Instruments for Color Measurement

Instruments for Color Measurement
INSTRUMENTS FOR THE MEASUREMENT OF COLOR
By Dan Randall
Datacolor International
Charlotte, NC
Introduction
solution up to a light and judging whether they are of
equal strength and shade, except that the absorptiometer
provided a method of adjusting the thickness or path
width so that this change in width could be read from a
scale.
In measuring reflected light from opaque
materials such as textiles, the first instruments were
reflectometers developed around 1915-1920.
These
early instruments were designed to closely simulate the
visual process as depicted in Fig. 1
Color is defined as “ the sensation experienced or
caused by light reflected from or transmitted through
objects”. In the strict sense, we cannot directly measure
perceived color, however we can measure and
subsequently calculate certain factors which are
responsible for producing this sensation of color. The
quantification of the color properties of textile materials
is of great economic value in industry and instruments
are employed to some degree in almost every textile
operation involved in textile coloration.
Color instrumentation has experienced a tremendous
advancement in technology during the past 40 years. In
the previous publication of Color Technology in the
Textile Industry, Roland Derby1 stated that in 1954 there
were no more than 10 instruments in common use in the
textile industry. Today there are several thousand in use
throughout North America alone as these instruments
have become indispensable in areas such as quality
control of processes, quality assurance of finished
products, color formulation, and color sorting of piece
shipments.
eye
lamp
Object
During this 40 year growth in color measurement, the
instruments have become more accurate, reliable,
flexible, smaller, and faster than their predecessors, at
significantly lower cost to the user.
The variety of
designs and features available to the prospective buyer
can be overwhelming.
This paper is written and
dedicated to providing a series of practical guidelines for
the technician, colorist, or manager, to better understand
the basics of color instrumentation with the hopes that
the instrument chosen will meet the requirements
demanded by the task.
Fig. 1 Visual Process requires light source, object, and a
receptor (eye)
Historical Perspective
Three colored filters, Red, Green, and Blue were used to
directly measure three Reflectance factors which ideally
fit the CIE Standard Observer functions for a given
Standard Illuminant, usually Daylight C or D65. Soon
afterwards, the reflectometer was further refined to
provide output of the tristimulus values X,Y,Z at which
point it became known as a Tristimulus Colorimeter.
One of the most widely used colorimeters in textiles is
the Hunterlab D-25 Color Difference Meter.
The first devices for measuring color were
absorptiometers which were used to determine by visual
inspection whether two solutions were of equal color.
This is very similar to holding two glass cylinders of dye
The beginning of reflectance spectrophotometry dates
back to 1928 when A.C. Hardy2 , Professor of Optics at
M.I.T. began a project to produce the first
spectrophotometer specifically for the measurement of
1
reflectance. Commercialization of Hardy’s design began
in 1935 when General Electric introduced the General
Electric Recording Spectrophotometer (GERS). This
instrument became the much needed reference
spectrophotometer and provided the basis for modern
industrial color measurement.
The elements of the
original “Hardy” are essentially the same as those used
today even though dramatic changes have taken place in
their design. As in the visual model , the basic elements
of the spectrophotometer are the light source, the object
being measured, a means of dispersing the light, and a
detection system as shown in Fig. 2.
Applications for Colorimeters
Since colorimeters are overall simpler to build than
spectrophotometers, they are usually lower in cost. As
such, they are commonly used as quality control
instruments in applications such as color difference,
strength determination, fastness determination, shade
sorting, to name a few.
The cost advantage and
simplicity must however be weighed against several
serious disadvantages. Firstly, the colorimeter measures
the tri-stimulus values for one illuminant and one
observer. As such, it is not possible to detect and
quantify metamerism. Thus in practice, the colorimeter
is used in areas where the standard and the measured
batch are non-metameric such as in checking production
batches against a production standard made with the
same dyes.
lamp
detector
slit
X
Y
Z
lamp
prism
photo-detectors
a
r
g
object
b
filters
45
Fig. 2 Key components of a spectrophotometer - lamp,
object, dispersing mechanism, and detector
sample
Colorimeters
.
