
J Insert polariser (condenser side) and analyser (objective side) into the light path.
J Remove specimen out of the light path.
J Turn the polariser until the image gets to its darkest view (Cross-Nicol position).
J Insert the DIC prism at the condenser corresponding to the objective and slider type in use, immediately
the image is bright again.
J Insert the DIC slider at the objective side. By rotating the fine adjustment knob at this slider the amount of
contrast of the DIC can be varied.
J The best contrast is achieved when the background shows a grey colour and the specimen is clearly
pseudo three dimensional.
Another Way to Achieve this Setting Is the Following:
J Use proper Köhler positioning of the microscope with a specimen in focus (best with 10x objective).
J Insert polariser (condenser side) and analyser (objective side) and the prism slider into the light path.
J Remove specimen out of the light path.
J Remove one eyepiece and if available look through a centring telescope, rotate the slider
adjustment knob until a black interference line is visible (box 11- Figure ).
J Rotate the polariser until the black line becomes darkest.
J Insert the DIC prism at the condenser and insert the eyepiece for normal observation.
J Fine adjustment of DIC-slider can be done for selecting the amount of interference contrast.
pret. Structures within a specimen can
be identified and even though they are
only two dimensionally displayed they
look three dimensional.
Real three dimensional images can
only be observed at a stereo microscope
where two light paths of two combined
microscopes are used, sending the image
to our eyes at a slightly different angle.
But this will be a topic to be described
later.
Simple from One Side
The simplest way to achieve a contrast
method that results in a relief-like image
is by using oblique illumination: however,
the theory of oblique illumination is more
complex. Details can be found at: www.olympusmicro.com/primer/ techniques/oblique/obliquehome.html. Special condensers are available to handle this technique
on upright microscopes with a lot of comfort. Oblique illumination is also often used
on stereo microscopes to enhance the contrast of a 3D surface simply by illuminating the specimen from one side. For reflected light, this can be done with flexible
cold light fibres, or with LED ring light systems that offer reproducible illumination
of segments that can even be rotated.
Fig. 34: Simple principle of Nomarski DIC microscopy.
Basics of light Microscopy & Imaging • 27
Fig. 35: Specimens imaged with different shearing values. Left side shows thin NG108 cells imaged
with the high contrast prism set, the middle image shows a thick diatom specimen imaged with the
high resolution prism set and the right image shows PtK2 cells imaged with the general prism set.
In transmitted microscopy the oblique
condenser (e.g. the Olympus oblique condenser WI-OBCD) has an adjustable slit
that can be rotated. After Köhler alignment of the microscope this slit is inserted in the light path and results in illumination from one side so that
specimens that vary in thickness and
density are contrasted. Rotating the slit
enables us to highlight structures from
every side. The contrast itself is produced
by the complete thickness of the specimen and the resolution of the image is
limited due to the oblique illumination
(fig. 33c).
To overcome the limitations of oblique
contrast, Nomarski Differential Interference Contrast (DIC) is commonly used for
high resolving images. The benefit of this
method is that the relief like image is
only contrasted at the focus area (depth
of field). The user can optically section a
thicker specimen by changing the focus
level.
As shown in fig. 33 the contrasted focus layer can be restricted to the layer of
the somata (fig. 33a) or the superficial
part of these cells (fig. 33b). In addition,
using infrared light (mostly used around
700 or 900 nm) instead of white light,
this technique allows a very deep look of
more than 100 µm into thick sections,
which is often used in neurobiological research. Nomarski DIC creates an ampli-
fied contrast of phase differences which
occurs when light passes through material with different refractive indices. Detailed information about the theory and
use of the DIC method can be found at
www.olympusmicro.com/primer/techniques/dic/dichome.html. Here we will
concentrate on the basic introduction
and the practical use.
Fig. 36: Alignment of Olympus Relief Contrast or
Hoffmann Modulation condenser insert, grey
and dark areas are located within the objective
and areas A and B are located as an insert within
the condenser and can be aligned accordingly.
To achieve transmitted Nomarski DIC
images, four optical elements are needed:
a polariser, two prisms and an analyser.
For the reflected DIC setup only one DIC
prism (the slider) is required.
Let us have a closer look at transmitted DIC. The wave vibration direction of
light is unified by a polariser located between the light source and condenser
Fig. 37: Transmitted polarising microscopy; variation of melted chemicals viewed with crossed polariser. Images courtesy of Norbert Junker, Olympus Europa GmbH, Hamburg, Germany.
28 • Basics of light Microscopy & ImagingContrast and Microscopy
(fig. 34). In the condenser, a special insert – a Wollaston prism (matching the
magnification of the objective) – divides
every light ray into two, called the ordinary and the extraordinary, which vibrate at a 90 degree angle to each other.
Both light rays then travel a small distance apart, the so called shearing distance. At the specimen, the ray that is
passing through e.g. a cell part is delayed
compared to the one passing through the
surrounding medium. This result in a
phase shift of both rays, which are recombined with the help of a second prism
located at the objective revolver. Only
those combined rays with a phase shift,
interfere in a way that they contain vibration planes that pass through the analyser. To create an easily observable
pseudo 3D image, a prism can be moved
in and out to enhance the phase shift between ordinary and extraordinary ray. At
a mid position the background should be
dark and the specimens should be visible
as if illuminated from the equivalent
North-West and South-West simultaneously. When the slider is screwed more to
one direction the typical three dimensional view of the specimen will come up,
either having the sun in North-West or
East-South location. This will only work
if no depolarising material (e.g. plastic) is
used within the light path. Therefore, a
real high resolving DIC method can not
be used with plastic Petri dishes or multi
well plates. In those cases other methods
like Hoffmann Modulation Contrast or
the analogous Olympus Relief Contrast
are commonly used.
The DIC method does not require special objectives, but the prism has to match
the objective used. Suitable prisms are
available for most fluorite and apochromat objectives. DIC contrast can be offered with shearing values for every optimisation, a high contrast set-up that is
best for very thin specimens (higher
shearing value, fig. 35 left), a standard
prism slider for general use (fig. 35 right)
and a high resolution slider for thick
specimens (lower shearing value, fig. 35
mid) like C. elegans or zebra fish embryos.
Different names but the same principle:
Hoffman Modulation Contrast – Olympus
Relief Contrast
Whenever high resolution is needed and
plastic dishes are used, the Hoffman
Modulation or Olympus relief contrast
method are common techniques for inverted transmission light microscopes
e.g. for in vitro techniques (fig. 33d).
In principle it combines the oblique illumination of a specimen where the reContrast and Microscopy Fig. 38: Brightfield image of a wafer. Left side: The ring shaped background pattern is clearly visible.
Right side: The same image with real-time shading correction.
Box 12: How to describe image processing operations mathematically
From the mathematical point of view, an image processing function is a transfer function which is applied to
each pixel of the original image individually and generates a new pixel value (intensity/colour) in the resulting
imaging. The transfer functions can be roughly divided into point operations, local operations and global operations.
J Point Operations: The resulting pixel is only dependent on the intensity or colour of the original pixel, but
it is independent from the pixel places (x/y) and the pixel intensities/colours of the neighbouring pixels.
Point operations can be monitored, defined and executed via the histogram. All intensity and contrast operations are point operations. To be exact, point operations are not filters in the more specific sense of the
word.
J Local Operations: These look not only at
the pixel itself but also at its neighbouring pixels to calculate the new pixel
value (intensity / colour). For example,
convolution filters are local operations.
Many well known noise reduction,
sharpening and edge enhancing filters
are convolution filters. Here, the filter’s
transfer function can be described as a
a)
b)
matrix consisting of whole positive or negative numbers, known as weight factors. Any original pixel is situated at the centre of the matrix. The matrix is applied to the original pixel and its neighbours to calculate
the resulting pixel intensity. See box 12 – Figure 1a as illustration. The 3x3 matrix has nine weight factors
W1 – W9. Each weight factor is multiplied with the respective intensities I1 – I9 of the original image to
calculate the intensity of the central pixel I5 after filtering. See a concrete numerical example in box 12 –
Figure 1 b. In addition to this calculation, the resulting intensity value can be divided by a normalisation
factor and added to an offset value in order to ensure that the filter operation does not alter the average
image brightness. The matrix weight factors determine the function and the strength of the filter. For example, a 3x3 matrix with all weight factors being one makes a simple average filter used for noise reduction. Sharpen filters consist of matrices having a positive central weight factor surrounded by negative
weight factors. The unsymmetrical matrix in fig. 43b creates a pseudo topographical contrast in the resulting image.
J Global operations: All pixels of the original image with regard to their intensity / colour and position (x/y)
are used to calculate the resulting pixel. Lowpass and bandpass noise reduction filters are examples. In
general, all Fourier Transform filters are global operations.
Basics of light Microscopy & Imaging • 29
fraction differences at various parts of
the specimen shape are used for contrast
enhancement. It allows semi-transparent
specimens structures to be analysed in a
manner difficult to achieve using bright
field microscopy. (See also
www.olympusmicro.com/primer/techniques/ hoffmanindex.html).
For this contrast technique a special
condenser and objective are needed. In
advanced systems the condenser is
equipped with a polariser and a condenser slit plate (according to the objective in use, fig. 36 A and B area). The
achromatic or fluorite objective contains
a modulator insert at one side of the back
focal plane (fig. 36 dark and grey area).
The slit plate at the condenser has to be
aligned according to the modulator
within the objective (fig. 36). This can be
done by visualising these elements via a
focused centring telescope (remove one
eyepiece and insert the telescope). The
resulting contrast can be varied by rotating the polariser at the condenser.
The interpretation of DIC and relief
contrast images is not intuitive. These
techniques contrast different refractive
indices within a specimen into a pseudothree-dimensional image. This means
that specimen details which look like
holes or hills on the surface of a structure (see fig. 35 left and right side) may
simply be areas of different refraction index but not necessarily different in
height.
Fig. 39: Fluorescence image of stem cells. The detail
zoom better reveals the noise level.
