|
 Color
 Formation
of the color impression, Standard illuminants, CIE Color
Systems
Visible-region
spectrophotometers
Spherical
based instruments, 0/45, Multi-Angle, Colorimeter
Applications
Hunter
L,a,b, CIELAB, CIELCH, Delta CIELAB and CIELCH, XYZ, Yxy,
CIELAB
Tolerancing, CIELCH Tolerancing, CMC Tolerancing, Visual
Color and Tolerancing, CIE94 Tolerancing,
Visual
Assessment vs. Instrumental, Choosing the Right Tolerance,
White and Yellow Indices
X-Rite
- Optronik's focus: Non-Contact Color Measurement - Teleflash
 Glossary
 top
Color measurement is an indispensable technology for
quality control and quality estimation in color industry.
Color
Light belongs to the electromagnetic waves. Within their
spectrum, the human eye captures visible light in the range
between about 380 nm and 700 nm. In addition to brightness
and darkness perception the eye captures three different
color stimuli: blue, green and red. The color impression
is achieved by addition of these three stimuli in the brain.
From this it follows that any color can be composed by adding
red, green and blue.
Formation of the color impression
The color impression an observer gets from a sample depends
on three interrelated factors.
1. Light source
Different light sources (e.g. daylight, filament lamp) feature
different intensities of their individual spectral components
and therefore produce different color impressions.
2. Sample
The composition of the sample defines the components of
reflection, absorption and refraction and thus the entire
spectral composition of the reflection spectrum.
3. Observer
Different sensitivities of the three light-sensitive receptors
on the retina convey different color impressions with different
observers.
Standard illuminants
For the illumination conditions for color measurement to
be clearly defined, the spectral composition of the light
sources must be known and be included in the measurement
as a constant value. As different fields of application
of a measuring instrument require different illumination
conditions, the spectral composition of some typical light
sources was analyzed and defined as so-called standard illuminants.
Standard illuminant A = standardized filament lamp light
(2856 K)
Standard illuminant C = medium daylight, without UV component
(6750 K)
Standard illuminant D65 = medium daylight, with UV component
(6500 K)
Standard illuminant F2 =CWF
Standard illuminant F11= fluorescent lamp
The D65 standard illuminant is very often used. It corresponds
to the spectral composition of medium daylight and also
includes the corresponding UV component of this light. The
color of objects looks different in different light sources,
therefore the type of light source must always be indicated
CIE Color Systems
The eye has three types of receptors (cones) on the retina.
These differ in their spectral sensitivity. One type reacts
with special sensitivity to red-orange (x), the second type
to green (y) and the third type to blue (z-). This allows
"standard spectral functions" to be assigned to
the eye. In order to vividly represent the individual colors,
the red and green color components x and y are illustrated
in a coordinate system. This illustration is independent
of the color's brightness and shows all possible body colors.
The CIE, or Commission Internationale de l'Eclairage (translated
as the International Commission on Illumination), is the
body responsible for international recommendations for photometry
and colorimetry. In 1931 the CIE standardized color order
systems by specifying the light source (or illuminants),
the observer and the methodology used to derive values for
describing color.
The CIE Color Systems utilize three coordinates to locate
a color in a color space. These color
spaces include:
- CIE XYZ
- CIE L*a*b*
- CIE L*C*h°
To obtain these values, we must understand how they are
calculated. Our eyes need three things to see color: a light
source, an object and an observer/processor. The same must
be true for instruments to see color. Color measurement
instruments receive color the same way our eyes do - by
gathering and filtering the wavelengths of light reflected
from an object. The instrument perceives the reflected light
wavelengths as numeric values. These values are recorded
as points across the visible spectrum and are called spectral
data. Spectral data is represented as a spectral curve.
This curve is the
color's fingerprint. Once we obtain a color's reflectance
curve, we can apply mathematics to
map the color onto a color space. To do this, we take the
reflectance curve and multiply the data by a CIE standard
illuminant. The illuminant is a graphical representation
of the light source under which the samples are viewed.