As the first measuring devices, colorimeters were crucial
in the development of the science of color, or
colorimetry. A colorimeter is a device of fairly simple
design based upon the visual concepts of color. The
sample is illuminated at a 45° angle relative to the
perpendicular line to the plane of the mounted sample.
The reflected light is measured directly perpendicular to
the sample through a series of three and sometimes four
colored filters which represent the relative amounts of
red, green, and blue light reflected from the sample.
More specifically these filters are designed to ideally
simulate the three functions, x,y,z for the Standard
Observer so that the instrument directly measures the
three Tristimulus values X,Y,Z for the specific illuminant
being used. This design is shown in Fig 3.
Fig. 3 Diagram of tristimulus colorimeter
Spectrocolorimeters
A spectrocolorimeter is somewhat of a hybrid instrument
which is capable of providing colorimetric data such as
X,Y,Z or CIEL*a*b* values for various standard
illuminants. In this regard, they are more capable qualtiy
control instruments than colorimeters.
They are
generally priced only slightly higher than tri-stimulus
colorimeters, but less than most spectrophotometers.
This price differential has been practically eliminated in
today’s spectrophotometers, and as a result the spectrocolorimeter does not enjoy the niche between the two.
The spectrocolorimeter is by design, a spectrophotometer
except that it does not output spectral data (%R) at the
2
various wavelengths. These instruments are almost
exclusively used for applications of quality control.
Spectrophotometers
ref. spectrometer
lamp
Spectrophotometers differ from colorimeters in that they
measure reflectance, transmittance, or absorbance for
various wavelengths in the spectrum. In the case of
reflectance measurement, the quantity measured is
termed Reflectance Factor and is defined as the
reflectance of the sample at a given wavelength
compared to the reflectance of the perfect diffuse white
measured under the exact same conditions. This is
expressed in the following equation:
RF(λ) =
mirror
reference beam
specular port
computer
sample beam
sphere
transmission
cell
sample
sample spectrometer
R(λ) (sample) / R(λ)(pwd)
Commonly expressed as a percentage, %R, the
reflectance factors are usually referred to as simply %
Reflectance.
In the measurement of transparent
materials such as dye solutions and films, the quantity
measured is Transmittance, usually expressed as %T.
This quantity is equal to the percentage of light, at a
given wavelength, transmitted through a given thickness,
usually 10mm, of the sample compared to the light
transmitted through the same path without the absorbing
sample in place. This may be written as:
Fig. 4 - Block Diagram of dual-beam spectrophotometer
Since reflectance curves are relatively smooth, it is
generally agreed that for most applications it is not
necessary to measure at 1 nm increments. For this
reason, most modern reflectance instruments measure a
band of a certain bandwidth which may be 5-20nm in
width. Instruments of this type are referred to as
abridged spectrophotometers.
Instrument Geometry
%T(λ) =
T(λ) (sample) / T(λ)(reference) x 100
The C.I.E.3 specified four geometric arrangements for
instruments used to measure color. These are (a) 0/45
(b) 45/0 (c) 0/Diffuse and (d) Diffuse/0 as shown in Fig
5.
In practice, the %T(ref) is measured by standardizing the
instrument with only the solvent in the glass cell or
cuvette.
When using reflectance instruments for
transmittance measurements, it is essential that the
reflectance port be covered with the white standard.
Designs of Spectrophotometers
45
All spectrophotometers must have certain key
components - Light source,
method of spectral
separation or dispersion, and a detection system. As a
fourth component, most all instruments have a microprocessor on board for data handling and computations.
The positioning of these elements and their mode of
operation determines the optical geometry of the
instrument as shown in Fig. 4. Many of the earlier
reflectance spectrophotometers such as the Hardy were
designed in a similar fashion to UV/VIS absorbance
spectrophotometers used for chemical analysis of liquids
in that they employed a scanning mechanism. This
provided wavelength by wavelength measurement and
data collection at each 1nm or lower if desired.