Get more then expected – Polarisation
DIC equipment on a microscope allows
the user to employ a totally different microscopic method, simple polarisation. In
this case no prisms are used and only the
first settings of aligning polariser and analyser in crossed positioning (see box 11)
are needed. This setting will generate a
dark image because the vibration direction of light that is travelling through the
specimen is exactly the vibration direction that is totally blocked by the analyser. However, if the specimen contains
material that is able to turn the light,
some light can pass the analyser and is
observed as a bright detail on a dark
background. Examples of such anisotropic materials are crystalline Vitamin
C, emulsions of butter, skeletal muscles,
urate crystals (Gout inspection), amyloid,
rocks and minerals, as well as metal surfaces or the DIC prism itself. The list is
very long and often the combination of
simple polarising microscopy and DIC
can offer a more detailed analysis of the
specimen, simply by using equipment
that is already available. Beside all the
analytical advantages, polarising microscopy also offers images of high aesthetical value (fig. 37).
Revealing structures with imaging
filter techniques
We are now at the point where the suitable microscope contrast method has
been selected and the optimum camera
settings have been made. So the field has
been tilled and digital filter and image
processing techniques come into play.
This is explained in more detail below.
Along a thin line
Image processing can turn the photograph of an ordinary face into an unbelievable beauty or create images that
have no counterpart in the real world.
Knowing this, digital image processing is
often compared with manipulation of results. It might be true for many microscopists that they enjoy the amazing
beauty of hidden nature as being mirrored in the images they acquire, but the
focus of the daily work lies upon the actual information within the image. Yet
this information can be superimposed by
artefacts which might be created by
specimen preparation, illumination conditions, camera settings or display parameters. At this point, filter techniques
come into their own right.
Depending on the reason for imaging,
a wide variety of processing steps can be
applied to reveal new information or to
enhance image clarity for the details that
are under observation. What kind of artefacts may appear in a digital image?
Imperfect digital images may look weak
in contrast, unevenly illuminated,
wrongly coloured, diffuse, blurred, noisy,
dirty etc. These distortions might make
the image look unprofessional. But what
is more, they might make any automatic
measurement routine based on threshold
values or edge detection difficult and
sometimes even impossible to apply.
30 • Basics of light Microscopy & ImagingContrast and Microscopy
Where there is light, there is also
shade
Fig. 40: The same image as in fig. 39 after application
of the Sigma noise reduction filter. The detail zoom
makes the improvement better visible.
Many of these artefacts can be reduced
by using a suitable digital filtering technique. Often there exist several different
filters to improve the image quality. It is
not always easy to find the best method
and parameter settings to suppress the
image defects without disturbing the
“real” image information. This is the
highest standard which the optimum filter should meet: removing the image artefacts and keeping the specimen structures in the image. This goal cannot
always be accomplished. When doing image processing, there is often a thin line
between what needs to be done and what
would be good to accomplish. In addition, there is a thin line between scientific virtue and creative composition!
Now or later – both are possible
Digital image processing is usually applied after image acquisition and image
storage. But the latest image processing
systems support “Live Digital Image
Contrast and Microscopy Processing”, which means that numerous real-time functions can be executed
during image acquisition. These include
the online histogram, which is used to
monitor the image brightness and contrast. Additionally, the histogram function itself can be used to have the image
contrast improved automatically or to set
it in the live mode by hand. (How to use
the histogram to improve image contrast
see box 8.) Another real-time function is
the white balance, which corrects colour
shifts at the moment of acquisition. Nonuniform specimen illumination can also
be immediately corrected in the live image using the online shading correction.
This is described below. The operations
of contrast enhancement, white balance
and shading correction can be applied
during or after image acquisition. They
are point operations, mathematically
speaking (refer to box 12). This makes
them suitable for reducing image artefacts without distorting the image content itself.
One major source of distortion in a light
microscopic image is brightness fluctuation, which may arise from out-of-axis illumination conditions or when the microscope is not aligned optimally. Uneven
illumination artefacts may also appear
under regular observation conditions using low objective magnification and/or in
combination with low magnifying camera adapters. Then the shading artefacts
are manifested as a ring shaped shade
structure which becomes even darker
near the edges of the image (fig. 38 left
side). The minor shading within the background needs to be corrected. The operation applied here is called background
correction, flat-field correction or shading correction. (A simple tool for background correction can be downloaded at
www.mic-d.com.)
The best method is to record a background image and a dark image in addition to the “raw” specimen image. The
background illumination profile is simulated in the background image, whereas
the noise level of the camera system is
apparent in the dark image. The advantage here is that these images are gathered independently from the specimen
image.
The background image is usually obtained by removing the specimen and
leaving an area of mounting medium and
cover slip in place. If this is not possible,
the same result can also be achieved by
leaving the specimen in place but to defocus the microscope. It is important that
the background image does not show any
debris. The dark image is acquired by
closing the camera shutter. It should use
the same exposure time as the specimen
image.
The shading correction algorithm first
subtracts the dark image (d) from the
specimen image (s) as well as from the
background image (b). Then it divides
the corrected background image through
the corrected specimen image: (s–d)/(bd). This procedure can also be performed
automatically with the live image. Here,
the background and the dark images
have to be acquired only once for each
microscope objective using a standard
sample and standard acquisition parameters. This is how the image in fig. 38
(right side) was taken.
If the background and dark images
are not available, the shading correction
can also be derived from the specimen
image itself. For example, a very strong
smoothing filter averages out all structures and reveals the background. Or a
Basics of light Microscopy & Imaging • 31
Fig. 41: Comparison of different smoothing filters. See text for further explanation.
multi-dimensional surface function can
be applied to fit the background illumination profile.
Just too much noise
Random noise can be an annoying phenomenon which is encountered when the
specimen signal is low and/or was enhanced with a high gain factor. An example from fluorescence microscopy is
shown in fig. 39. In regular brightfield
light microscopy images random noise is
usually not visible. But it is often unsheathed when a sharpen filter is applied
to the image. See fig. 42, first and second
image as an example. There are different
smoothing filters which can be used to
reduce the noise. Smoothing filters are
also applied to suppress artefacts deriving from small structures like dirt, dust,
debris or scratches. Yet the challenge is
to find a filter which eliminates the noise
and the artefacts without smoothing the
object edges too much.
Fig. 41 shows an artificially created
image having only one real structure: a
simple rectangle of light shading upon a
dark background. This structure is superimposed by two different kinds of distortion: strong statistical noise interfer-
ence on one hand and so-called hot pixels
or shot noise on the other hand. The shot
noise is individual bright and dark pixels
and can come from a defective camera.
These pixels also mirror artefacts from
dust particles, dirt, debris or small
scratches.
The noise reduction filters applied to
the image are local or neighbouring operations, i.e., the neighbouring pixels are
taken into account to calculate the new
pixel value of the resulting image (see
also box 12). The Mean and NxN filters
just average everything and thus also
broaden the structure of the rectangle
(fig. 41). The more powerful the smoothing effect, the more noticeable the morphology of the object within the image
will be altered. Shot noise will not be removed. Whereas the Mean filter applies
fixed settings, the extent to which the
NxN filter averages depends on the parameters set.
The Sigma or Gaussian blur filter is a
special average filter which does not affect any pixels deviating greatly from
their surrounding area (fig. 41). So “real”
structures and edges like the rectangle
are not touched whereas the random
noise disappears. The Sigma filter is the
method of choice to reduce selectively
statistical noise without greatly broadening the specimen structures (see also example in fig. 40).
The Median and Rank filters eliminate
the shot noise widely (fig. 41). They are
especially suited to suppress dust and
dirt, in general all small structures which
stand out the underground. Comparing
the effects the different smoothing filters
have on the artificial sample image (fig.
41), the image’s two different distortions
could be eliminated best by applying two
filters successively: first the Sigma filter
against the statistical noise, and then the
Rank filter against the shot noise. This
leads to the somewhat philosophical rule
which says that it is best not to strike
everything all at once but to cut the problem into pieces and find the appropriate
measure to deal with each piece separately.
NxN-, Sigma- and Rank filters have
user-definable controls which make it
possible to adjust the smoothing effect
optimally to the image. There is another
option to reduce statistical noise, usually
with fixed parameters, which gives quite
good results: the lowpass filter. This filter
is a global operation (refer to box 12)
which filters out high frequency and periodic distortions. Yet here, the edges become somewhat broadened. An even better global operation is the bandpass filter
which reduces the noise and preserves
the steepness of edges.
Revealing the details
Another class of filters seen from the application point of view are the sharpen
filters. Sharpen filters can be used to enhance fine image details. After processing an image with a sharpen filter, the
image appears to look clearer. But
sharpen filters have to be applied somewhat carefully because they can create
artefacts themselves if overdone. The established sharpen filters are local neighbouring operations like the convolution
filters (see box 12). The newer filters use
the so-called unsharp mask algorithms.
(Please have a look at fig. 42.) Sharpening the first, original image, gives the
second image. Unfortunately, this enhances not only the structure but also
the noise present in the original image.
So the sharpen operation makes the
noise visible. Additionally, the filter parameters have been set too aggressively
so that the “lamellae” structures of the
diatom look artificial. This context has to
be kept in mind when applying a sharpen
filter: Before the sharpen filter can be
applied to an image, a noise reduction
filter must often be applied first. Any
32 • Basics of light Microscopy & ImagingContrast and Microscopy
Fig. 42: Brightfield image of a diatom. (a): original image. (b): after application of a strong sharpen filter. This increases the noise. (c): after application of the
Sigma noise reduction filter and a moderate sharpen filter. (d): after application of the Sigma noise reduction filter and the DCE sharpen filter.
sharpen filter with user controls should
be applied conservatively. Here, less is
often more. The third image in fig. 42
shows the result after noise removal via
a Sigma filter and successive moderate
sharpening.
The DCE filter is a specially designed
sharpen filter and is part of the Olympus
image processing software solutions. The
letters DCE stand for Differential Contrast Enhancement. The DCE filter enhances weak differences in contrast. It
selectively takes the lesser intensity modulations between neighbouring pixels
and enhances them, while greater intensity modulations remain as they are. So
the DCE filter renders image structures
visible which are barely distinguishable
in the original image. The filter works
the better the more fine structures the
image has. Resulting images are more
detailed and appear more focused. See
fig. 42, last image, and both images in
fig. 43 as examples.
Fig. 43: Epithelial cells viewed with phase contrast (left side) and transmitted darkfield contrast (right
side). The original images are acquired with optimised microscope settings. These have already been
shown in box 8 and fig. 30. The lower right parts of the images are each filtered with the DCE filter.