Although extremely accurate, these instruments were
slow, mechanical, and expensive.
(a) 0 / 45
(c) 0 / diffuse
(b) 45 / 0
(d) diffuse / 0
Fig. 5 Recommended C.I.E. Instrument Geometries
3
determination of color change such as fastness and
staining testing. It is often said that a 45/0 instrument
measures not only color difference but also some
attributes of appearance such as surface gloss because of
it’s directional illumination(45/0) or viewing (0/45).
While the instrument does not directly measure these
geometric attributes, it is no doubt more sensitive to
surface texture as is illustrated in the following example:
The first angle given is the angle of illumination relative
to a perpendicular drawn to the plane of the sample to
be measured. This perpendicular is the normal angle, or
0 deg angle. The second angle is the viewing angle
again expressed relative to the normal angle for the
sample being meaured. The term diffuse is used to
indicate that the illumination or viewing is not directional
but is rather diffuse, usually by the use of an integrating
sphere.
Take two samples A and B which are printed on the
same small flat-bed machine. To avoid discussions of
pigment printing density and penetration, we will print
both samples with acid dyes using the same dye mix
however, sample A is printed on a very low gloss (delustered) nylon, whereas sample B is printed on a highly
glossy nylon.
Now a 45/0 instrument will measure a
fairly large color difference (2-3 dE CIELAB) because
the glossy substrate will give much higher reflectances
but lower chroma or saturation.
On a diffuse/0
instrument with the specular component included, the
same samples will show little color difference (< 0.40 dE
CIELAB) because the diffuse illumination creates such
multiple reflectances that the effects of the gloss are
minimized. The question then becomes “ What do you
really want to measure?”.
For this reason, most
instruments for color formulation are diffuse/0 since the
colorist wants to measure strictly color, especially when
standards are often not dyed or printed on the same
substrate as requested for the match. Likewise, in many
inspection areas, it is necessary to verify both the
geometric quality and color, and in these cases a 45/0 or
0/45 will provide the best assessment.
While these are the official C.I.E. recommended
geometries, a great deal of variation is allowed in
commercial instruments. When A.C. Hardy built his
first spectrophotometer he found that the surface texture
of textile samples lead to poor reproducibility in
measurement. As a result, he developed an integrating
sphere which provided diffuse illumination thereby
reducing the variability due to surface texture. In many
45/0 instruments today, this problem has been resolved
by the use of a circumferential “ring” used either in the
illumination or in the detection mode as depicted in Fig
6. This geometry is termed 45/0 Circumferential to
differentiate it from the bi-directional 45/0 geometry.
spectrometer
illumination ring
450
Instruments with Diffuse Geometry
sample
Practically all Diffuse / 0 (or 0/ Diffuse) instruments are
not truly 0 degree instruments, but are closer to 6-8
degrees off from the normal. This is done to allow for
the inclusion of a specular opening within the integrating
sphere. The specular component of reflectance may be
excluded, although not entirely, by allowing a portion of
the gloss to escape through the specular port.
The
efficiency of this specular port is determined by the
overall gloss of the sample and the size of the port
relative to the size of the sphere. Measurements with
sphere instruments are then designated as either Specular
Included (SCI, SPIN) or Specular Excluded (SCE,
SPEX). In textile formulation, the normal mode is to
measure with the specular included, however in cases
where the standard is a glossy paint chip, then better
results are obtained by excluding the specular gloss.
Fig. 6 45/0 Circumferential Geometry
45/0 or 0/45 Instruments
Instruments utilizing 45/0 or the 0/45 were the first to be
developed and are believed to closely represent the
visual viewing conditions, especially in a light cabinet.
There is considerable debate that in most room, office, or
retail environments, the illumination is rarely directional,
but is rather more diffuse.
Instruments with such
directional geometry are most widely used in
applications of quality control such as pass/fail
determination, color difference, shade sorting, or
4
measurement, it has much lower energy in the UV-Violet
region compared to daylight.
This could be a
disadvantage when measuring white samples treated with
fluorescent whitening agents where sufficient UV energy
is needed to excite the fluorescing agent.
For this
reason, some instruments have been fitted with a
secondary UV source such as deuterium to achieve neardaylight illumination.
The integrating sphere may be, in theory, of any
diameter provided the sample port is not more than 10%
of the total area of the sphere. Bench-top instruments
usually have a 3-6” diameter, whereas a portable may use
a sphere as small as two inches. Their sole purpose is to
create illumination which is uniformly diffuse at the
point at which at sample is placed. The inside of the
sphere is coated with multiple layers of Barium Sulfate
which is highly reflective (>90% Reflectance) at all
wavelengths.
Despite it’s high reflectance, Barium
Sulfate is not ideal in that the coating is not extremely
durable and tends to yellow over time. Instruments with
double-beams can compensate for this loss in sphere
efficiency, however re-coating the sphere is advisable
every few years.
Xenon Discharge Lamps
Xenon lamps have been in use since the 1970’s in
instruments made by Kollmorgen (Macbeth), Zeiss, and
Datacolor.
Xenon has many advantages and a few
disadvantages. Among the advantages, xenon is a good
daylight simulator as shown in Fig 7. In the UV region,
un-filtered xenon is much higher than daylight (D65) and
usually requires the use of a UV filter to approximate
daylight. If left un-filtered, xenon may over-excite a
fluorescent material, therefore most all instruments today
use a low wattage xenon lamp, or provide a means of
filtering the UV portion (360-400nm).
Light Sources in Instruments
For non-fluorescent materials, the reflectance factors are
independent of the illumination (lamp) since they are
ratios to the reflectance of the perfect white diffuser
(PWD) under the exact same illumination. The only
requirement is that the lamp possess sufficient radiant
energy throughout the visible spectrum. There are in
general two types of lamps used in instruments - tungsten
filament and xenon discharge lamps.
The early
instruments used tungsten filament, usually filtered to
simulate daylight.
Modern filament lamps are quartz
enveloped with a halogen to provide a very stable and
intense illumination from 400-700nm. This continuous
stable illumination was used extensively in single beam
instruments such as the Hunter D53, and the ACS
Spectro-Sensor. The lamps are very inexpensive but do
not last more than six months under normal conditions.
relative energy
filtered xenon
D65
There are however, some disadvantages with the
tungsten lamps which have contributed to the recent
increase in xenon lamps. Continuous tungsten lamps
create heat and must be cooled. Secondly, the heat and
continuous light exposes the sample which may lead to
variation in sample measurement due to such sensitivity.
The lamps are usually equipped with infra-red absorbing
filters and some models provide a shutter to open only
when measuring. In practice, the user should minimize
the amount of time a sample is exposed at the
measurement port.
For these reasons, some
manufacturers use pulsed tungsten illumination, however
these instruments must also be designed with dual beam
optics to account for illuminant fluctuations. The
spectral distribution of tungsten filament, xenon, and
Daylight are shown in Fig 7. Although tungsten has
adequate energy in the visible spectrum for most color
tungsten
wavelength nm
Fig. 7 Relative Energy Distributions for D65, xenon,
and tungsten filament lamps.
Xenon is an inert gas which when highly charged will
convert the electron build-up to photons, emitting a flash
for a fraction of a second. A sample being measured is
therefore not exposed to continuous light, nor is there
any heat to dissipate. Although the lamp is intense, it is
not as spectrally consistent or stable as a continuous
lamp such as tungsten. For this reason, all instruments
which use xenon must be dual-beam designs.
A
reference beam, usually aimed at a point inside the
integrating sphere, provides a reference measurement
5
be produced at various angles. A ruled grating with
about 300 lines per millimeter will produce a distribution
of visible light suitable for measurement.
against which the sample measurement is adjusted to
account for any change in the illumination.