Contrast and Microscopy Putting everything together
To integrate what has been said above,
there are some image processing operations which are meaningful or even necessary to apply to an image, either in real
time or after acquisition. These operations can reduce image distortions, prepare the image for automatic measurements or just make the image look better.
The following steps suggest a possible
strategy to proceed with digital image
operations when acquiring an image (see
also:
www.olympusmicro.com/primer/digitalimaging/imageprocessingsteps.html)
1.Shading correction is to equalise uneven background illumination.
2.Contrast enhancement is to optimise
brightness and contrast.
3.White balance is to adjust colour
shifts.
4.Smoothing is to reduce random noise
and suppress shot noise and small artefacts like dust and dirt.
5.Sharpening is to enhance fine edge
detail.
Here, we have covered a few of the wide
range of possible filter operations. There
are many more which go beyond scientific applications and transcend the pure
picturing of the visible reality towards
creative, funny, intelligent and beautiful
compositions.
Basics of light Microscopy & Imaging • 33
Shining
Fluorescence Details
34 • Basics of light Microscopy & ImagingShining Fluorescence Details
A first introduction
Most of the newly developed microscopic
techniques make use of fluorescence.
Fluorescence microscopy is more than
’just making colourful images in one,
two, three or even more colours‘, it is an
enormously powerful tool for investigations in the biological field. Fluorescence
techniques place numerous benefits in
the hands of researchers wishing to exploit the upper limits of sensitivity and
resolution in microscopy. Beyond the scientific benefits, just studying the fluorescence images can sometimes offer a new
insight into a reality which is usually hidden from the view of the world.
Fundamentals – a development both
remarkable and ongoing
Within the last few decades numerous
new techniques such as confocal, deconvolution, ratio-imaging, total internal reflection and applications such as the use
of fluorescent proteins (e.g. GFP) have
initiated a real renaissance in the microscopy field. All of these techniques make
use of fluorescence, a phenomenon first
observed by Sir George Gabriel Stokes in
1852 and physically described by Alexander Jablonski in 1935 (see box 13).
Compared with today, the number of
specific questions regarding life science
or materials science specimens and to
visualise the result in a specific colour.
For example, to identify the distribution
of a specific protein within a tissue, a
fluorochrome can be used to mark the
protein via an antibody (immunohistochemistry).
Histological staining procedures for
transmission light microscopy have a long
history in microscopy. One essential advantage of fluorescence microscopy, however, is the presence of fluorescent molecules themselves. Even if a structure is
too small to be resolved by a light microscope, the emission light remains visible.
Fluorescent molecules act like light
sources that are located within specific
areas of a specimen, indicating their location with light of a specific colour.
These indicators require energy to emit
light and this is given to the fluorochrome
by the excitation light, provided by the
microscope light source. A specific range
of wavelengths is needed to excite a specific fluorochrome. For example, a range
of blue wavelengths around 480 nm can
excite the FITC fluorochrome. This involves using two different light beams
and having to separate them. On the one
hand, we need to direct the light of the
microscope light source onto the specimen and on the other hand we have to
Fig. 44 a: Spectral diagram of a typical fluorochrome that can be
excited using blue light
and then emit green
fluorescence. This example shows that both
curves are close to each
other (small Stokes shift)
and also show an area of
spectral overlap.
Fig. 44 b: Cracks within
the resin of an electrical
circuit board.
widely used fluorochromes was restricted
to just a few in the 1990‘s. For example,
nowadays the fluorochrome FITC filter
set for fluorescence microscopy can also
be used for a wide range of different fluorochromes with green emission spectra.
Why use fluorescence?
Using fluorescence can be compared to
the situation where a teacher asks if the
students have done their homework. The
rapidly changing facial colours of the
“guilty” students provide conclusive “results”. However, fluorescence techniques
are not really for answering questions
such as the above. They help to address
Shining Fluorescence Details observe the light that is originating from
the fluorochromes. This separation is
possible due to the “Stokes shift”, which
describes the fact that the wavelength of
fluorescent light (emission) is always
longer than that of the excitation. Using
a blue excitation light will thus result in a
green emission for the FITC fluorochrome. Every fluorochrome has its own
excitation and emission spectra. The microscope must be perfectly equipped to
visualise this fluorescence accordingly.
Fluorescent molecules
There are two options for using fluorescent microscopy, depending on what is
Basics of light Microscopy & Imaging • 35
Fig. 45: Light path on a microscope equipped for fluorescence.
being investigated: either the specimen
itself already contains molecules that
show fluorescence; or specific fluorochromes have to be added to the specimen. Autofluorescence is widely found in
materials such as plant sections or electrical circuits, for example. The resin on
circuits is fluorescent and can easily be
inspected under blue excitation (fig. 44b).
The green emission of the resin enables
the detection of the tiniest cracks which
may influence material quality.
Fluorochromes themselves can be divided into at least three groups. The first
are fluorochromes that require other
molecules, such as antibodies or lectins,
to bind to specific targets. This rapidly
growing group of fluorochromes includes
longstanding ones such as FITC and
TRITC. Most of these fluorochromes are
sold together with the specific targetfinding molecule (e.g. a goat anti-mouse
IgG antibody Cy5 labelled). Quantum dots
are also partial members of this group
but different in structure and theory.
They are nanometre-sized crystals of purified semiconductors and exhibit longterm photo stability, as well as bright
emission. The main difference featurewise is their capacity to be excited by
wavelengths up to the blue range and
having different emission colours depending on their size. Due to their flexible capabilities they can also be used for
direct staining of cells (e.g. cell viability).
The second group contains fluorochromes that have inherent binding capacities, such as the DAPI nucleic acid
stain or the DiI anterograde neuron stain.
This group also contains fluorochromes
Box 13: What is fluorescence exactly?
Fluorescence activity can be schematically illustrated using the familiar Jablonski diagram, where absorbed
light energy excites electrons to a higher energetic level in the fluorochrome. These lose a proportion of their
energy by vibration and rotation and the rest is then given off as fluorescent light as they return to their original state after only about 10 ns.
Fig. 46: Characteristics of
an Olympus HQ filter set
optimised for GFP. By
using up to 100 layers of
an ion deposition technique with new substrates and coating
materials, filters can be
created with high transmission and exceptionally sharp cut-off (tolerances < 2 nm).
Autofluorescence is
significantly reduced and
the filters are equipped
with a stray light noise
destructor to enhance
the signal-to-noise ratio.
that change their fluorescent properties
when bound to different amounts of molecules such as calcium (e.g. Fura-2). This
means that these fluorochromes are used
directly and do not necessarily require a
transportation system such as an antibody.
The third group contains fluorescent
proteins produced by organisms themselves such as GFP. This makes it possible to set up experiments in an entirely
different way. It is most often used for
live cell imaging or developmental studies and molecular biology. All fluorochromes show distinct spectral properties (fig. 44a) and can often be combined
for a multicolour specimen analysis.
Requirements for a fluorescence
­microscopy system
The Light Source
To excite the fluorescence of a fluorochrome, an intense light source is needed
that provides the necessary excitation
wavelengths to excite the particular
fluorochrome. In the first chapter we described the most frequently used light
sources for light microscopy and their
alignment. A correctly aligned burner
plays an essential role in creating good
fluorescent images. If the burner is not
correctly aligned, the signal from the
fluorochrome may be excited in a very
weak way and the field of view will not
be homogeneously illuminated, or can
show a bad signal to noise ratio.
Due to the very different specimens
and applications that can be analysed using fluorescent techniques, there is no
one-size-fits-all strategy. All fluorochromes are subject to the process of
photo-bleaching, which is the chemical
destruction which takes place during excitation. Living cells, too, may be damaged by the intense light. This makes it of
supreme importance to restrict the excitation brightness and duration to the ex-
36 • Basics of light Microscopy & ImagingShining Fluorescence Details
act amount needed. The amount of light
can be efficiently modified with neutral
density filters or a motorised attenuator.
When the light is not needed for excitation, the shutter is closed. These features
can be predefined in motorised microscopes and help to optimise experiments.
The Filter Sets
In addition to the special light path within
a fluorescence microscope (fig. 45), another necessity is having a filter which
only permits the required range of excitation wavelengths to pass through. This
is achieved using an exciter filter with
what are referred to as bandpass filter
characteristics (box 14). After restricting
the light to the particular colour that is
needed to excite the fluorochrome, the
light is directed to the specimen via a dichromatic mirror (fig. 45).
As indicated by its name, the dichromatic mirror treats different colours differently. It reflects light below a given
wavelength and is able to let longer
wavelengths pass through. The excitation light travels through the objective to
the specimen, acting like a condenser.
This is where the fluorescence phenomenon takes place. Excited by the light,
the fluorochromes emit the fluorescence
light of longer wavelengths. This is captured by the objective, moving on to the
dichromatic mirror, now letting the
longer wavelengths pass through. The
last step of filtering is undertaken by the
emission filter (also called a barrier filter, see fig. 46). This filter restricts the
light colour to best fit the fluorochrome
emission and the question being investigated. It ensures that no unwanted wavelengths are observed and analysed. The
emission filter can be designed as a bandpass filter (precisely restricted to one
spectrum) or as a longpass filter (brighter
in effect, but less optimal due to a reduction in restricted wavelengths). To help
find the best filter combination for the
fluorochromes in use and the analysis in
mind, a variety of web pages is available
(e.g.
www.olympusmicro.com/primer/
java/fluorescence/matchingfilters/index.
html). A straightforward example (below) will demonstrate how the combination of filters may differ depending on
the context.
Using the correct filter set – an example
If you wish to make a vital analysis of a
cell culture you may choose a Fluorescein-diacetate (FDA) to stain vital cells.
This fluorochrome is excited by blue light
and will have a green emission. The vital
cells will be the ones appearing green.
Shining Fluorescence Details The filter set could thus be chosen as follows: an exciter with BP460-490 bandpath characteristics, a dichromatic mirror with DM505 characteristics and an
emission filter with LP510 long path
characteristics. This will result in a bright
green image of all green fluorescent molecules. So far, so good. The vital cells are
stained. To verify that non-labelled cells
are in fact dead, propidumiodide (PI) dye
may be used. This dye cannot pass
through intact cell membranes. The DNA
of dead cells only will be labelled and appear red. This means it can be used along
with the FDA. When doing so, however,
the excitation of the FDA with the filter
mentioned will cause some problems. PI
will already be excited by the blue light
and the red emission is also visible. This
is caused by the emission filter because
in this set up, all wavelengths above 510
nm are allowed to pass through. Both
dyes are thus excited and visible. A definite separation of both signals is not possible and as we will see later on, this can
cause problems during imaging.