Light Dispersion - Filters and Gratings
The earliest gratings were of the plane type in that they
were made using flat glass and etched. These gratings
produced a distribution which was detected by placing
photo-diodes along the distribution at certain
bandwidths, usually 10nm or 20nm. The plane grating
has been superseded now by a technique of laser etching
to produce a pattern of grooves in a concave glass
surface. This concave holographic grating has the
advantage of providing both the dispersing and the
collecting mechanisms into a single component. The
dispersed light can then be imaged or projected onto an
array of photo-diodes. The concave grating requires less
optical space and when combined with fiber optics, the
instrument can be made extremely small and lightweight.
This optical design is used in the Datacolor Spectraflash,
Dataflash, and Microflash instruments.
The earliest records of experiments involving the
separation of light into spectral colors were those of
Newton4 in 1730 when he used a prism to separate
sunlight into the seven spectral colors or bands. In
today’s instruments, there are primarily two types of
dispersing elements used - gratings and filters, with
gratings being the most commonly used.
It must first be pointed out that the quality or
performance of most dispersing elements such as filters
and gratings is determined by its ability to separate light
into bands of colors. These bands or spectral distribution
are measured in nanometers across the width of the
individual band (depending upon the detection type) at
the point of detection. The width is determined at 50%
of maximum peak height for the band measured.
Detectors
Just as gratings have improved in performance due to
microprocessor technology, the detector assemblies have
undergone similar revolutionary advancement. While
the later version of the Hardy, and the Diano Match-Scan
utilized the conventional analytical grade photomultiplier tubes (PMT), this highly accurate detector
required the use of a moving slit to bring monochromatic
light to the detector. Most modern instruments use fixed
gratings and an array of photo-diode detectors to achieve
the same purpose, but at a much lower cost of production
and lower cost of maintenance in the long term.
Interference Filters
The interference filter is very common in instruments,
especially those produced during the early rise of
industrial color matching in the 1970’s and 80’s. The
interference filter is mounted as a filter wheel which is
usually rotated by a small electric motor directly in line
with the sample and/or reference beam. This simple
design uses a single photodiode detector which measures
the dispersed light as the filter rotates resulting in bands
of variable width. Most are designed to provide an
average rather than a fixed bandwidth of about 10nm.
Many instruments still in use today are based upon this
interference filter such as the Hunter D53, D54, and the
ACS Spectro-Sensor, and the ACS Chroma-Sensor 5.
Interference filters may also be positioned statically in
sequence to provide the necessary spectral distribution.
The resulting bands are measured with diode array
detectors situated accordingly and are usually 12-15 nm
in bandwidth. Instruments of this type are the X-rite
portable spectrophotometers such as 968, and SP series.
The manufacturing of micro-processors and integrated
circuits has resulted in the development of high quality
photo-diodes built on a single solid state electronic
micro-chip. These silicon based diodes are ideal when
placed in an array across the spectral distribution from a
fixed diffraction grating or filter assembly. This optical
assembly consisting of both grating and detector is
referred to as the spectrometer shown in Figure 8.
Because the optical components are fixed, these
instruments are extremely stable exhibiting very little
short-term or long-term drift in accuracy or precision.
Another advantage is that these gratings and integrated
detectors are highly reproducible. This has resulted in
Diffraction Gratings
The first grating was produced in 1821 by Joseph von
Fraunhofer and is the most common light dispersing
mechanism used today in high performance instruments.
A grating is essentially a glass plane with a large number
of grooves etched or ruled into the surface. When light
strikes this grating, a pattern of diffraction and
interference will cause light of different wavelengths to
6
the life of the lamp or instrument. By always comparing
the standard to the batch, this variation is usually not a
problem provided the lamp has sufficient UV energy to
excite the fluorescence.
DIFFRACTION GRATING
LIGHT FROM
SAMPLE
LIGHT FROM
REFERENCE
DISPERSED LIGHT
(REFERENCE BEAM)
REFERENCE BEAM PHOTODETECTORS
OUTPUT
Many producers of white textiles prefer a more stable or
absolute determination of whiteness. Instruments are
available which are equipped with a filter calibrator for
controlling the ratio of UV to visible output so that the
illumination can approximate the distribution of standard
illuminant D65. The method of calibration is that of
Ganz and Griesser5 and after illuminant calibration, the
instrument is used to measure a more absolute whiteness,
namely the Ganz Whiteness index.