To separately identify both signals
from the cell culture, the emission filter
required for FDA is a bandpass filter
Box 14: How are filters described?
Filters for fluorescence microscopy are described
using letters and numbers: e.g. BA 460-480. In
this case, BA stands for bandpass filter and the
numbers indicate the 50 % cut on and the 50 %
cut off (fig. 46 and see boxfigure below). For a
longpass filter, just the number for the 50 % cut
on is indicated. Some companies use a different
description for the same overall filter characteristics (e.g. 470/20), 470 indicating the central wavelength and 20 indicating the range of the full
width at half maximum (FWHK). The correct transmission characteristics of the filter can only be
provided using a transmission/wavelength diagram (see boxfigure below).
Fig. 47: Upper row: Images of a double-labelled specimen (DAPI for DNA staining and Alexa 488labelled antibody for F-Actin staining). The first image shows the detection of the fluorescence using a
filter set with the appropriate excitation filter for DAPI and with a long pass emission filter. Analysed
with a monochrome camera, even the Alexa-488-labelled structures are visible and prevent effective
analysis. The blue and the green image show the same specimen with optimised emission filters,
allowing the separation of each signal.
The lower row shows an example of an image area analysed with an achromat (left), a fluorite (centre)
and an apochromat objective (right) at the same magnification.
Note that the signal intensity and colour transmission increases from achromat to apochromat due to
the increasing NA and quality of the objectives, whereas the background intensity in the upper left
corner remains unchanged.
Basics of light Microscopy & Imaging • 37
Box 15: How to measure FRET.
Ratio measurement
An epifluorescence microscope
equipped with highly specific fluorescence filter sets, a digital camera
system and image analysis software
is required. The ratio between the intensity of the sensitised acceptor
emission (= FRET signal) and the intensity of the donor emission is calculated (see figure). The results have
to be corrected for spectral cross-talk
by spectral unmixing software.
Acceptor photo-bleaching.
This effect occurs when the acceptor
is irreversibly destroyed by extensive
illumination from excitation light. After bleaching of the acceptor fluorophore, FRET is stopped. Excitation of the donor will result in donor fluorescence emission. After the acceptor
photo-bleaching, the donor shows more intensity than before.
Donor-bleaching.
The sample is irradiated with the specific excitation wavelength of the donor. Images of the donor are continuously acquired and the intensity decay is quantified. FRET decreases the mean lifetime of the donor and
protects the donor from photo damage. As a result, when using FRET, the rate of photo-bleaching is lower;
without FRET, the rate of photo-bleaching is higher.
(e.g. BP510-550). This filter will only allow the green emission of the FDA to
pass through and the emission of the PI
will be blocked. A second filter set can
then be used to analyse the PI signal efficiently (e.g. BP530-550 excitation filter,
LP575 emission filter, DM570 dichromatic mirror). This principle also applies
to other multicolour staining procedures.
Best results are achieved when the emission filter for the detection of the fluorochrome with the shorter wavelength is a
band pass filter (fig. 47 upper row).
The objectives
The light gathering capacity of the objective plays an essential role in fluorescence microscopy. For optimal signal
strength, a high-numerical aperture
(high NA) and the lowest useable magnification should be employed. For example, using a 1.4 NA aperture lens instead
of a 1.3 NA lens of the same magnification will result in a 35 % increase in light
intensity, assuming that all other factors
are the same. Furthermore, the type of
glass that is used requires good transmission for the wavelengths used. Fluorite or apochromat objectives are used for
that reason. Further enhancement can
be achieved by selecting objectives with
extremely low autofluorescence of the
glass material used (Olympus UIS2 Objectives). When all of these factors are
provided for – high NA, good transmission and low autofluorescence – this en-
sures a perfect signal-to-noise ratio (i.e.
a strong signal with low background intensity (fig. 47 lower row)). Background
noise can also be introduced by the specimen itself due to fixation, autofluorescence of the specimen or non-optimised
staining procedures.
Type of camera
The imaging device is one of the most
critical components in fluorescence microscopy analysis. This is because the
imaging device used determines at what
level specimen fluorescence may be detected, the relevant structures resolved
and/or the dynamics of a process visual-
ised and recorded. Numerous properties
are required to use fluorescence microscopy effectively. These include: high resolution, extreme sensitivity, cooling, variable exposure times and an external
trigger function. Generally no single detector will meet all these requirements in
fluorescence microscopy. Consequently,
the fluorescence microscopist frequently
has to compromise. However, the cameras used in fluorescence microscopy
should at the very least offer high signal
sensitivity, low noise and the ability to
quantify intensity of intensity distribution.
Colour cameras are less sensitive than
their monochrome counterparts because
of the additional beam-splitting and
wavelength selection components. Therefore, monochrome cameras are preferable. They image the fluorescence intensity of each fluorochrome separately and
can handle the respective images later on
within a specific colour space using the
appropriate software. The resulting multicolour images can be displayed, printed
out and analysed or further processed.
Every channel of colour represents the
result of one fluorochrome. This result
can be obtained if the fluorescence filters
are chosen correctly. Returning to our example of the FDA and PI double labelling:
when using the longpass emission filter to
detect the FDA signal, the red emission of
the PI will also contribute to the resulting
digital image. A monochrome camera
would not differentiate between red and
green or between blue and green as
shown in fig. 47 (first image) – it will only
show intensity in grey values. Therefore,
the image will represent the resulting distribution of both fluorochromes within
the same colour. The use of the bandpass
emission filter as described above will
help in this respect.
Fig. 48: Multi-colour image of a GFP/YFP double-labelled sample, before (left) and after spectral
unmixing (right). In this sample, GFP was fused with the H2B histone protein and YFP with tubulin. The
visible result is a pronounced chromatic resolution of both structures now displayed in green and red.
The tubulin on top of the bright green nuclei is even detectable.
38 • Basics of light Microscopy & ImagingShining Fluorescence Details
How to use light as a tool
Fig. 49: A climate chamber offers features like
control of temperature, humidity and CO2 content of the atmosphere.
Software-tools: Spectral unmixing
A further problem can occur when using
different fluorochromes with overlapping
spectra within one sample. The considerable overlap of excitation and emission
spectra of different fluorochromes is exemplified by the spectra of enhanced
green fluorescence protein (eGFP) and
enhanced yellow fluorescent protein
(eYFP), two commonly used variants of
the green fluorescent protein (fig. 48 left
side). Even with the use of high quality
optical filters it is not possible to separate the spectral information satisfactorily. This means that YFP-labelled structures are visible with a GFP filter set and
vice versa, affecting the resulting images
significantly and detrimentally. This phenomenon, known as “bleed-through”,
strongly reduces colour resolution and
makes it difficult to draw accurate conclusions. The solution to overcome this
effect is called spectral imaging and linear unmixing.
Spectral imaging and linear unmixing
is a technique adapted from satellite imaging to wide-field fluorescence microscopy. Using this highly effective method,
it becomes possible to ascertain the specific emission of the different fluorochromes to the total signal and to restore
a clear signal for each colour channel,
unaffected by emission from the other
fluorochrome. This is achieved by redistribution of the intensity (fig. 48 right
side). It is important to note that original
data is not lost during linear unmixing
nor is any additional data added to the
image. The original image information is
all that is used in this procedure. The
overall intensity of pixels is maintained.
Thus, the technique does not result in artificially embellished images. After unmixing, quantification analysis not only
remains possible, but also becomes more
precise.
Shining Fluorescence Details After having laid the fundamental principles and the microscopic requirements
we can now go deeper into different
state-of-the-art qualitative and quantitative imaging methods for fluorescence
microscopy. These approaches require
advanced microscopic and imaging
equipment which varies from case to
case. So only general statements can be
made here about the hardware and software used, along with a description of
the applications and methods.
Life under the microscope
Wherever there is life there are dynamic
processes – observation of living cells
and organisms is a tremendous challenge
in microscopy. Microscopy offers different techniques to reveal the dynamic
processes and movements of living cells.
The use of fluorescent proteins and live
fluorescent stains enable the highly specific labelling of different organelles or
molecules within a living cell. The intensity of the emission light of these markers can be used to image the cells. In addition to the application protocol and the
fluorescent technique to be used there
are further general considerations to be
aware of.
First of all, there is the definition of
the needs for the specific environmental
conditions and controlling these with regard to specimen handling. Assuming
you are using a cell line and would like to
analyse processes over time, it may be
necessary to provide appropriate environmental conditions for these cells. Dynamic processes in single cells can occur
within the millisecond range – such as
shifts in ion concentrations. Or they may
take minutes – such as the active or passive transport of proteins or vesicles. Microscopes can be equipped with heating
stages and/or minute chambers, or with
complete cultivation chambers to ensure
cultivation of living cells with all the appropriate parameters on the microscope
while observation is conducted for hours
or days (fig. 49).
toxymethylester of the dyes can be added
to the medium, loaded passively into the
cells and cleaved enzymatically to produce cell-impermeable compounds.
For the analysis of a typical two channel FURA experiment it is necessary to
switch between the excitation of 340nm
and 380 nm. When observing FURA
loaded cells without any stimulus, Ca2+ is
bound in cell compartments. The FURA
molecules show strong fluorescence at
an emission of 510 nm when excited with
380 nm, and weak fluorescence when excited with 340 nm. As the cell releases
Ca2+ from storage compartments due to a
reaction to a stimulus, FURA molecules
form complexes with these released Ca2+
ions. The fluorescence signal in the emission channel of 510 nm increases when
excited with 340 nm, and decreases when
excited with 380 nm. The ratio between
the signals of the two excitation channels
is used to quantify the change of intensity.
Why use a ratio? Lamp fluctuations or
other artificial intensity changes can
cause a false signal which can be misinterpreted as a change in ion concentration when intensity is measured in one
channel only. The second drawback of a
single channel analysis is that it displays
the amount of the fluorescence only.
Therefore, thicker parts may look
brighter than smaller parts of a cell;
however, they simply contain more fluorochromes due to the larger volume. Physiologically relevant changes in ion concentration in small areas such as growth
cones of a neuron may then not be visible
over time because they are too dim in
fluorescence compared to the bright centre of the cell. After background subtraction, the calculation of a ratio between
Where do all the ions go?