This method of
illumination control is also useful for the measurement of
visible fluorescent materials since the calibrated
illumination leads to improved long-term repeatability.
DISPERSED LIGHT
(SAMPLE BEAM)
SAMPLE BEAM PHOTODETECTORS
OUTPUT
TO
COMPUTER
Fig. 8 Dual Beam Spectrometer with twin detectors
Instruments for Measuring Transmittance
instruments which have excellent absolute agreement.
This agreement between instruments is becoming much
more important in global economies and many textile
manufacturers and retailers are stipulating specific
minimum color tolerances on goods to be shipped.
Most general purpose bench-top instruments have
provision for the measurement of transmitted light as
well as reflectance.
The measurement of dyes in
solution to verify the color quality and strength is the
most common application, although the measurement of
transparent films is also used. Most spectrophotometers
for measuring liquids are designed such that a
transmission cell or cuvet is inserted between the
detector and the integrating sphere as shown in Fig 9.
Instrument Considerations
Although many bench-top instruments are capable of
measuring a diverse range of samples, there are some
special considerations that the user should be aware of
when choosing instruments. Some of these are covered
in the next sections.
lamp
total transmittance
Measurement of White Textiles
regular transmittance
Many textile companies produce white fabrics most of
which is finished with fluorescent whitening agents
(FWA) to achieve the desired bluish-white brightness.
Many indices of whiteness have been developed which
are suitable for measurement using any colorimeter or
spectrophotometer. The AATCC recommended method
is the C.I.E. Whiteness Index adopted as the AATCC
Test Method 110. The method specifically states that
the whiteness indices are relative and that the standard
and batch are measured at about the same time on the
same instrument. The reason for this is that in the case
of fluorescent materials, of which FWA certainly
qualifies, the emitted fluorescence is proportional to the
overall intensity, or absolute number of photons, of the
instrument illumination.
Due to some variation in
lamps, and the aging of such lamps, the whiteness values
for FWA-brightened textiles tend to decrease throughout
to spectrometer
sphere
white tile
cell positions
Fig. 9 Measurement positions for transmittance using
Diffuse / 8 instrument
The standardization for the measurement of tranmittance
is generally performed by placing the white calibration
tile at the sample reflectance port and setting the 100%
transmittance with the solvent only in the cell. The zero
(0%) transmittance is standardized by blocking the lens
7
or detector so that no light is allowed to enter the
detector.
Wavelength Accuracy - the average difference in
nanometers between an instrument’s
working
wavelength scale and the absolute scale as determined by
the spectral emission lines from a discharge lamp.
The measurement of transmittance may be measured in
two ways on sphere type instruments as either the total
transmittance or regular transmittance.
Total
transmittance is measured by placing the cell flush
against the sphere as shown in Fig. 9 . In this way, the
forward as well as side-scattered light is collected by the
detector. In the other mode, the cell is positioned away
from the sphere and closer to the detector. This
measurement excludes all scattering except forward. In
the measurement of transparent dyes in solution, the two
methods yield identical results, however the total
transmittance is most commonly used.
Photometric Accuracy - the accuracy in % Reflectance
of the reflectance scale - usually 0-100% range. This is
usally determined by measuring neutral tiles of known
absolute reflectance.
Performance Specifications
Measurement speed - the time required to measure a
sample including the time of actual data collection and
the processing time to send the corrected data to the
computer.
Guide to Instrument Specifications
Inter-instrument agreement - the average color
difference expressed in either CIELAB dE or CMC dE
between the instrument and a theoretical or real master
instrument. This is usually determined in the factory by
measuring a set of BCRA ceramic tiles on each
instrument and calculating color differences from the
master instrument. The BCRA tiles are suitable working
standards although they are known to be thermochromic
and manufacturers must work within controlled
conditions.