Fluorescent dyes such as FURA, INDO or
Fluo show a spectral response upon binding Ca2+ ions and are a well established
tool to investigate changes in intracellular Ca2+ concentration. The cells can be
loaded with a salt or dextran conjugate
form of the dye – e.g. by microinjection,
electroporation or ATP-induced permeabilisation.
Furthermore,
the
ace-
Fig. 50: Autofluorescence of an apple slice
(Lieder). The image was taken with an Olympus
FluoView FV1000 and the TCSPC by PicoQuant.
The colour bar in the lower right corner is a key,
showing the distribution of the various lifetimes
within the image.
Basics of light Microscopy & Imaging • 39
pole orientation, insufficient spectral
overlap between the emission spectrum of
the donor and the excitation spectrum of
the acceptor. See box 15.
How long does a fluorochrome live? –
count the photons!
Fig. 51: Photoconversion: In Miyawaki‘s lab, Ando et al [1] succeeded in cloning the gene that encodes
the Kaede fluorescence protein of a coral. By irradiation with UV light, the green fluorescence can be
converted to red fluorescence. By using excitation wavelengths in the blue and green spectral range,
the movement and distribution of the protein can be observed in a living cell. The figure shows the
protein diffusion in Kaede-expressing HeLa cells. Stimulation via 405 nm laser in the region of interest
converts the Kaede protein from green to red emission. Confocal images were recorded every three
seconds using 488/543 nm laser excitation, showing the activated Kaede protein spreading throughout the HeLa cell. By using two synchronised scanners (one for photo-conversion, one for image acquisition), fluorescence changes that occur during stimulation can be observed.
Data courtesy of: R. Ando, Dr A. Miyawaki, RIKEN Brain Science Institute Laboratory for Cell Function
Dynamics.
two channels corrects the result for overall, artificial fluctuations and specimen
thickness. Following a calibration procedure, even quantitative results are obtainable. There is a range of fluorochromes,
with
different
spectral
properties for numerous different ions,
available on the market. Microscopical
systems with fast switching filter wheels
and real time control permit investigations even in the millisecond range.
Light as a ruler
Many processes in a cell are controlled by
inter- and intra-actions of molecules: e.g.
receptor-ligand interactions, enzyme-substrate reactions, folding/unfolding of molecules. Receptor-ligand interactions, for
example, occur in the very close proximity of two proteins in the Angstrøm range.
Colocalisation studies do not reveal interactions of molecules in the Angstrøm
range because the spatial resolution of a
light microscope is limited to 200 nm.
When using a light microscope, how can
the proximity of two molecules in the Angstrøm range be proven beyond the physical limits of light microscopy? Fluorescence Resonance Energy Transfer (FRET)
helps to find an answer to this question.
FRET is a non-radiative energy transfer
between two different fluorophores. The
first fluorophore (the donor) is excited by
light. The donor transfers its energy to the
second fluorophore (the acceptor) without
radiation, meaning without any emission
of photons. As a result, the acceptor is excited by the donor and shows fluorescence
(“sensitised emission”). The donor is
quenched and does not show any fluorescence. This radiation-free energy transfer
occurs within the very limited range of 1–
10 nm distances between the donor and
the acceptor. A positive FRET signal provides information about the distance between the FRET partners and can be
quantified as FRET efficiency. When no
FRET signal is achieved, there may be
many reasons for that: e.g. too much distance between the FRET partners, insufficient steric orientation, insufficient di-
When a fluorochrome is excited, it is
shifted to a higher energy level. The lifetime of a fluorophore is the average
amount of time (in the nanosecond/picosecond range) that it remains at the higher
energy level before it returns to the
ground state. A fluorochrome‘s lifetime is
a highly specific parameter for that particular fluorochrome. It can be influenced
easily by changes of environmental parameters (e.g. pH, ion concentration, etc.),
by the rate of energy transfer (FRET) or
by interaction of the fluorochrome with
quenching agents. Fluorochromes often
have similar or identical spectral properties, therefore, analysing a fluorochrome‘s
lifetime is critical to distinguishing the localisation of those fluorochromes in a cell
(fig. 50). Here, the different experimental
setups are referred to as FLIM – Fluorescence Lifetime Imaging Microscopy.
Fluorescence Lifetime Imaging Microscopy –
FLIM.
There are different techniques for fluorescence lifetime imaging microscopy
available on the market, for both widefield and confocal microscopy. Here we
focus on Time-Correlated Single Photon
Counting (TCSPC). The fluorochrome is
excited by a laser pulse emitted by a
pulsed laser diode or femto-second
pulsed Ti:Sa laser. A photon-counting
photo- multiplier or single photon avalanche diode detects the photons emitted
from the fluorophore. The time between
the laser pulse and the detection of a
photon is measured. A histogram accumulates the photons corresponding to
the relative time between laser pulse and
detection signal. Every pixel of the FLIM
image contains the information of a complete fluorescence decay curve. If an image is composed of three fluorochromes
with different lifetimes, distribution of all
Fig. 52: Photoactivation of PAGFP expressing HeLa cells. The
PA-GFP was activated by 405
diode laser (ROI) and images of
the PA-GFP distribution within
the cell were acquired at 1 second intervals using the 488nm
laser.
Data courtesy: A. Miyawaki, T.
Nagai, T. Miyauchi,
RIKEN Brain Science Institute
Laboratory for Cell Function
Dynamics.
40 • Basics of light Microscopy & ImagingShining Fluorescence Details
dyes can be shown as three different colours (fig. 50).
Time-resolved FRET microscopy
With FRET, the lifetime of the donor depends on the presence or absence of radiation-free energy transfer (see above).
Therefore, time-resolved FRET microscopy is a technical approach for quantitative measurement of the FRET efficiency.
FRAP, FLIP and FLAP
FRAP, FLIP and FLAP are all photobleaching techniques. By using the laser
scanner of a confocal microscope, fluorochromes (which are bound to a specific
protein, for example) in a selected region
of a stained cell can be bleached (destroyed). As a result, the fluorochrome
does not show any appropriate fluorescence. Other proteins that are also labelled, but where the fluorescence was
not bleached, can now be observed during movement into the previously
bleached area. Dynamic processes, such
as active transport or passive diffusion in
a living cell cause this movement. Therefore the intensity of the fluorochrome recovers in the bleached area of the cell.
bleached, the other remains undamaged
and acts as a reference label. The population of bleached fluorochromes can be
identified by subtraction of the image of
the bleached dye from the image of the
unbleached dye.
FAUD = Fluorescence Application Under
­Development!
We are looking for inventors of new fluorescence techniques – please keep in
touch!
From black to green, from green
to red …
If we sunbathe for too long, the colour of
our skin changes from white to red and
is referred to as sunburn. If we take a UV
laser and irradiate a cell with Kaede protein, the colour of the Kaede protein
FRAP = Fluorescence Recovery After
­Photobleaching
After bleaching of the fluorochrome in a
selected region of a cell, a series of images of this cell is acquired over time.
New unbleached fluorochromes diffuse
or are transported to the selected area.
As a result, a recovery of the fluorescence signal can be observed and measured. After correction for the overall
bleaching by image acquisition, a curve
of the fluorescence recovery is obtained.
FLIP = Fluorescence Loss In Photobleaching
The fluorophore in a small region of a
cell is continuously bleached. By movement of unbleached fluorophores from
outside of the selected region into the
bleaching area, the concentration of the
fluorophore decreases in other parts of
the cell. Measuring the intensity loss,
outside the bleached area results in a decay curve. A confocal laser scanning microscope which incorporates one scanner for observation and one scanner for
light stimulation is especially appropriate for this technique. A simultaneous
scanner system allows image acquisition
during continuous bleaching.
FLAP = Fluorescence Localization After
­Photobleaching
The protein of interest is labelled with
two different fluorochromes. One is
Shining Fluorescence Details Fig. 53: Imaging at the Outer Limits of Resolution.
(a)
Dual emission widefield
(b)TIRF images acquired with a 488nm laser
green Fitc-Actin
red
DyeMer 605-EPS8.
Courtesy of M. Faretta, Eup. Inst. Oncology, Dept.
of Experimental Oncology, Flow Cytometry and
Imaging Core, Milan, Italy.
changes from green to red – and we call
this photo-conversion (fig. 51). A confocal microscope can be used to stimulate
fluorochromes in selected parts of the
cell. The Olympus FluoView FV1000 even
offers stimulation of one area with one
laser whilst observing the result in a different channel with a second laser simultaneously. A recently developed photoactivatable GFP mutant, PA-GFP, can be
activated by irradiation using a 405 nm
diode laser (fig. 52). The intense irradiation enhances the intensity of the fluorescence signal of PA-GFP by 100 times. PAGFP is a powerful tool for tracking
protein dynamics within a living cell
(fig. 52).
Imaging at the upper limits of
­resolution
The bleaching and photoactivation techniques described above can be undertaken easily using a suitable confocal laser scanning microscope. The laser is a
much more powerful light source than a
fluorescence burner based on mercury,
xenon or metal halide. The galvano mirror scanners, in combination with acousto-optically tuneable filters (AOTF), permit concentration of the laser onto one
or more selected regions of the cell without exciting any other areas in that cell.
This makes the laser scanning microscope the preferred tool for techniques
which use light as a tool. Additionally,
the confocal principle removes out-of-focus blur from the image, which results in
an optical section specifically for that focal plane. This means that high resolution along the x, y and z axes can be obtained. A confocal microscope is an ideal
system solution for researchers who
want to use light not only for imaging,
but also as a tool for manipulating fluorochromes.
To achieve an even thinner optical section (for a single thin section only and not
a 3-D stack), Total Internal Reflection Microscopy (TIRFM) may be the technique of
choice (for more information on TIRFM,
see box 4 or at www.olympusmicro.com/
primer/techniques/fluorescence/tirf/tirfhome.html). The evanescent field used for
this technique excites only fluorochromes
which are located very closely to the coverslip (approx. 100–200 nm). Fluorochromes located more deeply within the
cell are not excited (fig. 53). This means
that images of labelled structures of membranes, fusion of labelled vesicles with the
plasma-membrane or single molecule interactions can be achieved with high z
resolution (200 nm or better, depending
on the evanescent field).