The selection of an instrument for color measurement
can be a rather confusing and time consuming task since
there are many varieties in models with differing features
and options. The following sections are given as a help
to the colorist or lab manager in understanding some of
the terms likely to appear on the technical brochures.
More detailed descriptions of color terminology are
given in ASTM E-284-93a Standard Terminology of
Appearance.6
Repeatability - the color difference obtained when
measuring a stable sample (usually a BCRA ceramic tile)
repeatably on the same instrument, usually over a short
period of time.
Geometry - the angle of illumination / angle of detection
in the optical system of the instrument
Wavelength Range - total range (in nanometers nm) in
which the instrument is capable of measuring, generally
somewhere between 360-750nm, with 400-700 most
common.
Reproducibility - the color difference obtained when
measuring a stable sample (usually a BCRA tile) over a
longer period of time. This term usually includes
variables such as time, operator, and conditions of the
instrument. Some manufacturers report reproducibility
as the color difference obtained on a single standard
when measured on different instruments of the same
model or type.
Bandwidthin
abridged
or
scanning
spectrophotometers, the width of the measured band at
1/2 peak height used as a single point in the calculation
and reporting of reflectance factors.. Bandwidths may
range from 5nm - 20nm and is an important parameter in
achieving good agreement between two instruments.
Instruments for Special Purposes
Spectral Resolution - very similar to bandwidth, but
indicates the actual spectral width being measured but
not necessarily reported as a single point. An instrument
may have diodes placed every 1nm however the data is
integrated for every 10 diodes to give a bandwidth of
10nm but a spectral resolution of 1nm. This term is
sometimes called sampling interval.
There are a variety of other insruments which have been
developed for special purposes such as portability,
continuous on-line measurement, goniophotometers, and
extended wavelength instruments. A more complete
description of instruments for the measurement of
geometric and chromatic attributes of appearance is
given by Hunter and Harold7 .
8
reversible optics also allow for the measurement of
fluorescence by the two-mode method8.
Portable Instruments
The recent advances in integrated electronics and smaller
optical components have lead to another revolution in
color technology - the portable instrument. The variety
in models and geometries are as diverse as in the benchtop models.
Besides being completely portable, their
attraction is that they can meet most quality control
requirements without the use of an accompanying
computer system. Their micro-processors are capable of
calculating color differences, pass/fail, shade sorting,
whiteness, grades of fastness, and many other indices of
color and appearance. Their simplicity and lower cost
relative to bench-tops have resulted in widespread use in
quality inspection areas, retail, fabric and garment
sourcing, and other areas which were essentially not
using instruments and numerical methods previously.
Goniophotometers
This instrument is designed for measuring reflectance at
various and sometimes selectable viewing angles. They
are used in areas where there is surface or internal
scattering which changes the reflectances depending
upon viewing angle. Examples in textiles are pigmented
fibers and pile fabrics.
On-Line Continuous Instruments
Instruments for on-line color monitoring are used in
carpeting and other continous wet processing, and are
finding their way into the inspection area as well. These
instruments are usually quite different from the benchtop models in the lab or dyehouse office. The most
obvious difference is that most measure color without
physical contact with the fabric since they are positioned
above the web from 3 inches to 8 feet.
On-line
instruments must be very robust and capable of
withstanding production environments as well as
measuring while being traversed across the width of the
frame.
While these advantages have provided many users with
the opportunity to now use color instrumentation, one
must be aware of some limitations.
Portable
colorimeters, as with all colorimeters, are not capable of
detecting metamerism. Many portables do not meet the
same performance specifications as bench-top models in
areas such as spectral resolution, bandwidth, and largeto-small viewing areas. Due to their size, many other
features of bench-tops are not available, such as
transmission measurement, and adjustable UV filters.
Another key difference is sample measurement and
presentation, a factor worth considering in textiles.
Every bench-top instrument uses a sample holder such as
a spring loaded plunger or air cylinder which applies a
consistent amount of pressure to the back of the sample.
This reduces the variability in measurement and thus
lowers the number of reads required to achieve
acceptable repeatability. Since most portables are handheld while measuring, the variability in pressure leads to
higher variability in measurement. This is especially
true when measuring fabrics or materials which have
texture, pile, or are multi-layered allowing some
pillowing of the fabric when measuring.
Concerning measurement geometry for on-line, it is not
practical to use an integrating sphere except as a
reference beam. Most instruments are bi-directional and
actual illumination and viewing angles are determined by
the position of the instrument when mounted above the
plane of the fabric. For this reason, it is usually accepted
that on-line measurements will not agree with
measurements made off-line on samples taken to the lab,
unless artificial correlation methods are applied.
On-line measurement offers many advantages in color
control such as real-time data allowing for immediate
adjustment of pad roller pressure to correct for sidecenter-side variation. When used for monitoring, the
system may be linked with a yardage meter and
traversing frame to provide detailed color mapping of the
roll of fabric or carpeting. Since the temperature of the
fabric may be variable as it exits from a dryer, a
pyrometer may be required which will allow for
correlation between production conditions and some
standard environment.
Reversible Optics
This term refers to an instrument which is capable of
measuring in two modes - polychromatic illumination or
monochromatic illumination. The Diano Match-Scan is
the best example of this type and consists of two lamps
and two detectors which allow both diffuse/0 and
0/diffuse geometries within the same instrument. The
0/diffuse mode is useful in measuring dye solutions or
opaque samples which are light sensitive.
The
9
Extended Wavelength Instruments
References
Instruments have been developed for measuring
reflectance and transmittance at wavelengths other than
the visible (400-700 nm).
Reflectance in the near
infrared region of 700-1100 nm is of interest to those
providing textiles for military use such as uniforms,
tents, and vehicle fabrics.
Although not visible, this
reflected light is detectable using infra-red sensitive
photography and filters. Instruments which are capable
of measuring 700-1100nm are equipped with a special
grating or interference filters.
1.
Derby, Roland E. Jr, (1983), Color Technology
in the Textile Industry, AATCC, Research
Triangle Park, NC.
2.
Hardy, Arthur C., (1935), Journal of Optical
Society of America, Vol. 25, p 305.
3.
Commission International E’clairage, (1986),
Colorimetry, Publication 15.2
4.
Newton, Sir Isaac, (1730), OPTICKS, Reprinted
by Dover Publications, New York, N.Y., Fourth
Edition
5.
Griesser, Rolf, (1981), Rev. Prog. Coloration,
Vol 11, p 25-36
6.
American Society for Testing and Materials
(ASTM), Publication ASTM Standards on
Color and Appearance Measurement, (1994)
7.
Harold, Richard and R.S. Hunter (199?), The
Measurement of Appearance, 2nd Edition John
Wiley and Sons, New York, N.Y.
8.
Simon, Fred T., (1972), Journal of Color
Appearance, Volume 1
9.
Johnston-Feller,
Ruth,
(1979),
Color
Technology in the Textile Industry, AATCC,
Research Triangle Park, N.C.
Likewise, those involved in measuring fluorescent
whites may want to measure the near ultraviolet
reflectance or transmittance below 400nm. For liquids,
a precision UV/VIS analytical spectrophotometer is
recommended.
For reflectance measurement below
400nm, some manufacturers provide measurement down
to about 350nm.
If the instrument uses a tungsten
filament lamp, a supplemental UV lamp, such as a
deuterium gas lamp, may be installed to provide the
required output in the UV region.
Conclusion
The words of Ruth Johnston-Feller9 in 1979 still hold
true today in that “a whole generation of instruments is
available” with “far greater speed, better precision,
better short-term repeatability and more flexibility in
application”. These high standards of performance in
instrumentation, the powerful new software programs,
and increased flexibility continue to provide tremendous
tools toward achieving total color control. As a result,
those companies who have invested in color
instrumentation in the past, as well as those who will
invest in the future, are certain to reap the rewards of
effective color measurement and control.
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