References
[1] R. Ando, H. Hama, M. Yamamoto-Hino, H. Mizuno, A. Miyawaki , PNAS Vol. 99, No. 20,
12651–12656 (2002)
[2] G. H. Patterson, J. Lippincott-Schwartz, Science, Vol 297, Issue 5588, 1873–1877 (2002)
Basics of light Microscopy & Imaging • 41
3D Imaging
42 • Basics of light Microscopy & Imaging
3D Imaging
Reality is multi-dimensional
What do we see when we observe a sample through a light microscope? What information do we perceive? Most light microscopes give us a two-dimensional view
of physical reality. What we usually observe in the widefield microscope is the
projection of a three-dimensional physical structure down to a two-dimensional
image. This means one dimension is lost
which significantly restricts our perception of the physical reality viewed.
When looking at two dimensional images we often reverse the process and,
based on experience, more or less unconsciously extend the two dimensional
view into the third dimension. But how
can we be sure that this interpretation is
accurate? While watching the night sky
we cannot tell whether two stars are
close to each other or are in fact hundreds of light years away in space. Painting techniques, however, offer numerous
examples of how to create a distinct impression of a third dimension using different lighting and shading of contrast,
as well as well-placed usage of objects
with familiar proportions. The fact that
we are accustomed to recognising different lighting on a structure in a three dimensional way is also how relief is shown
– such as in contrast methods used in microscopy (e.g. Differential Interference
Contrast (DIC)).
The stereo view
Our eyes always view objects from a
slight angle defined by the distance of
our eyes and the distance between the
object and us. Together with the interpretation that our brain provides us with,
this creates a three-dimensional view
with these two sources of perception. To
see objects within a microscope in a similar way, both eyes have to each be provided with a separate light path to enable
observation of the specimen at an angle
similar to the one our eyes see from naturally. This is exactly what stereo microscopy is all about. These microscope
types, often called “binos”, provide a
view of a specimen that looks natural, including a high depth of field and working
distance. With a convergence angle of
10–12°, the left eye and right eye receive
views of the same object from a slightly
different perspective (fig. 54).
For viewing surfaces, the stereo microscope provides a reliable three-dimensional impression of the object. We
get an idea of how the surface is shaped
and which structures are higher or lower.
What we are investigating here is a
3D Imaging curved, two-dimensional surface expanded into three dimensional space.
However, one general restriction is still
the limited depth of field which makes it
often impossible to obtain a completely
sharp image. Additionally, stereo microscopes are restricted in magnification
and resolving power due to their general
design when compared with upright or
inverted widefield microscopes. Furthermore, when using a stereo microscope
for documenting images, only one of the
microscope pathways will be used for the
camera in most cases. This means the
resulting digital image is only two-dimensional – there is no third dimension.
Fig. 54: The stereomicroscopic (Galileo type)
light path – two microscopes in one – offers
observation of specimens at the natural conversion angle of our eyes. This makes the 3D topography visible. However, when the specimen is
documented the image will nevertheless remain
a two-dimensional view.
Thus, when we want to have a sharp
view of a topographic structure in light
or stereo microscopes – or in general,
when we want to know more about the
three-dimensional internal set-up of
structures, several sections or slices have
to be acquired. All at a different z position or on a different focal plane. Have a
look at an image stack containing several
such sections of the human head acquired via Magnetic Resonance Imaging
(MRI) (fig. 55).
Not too few – Not too many – just
enough
Using the light microscope, optical sections are usually acquired using a motorised focus. First the maximum and the
minimum position along the z axis of the
stage or the nosepiece are defined. Then
you either set the distance of two z-positions (i.e. the focal change (Δz) or z spacing), or you set the total number of sections to acquire. Depending on the
application, a minimum number of secBasics of light Microscopy & Imaging • 43
Fig. 55:
Left:
Right:
Stack of images taken via MRI (Magnetic Resonance Imaging).
The head structures are extracted from the individual image sections.
tions with the optimum z spacing (Δz)
will be necessary.
One application in light and stereo microscopy is the acquisition of z stacks for
overcoming the limited depths of field of
a single frame. This is where the focal
change (Δz) between two sections should
be the same or somewhat smaller than
the depth of field. As described above,
the depth of field depends mainly on the
numerical aperture of the objective. In
practice, a usable Δz value can be easily
derived by lifting the stage slowly. At the
same time, you observe at which focal
change a sharp structure goes out of focus. You apply this value to all the sections over the whole height range of your
sample (fig. 56). With digital imaging, the
focus parameters can be entered; the
software takes control of the motorised
focus and acquires the z stack. The focused areas through the sections can
then be composed to generate an image
which has an unlimited depth of field in
principle (fig. 56). The method referred
to is called EFI (Extended Focal Imaging). (See also box 6)
In addition to the resulting image (fig.
56) with an unlimited depth of field, a
height map can be generated (fig. 57).
The software algorithm knows which
section to obtain the sharp pixels from to
compose the resulting image. This information is used to create an image where
each grey value represents a specific
height. This height map contains as many
grey value levels as sections have been
acquired. This means that the smaller
the depth of field of your objective and
the more sections actually acquired, the
more exact the height information will
be. This is significant when the height information is represented in a three-dimensional view (fig. 57). When a sufficient number of sections are acquired,
the viewer receives a clear spatial impression of the surface. To smoothen the
height map, peaks and steps may be flattened somewhat. The three-dimensional
view becomes quite a realistic representation of the actual object structure when
the sharp, composite image is placed
over the surface as a texture (fig. 57).
Just the right number
In life science applications and fluorescence microscopy, the appropriate
number of sections for three-dimensional
imaging can also be derived from the experimental set-up. It is well known that
out-of-focus haze can significantly diminish fluorescence images‘ scientific value,
as well as the apparent clarity in fluores-
cence images acquired in standard fluorescence widefield microscopes. There
are two main techniques used to remove
this out-of-focus blur and to restore the
images: the deconvolution computational
technique and confocal microscopy. One
of these approaches is desirable to create a reasonable three-dimensional representation of the fluorescence signals.
Deconvolution requires a three-dimensional image stack of the observed
sample, having a minimum focal change
(Δz) for obtaining maximum effect. The
focal change (Δz) depends on the numerical aperture (NA) of the objective, the
sample fluorochrome’s emission wavelength (λ) and the refractive index (n) of
the immersion medium, and can be calculated using the following equation:
Δz ~ (1.4*λ*n)/NA2
(7)
To get a better idea of the actual numbers, an example will be calculated. A
Plan Fluorite 40x objective has an NA
value of 0.75. The refractive index of air
is 1. At 540 nm emission wavelength, this
yields a minimum required focal change,
i.e. z spacing (Δz) of about 1.3 µm. The
numerical aperture mostly influences the
z spacing. When using a higher quality
Plan Apochromat objective with a numerical aperture (NA) of 0.9, a z spacing
(Δz) of about 0.9 µm is required. When
observing cells, a height range of 5 to 10
µm must be covered depending on cell
type and preparation. So when using a
Plan Apochromat 40x objective, up to 10
sections or frames should be acquired.
Fig. 58 shows an example.
Software solutions available today offer either definition of the numerical aperture and the refractive index. With
fully motorised systems, the software
also reads out the numerical aperture
and the refractive index of the current
objective. When the fluorochrome is selected, the z spacing is automatically calculated. You define the maximum and
Fig. 56:
Left and middle: This is an eye of a beetle acquired at two different focal planes. The focal change (Δz) is such that no structure between these focal planes
is not sharply in focus.
Right: Seven optical sections acquired at different focal planes are combined to create one sharp image with unlimited depth of field.
44 • Basics of light Microscopy & Imaging
3D Imaging
Fig. 57: Left: Height map. Middle and right: Three-dimensional view of the surface. The surface shape can be textured with the composite image.
the minimum lift of the stage and the z
stack is acquired.
The deconvolution filter can be directly applied to the z stack image object.
The most powerful deconvolution filter is
called blind deconvolution. It does not
assume any prior knowledge of the system or sample. Instead, it uses elaborate
and time consuming mathematical iterative approaches. These include the constrained maximum likelihood estimation
method to adapt to the actual three-dimensional point spread function (PSF) of
the system (for a more detailed description see the paragraph „What exactly is a
point spread function“ in the chapter
„The Resolving Power“). Knowing the
three-dimensional PSF of a system, the
blind deconvolution takes the blurred z
stack and essentially provides a three-dimensional image restoration of the sample (fig. 58, 60, 62). Deconvolution algorithms are applied to a lesser degree in
confocal microscopy image stacks and to
images from transmitted light widefield
microscopy as well.
The confocal view
The second way to create optical sections
in a fast and high resolution manner is
via confocal microscopy. Here, a light
beam, usually a laser, is concentrated on
a small area of the specimen. The light
originating from this spot (emission light
of the fluorescence or reflected light of
opaque materials) is captured by a sensitive detector. Within the confocal optics,
an aperture (usually a pin hole) is placed
at a position which is optically conjugated with the focusing position. This
conjugated focal position set-up is why
the term “confocal” is used (see also
Box 4). The system is meant to eliminate
light from all areas other than that on
the focal plane. Therefore, while scanning along x, y axes, an optical section of
the focused area can be obtained without
requiring software algorithms.
3D Imaging One for all
Today‘s software generally handles the z
stack as one object. All sections and
frames are saved in a single file. The information about the single frames’ x-y
resolution and calibration, as well as the
z spacing and the other meta-data such
as the channel (fluorochrome and wavelength), point in time acquired, numerical aperture and refractive index are
saved in the same file as well. The single
layers can be viewed separately or extracted. A quick way to display all the
significant structures through the whole
stack is the maximum intensity projection. This is where the brightest pixel
value at each x-y position throughout all
layers is shown in the projection image
(fig. 58, 60, 62). Other projection methods are mean intensity and minimum intensity projections.
Projection methods have to be used
with care because the lack of out-of-focus blur shows all details from the various z-levels as if they were all located on
the same horizontal plane – which is obviously not the case. For readers viewing
such images in a scientific journal, a
seemingly satisfactory multicolour confocal two-dimensional image will in fact
reveal less valid information than a “normal” widefield fluorescent image. Another way of presenting stacks is by animating them – like watching a movie.
Image objects are truly multi-dimensional. In addition to the third axis in
space, they may also contain different
colour channels of multiply labelled fluorescent specimens – the fourth dimension (fig. 59, 60 and 62). Image objects
may also be comprised of a fifth dimension – time: where slices are acquired at
different points in time.
It‘s all spatial
Z stacks and multidimensional images
can be visualised as three-dimensional
objects via three-dimensional volume
rendering, or voxel viewing. Voxel stands
for Volume Element. Fig. 61 shows the
correspondence between pixel and voxel.
A voxel is a pixel which has a height value
additionally assigned to it. The height assigned corresponds to the z spacing of the
frames. In this way, voxels are reconstructed from the frames of the z stack.
Fig. 58: Human breast cancer tissue labelled with a probe against the HER2-gene (Texas Red – not
shown here, see fig. 62) and Chromosome 17 (FITC).
Uppler left: Maximum intensity projection of 25 frames, l(FITC) = 518 nm, numerical aperture NA = 1,
z spacing (Δz) = 0.18.
Lower Left: The same z stack after applying the blind deconvolution algorithm. Right: The three-dimensional slice view gives a good impression of the proportions and helps to spatially locate the fluorescence signals.
Basics of light Microscopy & Imaging • 45
Each voxel of a three-dimensional object has a colour. There are several wellknown methods for volume rendering,
such as projection and ray casting. These
approaches project the three-dimensional
object onto the two-dimensional viewing
plane. The correct volume generating
method guarantees that more than just
the outer surface is shown and that the
three-dimensional object is not displayed
simply as a massive, contourless block.
Inner structures within a three-dimensional object, such as fluorescence signals can be visualised. The maximum intensity projection method looks for the
brightest voxel along the projection line
and displays only this voxel in the two-dimensional view. Fig. 62 shows the maximum intensity projection of two colour
channels of 25 frames each when seen
from two different angles. Changing the
perspective helps to clearly locate the fluorescence signals spatially. The three-dimensional structure will appear more realistic when the structure rendered into
three-dimensions is rotated smoothly.
How many are there – One or Two?
Managing several colour channels (each
having many z layers) within one image
object can be deceptive. The question is
whether two structures appearing to
overlap are actually located at the same
position spatially or whether one is in fact
behind the other. The thing is, two fluorescence signals may overlap – because
their mutual positions are close to each
other within the specimen. This phenomenon is called colocalisation. It is encountered when multi-labelled molecules bind
to targets that are found in very close or
spatially identical locations.
Volume rendering makes it possible to
locate colocalised structures in space visually. For better representation and in
order to measure colocalisation in digital
image objects, the different fluorescence
signals are first distinguished from the
background. This may be done via
Fig. 59: Confocal images of pollen. The upper rows show the first 12 images of a series of 114, that
can be used to create either two-dimensional projections of parts of the pollen or create a 3D view of
the surface structure. This three-dimensional projection shown here is accompanied by two-dimensional projections as if the pollen was being viewed from different perspectives. Images were created
using the Olympus Fluoview 1000.
threshold setting in each of the colour
channels. A new image object can be created with only those structures that were
highlighted by the thresholds. The colocalised fluorescence signals are displayed
in a different colour. Rendering these image objects clearly reveals the colocalised signals without any background disturbance. To obtain quantitative results,
the area fraction of the colocalised signals compared to the total area of each
of the two fluorescence signals can be
calculated throughout all image sections
(see table 4).
Table 4:
Layer
Red
Area[%]
Green
Area[%]
Colocalisation
Area[%]
Colocalisation/Red Colocalisation/Green
Ratio[%]
Ratio[%]
1.00
2.00
3.00
4.00
5.00
6.00
7.00
0.11
0.15
0.20
0.26
0.35
0.43
0.38
0.20
0.26
0.36
0.53
1.34
2.07
1.96
0.04
0.07
0.10
0.13
0.20
0.26
0.24
39.60
47.76
49.93
51.84
57.39
61.00
61.66
21.22
27.92
28.08
25.32
14.87
12.81
12.07
Colocalisation sheet: A ratio value of about 52 % in the Colocalisation/Red column in layer 4 means
that 52 % of red fluorescence signals are colocalised with green fluorescent structures here.
46 • Basics of light Microscopy & Imaging
3-D reconstructions
Now let us draw the net wider and move
for a minute beyond the light microscopy
approaches in imaging three-dimensional objects. In general. determining
three-dimensional structures from macro
to atomic level is a key focus for current
biochemical research. Basic biological
processes, such as DNA metabolism, photosynthesis or protein synthesis require
the coordinated actions of a large number
of components. Understanding the threedimensional organisation of these components, as well as their detailed atomic
structures, is crucial to being able to interpret what their function is.
In the materials science field, devices
have to actually „see“ to be able to measure three-dimensional features within
materials, no matter how this is done:
whether structurally, mechanically, electrically or via performance. However, as
the relevant structural units have now
moved to dimensions less than a few
hundred nanometres, obtaining this
three-dimensional data means new
methods have had to be developed in order to be able to display and analyse
3D Imaging
Fig. 60: Ovariole from Drosophila melanogaster
mutant bgcn. Multicolour channel and z stack
images can be managed in one image object.
This image object contains three colour channels
each consisting of a 41 frame z stack. This image
is a maximum intensity projection.
Red: Fascilin III, Green: F-Actin, Blue: Nuclei.
Above right: Single z layer after applying the
blind deconvolution algorithm to all three channels of the image object. Left: The three-dimensional, volume-rendered structure.
Image courtesy of: Dr. Ralph Rübsam, Institute
for Biology, University of Erlangen-Nürnberg,
Erlangen, Germany.
three-dimensional structures. Among
these methods, different 3-D reconstruction approaches have become a useful
tool in various applications in both the
life and materials sciences. They result
in three-dimensional displays which are
particularly illustrative and instructive.
Therefore, we now take a closer look at
two of these 3-D reconstruction methods,
one using an image series of specimen
sections acquired in a light microscope,
the second using electron tomography in
a transmission electron microscope.
3-D Reconstruction using an image series of
specimen sections
One commonly used method when dealing with 3-D reconstruction is to make a
series of ultra-thin slices of the specimen
being investigated. These slices are then
dyed. The dye makes structures more
easily recognisable and/or highlights specific parts. The slices are viewed one at a
time beneath the microscope and drawn.
These drawings are then enlarged to
scale. In the past, a 3-D model was reconstructed layer by layer from pieces of
cardboard cut according to the drawings.
Alternatively, wax or plastic was used to
3D Imaging make the model. Today, this is greatly
simplified by the computer assembling
the virtual model. Special 3-D programmes calculate the three-dimensional model based on digitised image
slices. This model is referred to as a 3-D
reconstruction.
Today‘s powerful software offers 3-D
processing, display and evaluation of
two-dimensional image information. Ergonomic functions and flexible project
management provide the user with the
ability to reconstruct even the most complex models, and allow easy access to
spatial views of the interiors of such
structures. Up until now, these interior
views were extremely difficult to generate. Box 16 shows the necessary steps of
3-D reconstruction, such as those used
for displaying embryonic growth processes in three dimensions. This approach
is attractive for research conducted in
other areas as well – such as the materials sciences.
3-D reconstruction by tilting the specimen
Tomography is another way of reconstructing the three dimensional (3-D)
structure of a specimen from a series of
two dimensional (2-D) micrographs. Some
sample tomography applications include
not only those for electron microscopy
(called electron tomography) but also
* eucentric is derived from the Latin for well-centered.
It implies, e.g., in a goniometer and especially in a specimen
stage, that the specimen can be tilted without moving the
field of view. This capability is highly desirable for microscope stages.
Fig. 61: This is how a two-dimensional pixel (picture element) is expanded to a three-dimensional
voxel (volume element).
Basics of light Microscopy & Imaging • 47
Fig. 62: Human breast cancer tissue labelled with
a probe against the HER2-gene (Texas Red) and
Chromosome 17 (FITC).
Upper left: Maximum intensity projection of 25
frames, λ(FITC) = 518 nm, numerical aperture
(NA) = 1, z spacing (Δz) = 0.18.
Upper right: Blind deconvolution applied to both
channels. Left: Maximum intensity projection.
When the volume-rendered structure is seen
from two different angles, the spatial composition of the fluorescence signals is shown.
computerised axial tomography, (CAT) –
familiar to many in medical applications
as Computerised Tomography (CT).
Computer tomography has long been
a part of the standard repertoire for routine medical applications. Transmission
electron tomography, however, has
grown increasingly important as well in
the last few years. Transmission electron
tomography made possible what had
seemed impossible in biology: 3-D imaging of interior cellular structures, just
nanometres in size. This has become a
reality due to a combination of new tech-
48 • Basics of light Microscopy & Imaging
nologies – suitably equipped electron microscopes, high resolving and highly sensitive CCD cameras, increased computer
processor performance and improved
software.
The way in which electron tomography works is actually quite straightforward (see box 17). The specimen holder
is tilted along an eucentric* axis (if possible) and in the process, images are acquired from a wide range of angles. This
guarantees that the 3-D structure will
have adequate resolution along the three
axes. Usually the specimen is tilted along
a single axis from at least – 60° to + 60°
with images (projections) recorded at
equidistant increments of 1° or 2°. The
images are acquired using a CCD camera
and stored as a tilt stack. Due to effects
such as sample shift, tilt of axis position
or thermal expansion, the tilt stack has
to be aligned before further processing
can begin. Once the stack has been
aligned, it is ready to be used for reconstruction. Reconstruction can be performed using different approaches. Two
3D Imaging
Box 16: The steps involved in 3-D reconstruction using an image series of specimen sections
In the past, pieces of cardboard cut out by
hand had to be glued together to make a 3-D
model. Today this is all taken care of by the
computer. First, the software digitally acquires
microscope images of the series of image slices
(or drawings are scanned into the computer, as
in this case). The software then stacks these
digitised slices. Any displacements or misalignments can be corrected interactively or semiautomatically.
The contours of a tissue structure can be
traced with the cursor or are automatically
detected following a click of the ‚magic wand‘
tool (a local threshold approach). The software
then reconstructs the contours as polygons.
This is a scanned drawing done at a stereo
microscope. The procedure is exactly the same
when using digital microscope images.
The software joins the two-dimensional polygons of the component layers to form spatial,
three-dimensional structures. This is undertaken automatically. If necessary, manual
corrections can be made. Using navigation
tools, the object can be enlarged, reduced and
rotated spatially in any direction.
This is a complete reconstruction of the skull of
a quail embryo. To be able to investigate and
measure precisely it is extremely important to
be able to look inside the model. This is
achieved by making individual parts transparent or fading them from view entirely.
Virtual specimen preparation: the digitally reconstructed skull of a quail embryo can be reduced to
its component parts at any time. Virtual slices can also be generated in any direction.
Distance, polygonal length, area, volume and
absolute position can all be measured directly
within the 3-D view.
approaches available are called |w| filtering and (un-)weighted back projection
via FFT. The resulting reconstructed image is stored and visualised as a z stack.
Box 17 explains the steps necessary for
the electron tomography process.
The more dimensions – the more there
is to see
Many image-analysis methods, especially
two- and three-dimensional approaches,
have become well established and are
3D Imaging applied on a daily basis in both industrial
and research applications. Innovative
procedures involving even more dimensions with applications in the Life Sciences have also become available and
are certain to become just as well established in the foreseeable future.
Wavelength is viewed as a fourth dimension in the field of fluorescence microscopy, as described above. Being able
to differentiate wavelengths provides additional structural information. We have
seen many new applications using this
technique. How the object being investigated changes over time is also of decisive importance in the Life Sciences. One
particular reason for this is that this information may be crucial for assessing
cell culture growth or for assessing how
a fluorescent-labelled molecule behaves
over time (see the paragraph „Fluorescence Lifetime Imaging Microscopy“ in
the chapter „Shining Fluorescence Details“). In addition, there is further information which can be combined with x, y
information efficiently. One example is
Basics of light Microscopy & Imaging • 49
Box 17: The steps involved in 3-D reconstruction via Electron Tomography (TEM).
Data collection for automated electron tomography involves a complex setup of TEM, CCD camera and a
data gathering and processing system. Usually assembling the data would require a tilt of +/–90°. However, due to the geometry of the sample holder, or the thickness of the sample itself, the range of rotations
is limited. This means that in practice, tilt angles ranging from –70° to +70° are recorded at equidistant increments. The image shows only 6 of at least 144 acquired images of the complete image stack. (Image
courtesy by Oleg Abrosimov, University of Nowosibirsk, Russia.)
A key feature offered by automated electron tomography is automatic alignment of the tilt series. Various
algorithms can be used. The rigid axis is calculated using information such as magnification, tilt range and
increment. The images are then stored in an aligned tilt stack.
Special reconstruction routines are used to generate a z-stack based on the data contained by the tilt
stack.
Using visualising tools, 3-dimensional views inside the specimen becomes possible. The sequences can be
animated and stored as files in .AVI format. The image shows 4 different views clipped from one .avi file.
energy loss. Energy loss plays a key role
in fluorescence applications such as
FRET (see the paragraph „Light as a
Ruler“ in the chapter „Shining Fluorescence Details“). It is also used in several
electron microscopy methods. Let’s have
a look at two of these methods: X-Ray
Microanalysis
and
Energy-Filtering
Transmission
Electron
Microscopy
(EFTEM).
Combining morphological and
­chemical data
X-rays can be described as electromagnetic radiation with wavelengths ranging
from 10–9 to 10–11 m or energy in the 10
eV – 100 keV range, respectively. X-rays
are employed for element analysis in the
field of biology, as well as in materials research. Characteristic X-rays and radiative deceleration of electrons within matter are recorded using special detectors.
After correction for absorption, atomic
weight and fluorescence, each sample
section shows a characteristic spectrum.
By scanning the sample and simultaneous recording of the X-ray signals, an el50 • Basics of light Microscopy & Imaging
ement distribution image can be recorded and visualised. Using image
evaluation of these acquisitions, additional information becomes available
which has previously been unattainable.
This technique is in widespread use and
referred to as EDX (Energy Dispersive Xray Analysis) (fig. 63).
Combining digital image analysis with
X-ray microanalysis makes it feasible to
study and characterise the properties of
new materials. This involves examining
macroscopic material properties in relation to the microstructure. One application where this is essential is for improving and developing aluminium alloys,
products and fabrication processes. To
obtain an analysis that is statistically-reliable and representative requires a large
number of samples. A TEM (Transmission Electron Microscope) with an integrated scanning mode can be used for
analysing particles within the 10–300 nm
range. This ensures that local resolution
along with sufficient electron intensity is
as required. The software controls and
automates the entire acquisition and
analysis process. This includes simulta-
neous TEM and EDX analyser control, as
well as analysis of the images acquired.
The morphological data obtained within
the context of the analysis is saved and
displayed using tables and graphs. Subsequently, the automatic acquisition of
the EDX spectra – at the centre of the
particles being investigated – takes place.
Data is also saved and quantified automatically. The proportion of the exudation volume can then be determined in
conjunction with a layer-thickness measurement. These results can provide researchers with important micro-structural information (fig. 64).
Combining local data with energy loss
data
The capacity for analysing multidimensional images is becoming more and
more relevant. This includes image series of energy-filtering electron microscopy. EFTEM (energy-filtering TEM) image contrast occurs primarily due to the
electron scattering within the sample –
as is the case with the conventional TEM
(box 18). Whereas conventional TEM
3D Imaging
Box 18: How is an elemental map of iron generated?
EFTEM stands for Energy-Filtering Transmission Electron Microscope. The electron
beam of the EFTEM is transmitted through
the sample (here, an ultra-thin section of
brain tissue) and generates a magnified
image of the sample in the imaging plane.
The built-in energy filter makes it possible
to generate electron-spectroscopic images (ESI). If the energy filter is set to
725 eV, for example, only those electrons
reach the imaging plane that have an energy loss of 725 eV (while travelling through the sample). All other
electrons are blocked by the energy filter.
725 eV is the characteristic energy loss of an electron from the beam when it ‚shoots past‘ an iron atom at
close proximity and causes a specific kind of inner-shell transition. The energy filter allows such electrons to
pass through. These electrons contribute to image generation in the imaging plane. Unfortunately, other
electrons manage to make it through as well. These also contribute to the image. These electrons have, however, coincidentally lost 725 eV – due to getting ‚knocked about‘, i.e., multiple impacts occurring while the
electrons pass through the sample. Without these coincidental electrons, the ESI image at 725 eV would be a
straightforward elemental map of iron. This means that the image would show how iron is distributed
throughout the sample.
The elemental map can be calculated using multiple ESI images. Using what is known as the 3-window
method, 3 ESI images are acquired. The first one is at 725 eV, the second and third ones at somewhat lower
energy losses. The second and third images are used to compute a ‚background image‘. This is then subtracted from the first image. This is how any background interference is removed from image number one
(made up of all the contrast components caused by the coincidental electrons referred to above). The result
is the desired elemental map of iron. This can now be used for making quantitative measurements.
Iron apparently plays a significant role in Parkinson‘s disease. Using an electron microscope, it can be shown
how iron concentration has increased tremendously in the diseased part of the midbrain. The image on the
left is an electron-spectroscopical image of an ultra-thin section taken from the substantia nigra** of a Parkinson‘s patient (inverted display). The dark areas represent the neuromelanin. The image on the right is the
elemental map of iron computed using the 3-window method. The light areas show where iron has accumulated. The quantitative evaluation in the example shown above indicates a concentration of iron that is three
times as high as what it would be in the brain of a healthy person. (A LEO 912 AB EFTEM was used (acceleration voltage of 120 kV, electron energy loss of 725 eV) along with analySIS TEM EsiVision image-analytical
software. (by Prof. N. Nagai and Dr. N. Nagaoka))
**substantia nigra: is located in the midbrain and is a layer of large pigmented nerve cells. These cells produce dopamine and their destruction is
associated with Parkinson‘s disease.
Fig. 64: a) Acquisition of an aluminium exudation, b) image following detection and classification. Chemical analysis is conducted following
morphological evaluation.
contrast is caused by selecting the angles
of the scattered electrons, the electrons
within the EFTEM undergo an additional
energy selection: using a spectrometer.
Only electrons of a certain energy loss
are selected (from the spectrum of the
transmitted electrons) – the contrast-reducing portions are not visible. This
means that only those electrons with a
specific energy loss contribute to the acquisition. Thus the result offers enhanced
contrast for all types of acquisition. Because this method does not affect local
resolution, thin samples and those with
no contrast, as well as frozen or unconventionally thick samples can be acquired with excellent contrast. Furthermore, this approach enables one to also
select electrons with very specific scattering behaviour, thus yielding structural
or element-specific contrast within the
image. Images such as these provide
new information on the sample that used
to be inaccessible.
Control and remote control of the electron microscope for acquiring, displaying
and analysing the images and spectrums
is undertaken by special software. Much
of the information can only be obtained
via computer-supported processing and
analysis. This method is used in the biomedical field, for example to demonstrate the presence of mineral particles
in human lung tissue or when investigating diseases involving excessive accumulation of iron. This method has become
greatly prevalent in the materials sciences as well: e.g., primary influences on
the mechanical properties of steel – i.e.,
secondary phases, such as exudations or
grain-boundary phases (carbide, nitride
or also intermetallic phases). Determining the exact chemical composition, size,
distribution and density is most important. This information can be obtained
via the EFTEM method. Conventional
methods are useless in this context.
Fig. 63: Typical EDX spectrum.
3D Imaging Basics of light Microscopy & Imaging • 51
INCREDIBLY SIMPLE.
YOU JUST HAVE TO THINK OF IT : OLYMPUS ANALYSIS
IMAGING SOFTWARE.
At the end of the day, it’s results that count. Anyone measuring
and testing on a daily basis wants results directly, simply and quickly.
And this is exactly what Olympus now offers with its analySIS
imaging software for microscopic applications. Whether recording and
analysing data or archiving and reporting, analySIS simplifies your
daily procedures reliably, accurately and objectively. And the software
is so user-friendly and intuitive that you feel at home with it straight
away. Once configured for your application, analySIS delivers the
results you need with just one mouse click. In fact, it’s so simple that
you will wonder why you didn’t think of using it before. Olympus
helps you solve problems easily.
For more information, contact:
Olympus Life and Material Science Europa GmbH
Phone: +49 40 2 37 73 54 26
E-mail: [email protected]
www.olympus-europa.com
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Basics of
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Imaging
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SPECIAL EDITION OF
Imaging
&Microscopy
RESEARCH • DEVELOPMENT • PRODUCTION
www.gitverlag.com
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