Each light source has a power distribution that affects
how we see color. Examples of different illuminants are
A - incandescent, D65 - daylight and F2 - fluorescent. We
multiply the result of this calculation by the CIE standard
observer. The CIE commissioned work in 1931 and 1964 to
derive the concept of a standard observer, which is based
on the average human response to wavelengths of light. In
short, the standard observer represents how an average person
sees color across the visible spectrum. Once these values
are calculated, we convert the data into the tristimulus
values of XYZ. These values can now identify a color numerically.
A spectrophotometer measures spectral data - the amount
of light energy reflected from an object at several intervals
along the visible spectrum. The spectral data is shown as
a spectral curve.
Tristimulus values, unfortunately, have limited use as
color specifications because they correlate poorly with
visual attributes. While Y relates to value (lightness),
X and Z do not correlate to hue and chroma.
 |
As a result, when the 1931 CIE standard observer
was established, the commission recommended using
the chromaticity coordinates xyz. These coordinates
are used to form the chromaticity diagram. The notation
Yxy specifies colors by identifying value (Y) and
the color as viewed in the chromaticity diagram (x,y).
Hue is represented at all points around the perimeter
of the chromaticity diagram. Chroma, or saturation,
is represented by a movement from the central white
(neutral) area out toward the diagram's perimeter,
where 100% saturation equals pure hue.
|
To overcome the limitations of chromaticity diagrams like
Yxy, the CIE recommended two alternate, uniform color scales:
CIE 1976 (L*a*b*) or CIELAB, and CIELCH (L*C*h°). These
color scales are based on the opponent-colors theory of
color vision, which says that two colors cannot be both
green and red at the same time, nor blue and yellow at the
same time. As a result, single values can be used to describe
the red/green and the yellow/blue attributes.
Learn more chapter "applications".
top
Visible-region spectrophotometers
Today, the most commonly used instruments for measuring
color are spectrophotometers. Spectro technology measures
reflected or transmitted light at many points on the visual
spectrum, which results in a curve.
Since the curve of each color is as unique as a signature
or fingerprint, the curve is an excellent tool for identifying,
specifying and matching color.
Visible region 400-700nm spectrophotometry is used extensively
in colorimetry science. Ink manufacturers, printing companies,
textiles vendors, and many more, need the data provided
through colorimetry. They usually take readings every 10
nanometers along the visible region, and produce a spectral
reflectance curve. These curves can be used to test a new
batch of colorant to check if it makes a match to specifications.
Traditional visual region spectrophotometers cannot detect
if a colorant has fluorescence. This can make it impossible
to manage color issues if one or more of the printing inks
is fluorescent. Where a colorant contains fluorescence,
a bi-spectral fluorescent spectrophotometer is used. There
are two major setups for visual spectrum spectrophotometers,
d/8 or spherical and 0/45. The names are due to the geometry
of the light source, observer and interior of the measurement
chamber. Each has its own advantages and disadvantages,
but the spherical is a better match to the human eye for
most substrates
Spherical based instruments
Spherically based instrument have played a major roll in
formulation systems for nearly 50 years. Most are capable
of including the "specular component" (gloss)
while measuring. By opening a small trap door in the sphere,
the "specula component" is excluded from the measurement.
In most cases, databases for color formulation are more
accurate when this componentis a part of the measurement.
Spheres are also the instrument of choice when the sample
is textured, rough, or irregular or approaches the brilliance
of a firstsurface mirror. Textile manufacturers, makers
of roofing tiles or acoustic ceiling materials would all
likely select spheres as the right tool for the job.
0/45 (or 45/0)
No instrument "sees" color more like the human
eye than the 0/45. This simply is because a viewer does
everything in his or her power to exclude the "specular
component" (gloss) when judging color.
When we look at pictures in a glossy magazine, we arrange
ourselves so that the gloss does not reflect back to the
eye. A 0/45 instrument, more effectively than any other,
will remove gloss from the measurement and measure the appearance
of the sample exactly as the human eye would see it.
Multi-Angle
In the past 10 or so years, car makers have experimented
with special effect colors. They use special additives such
as mica, pearlescent materials, ground up seashells, microscopically
coated colored pigments and interference pigments to produce
different colors at different angles of view. Large and
expensive goniometers were traditionally used to measure
these colors until X-Rite introduced a battery-powered,
hand-held, multi-angle instrument. X-Rite portable multi-angle
instruments are used by most auto makers and their colorant
supply chain, worldwide.
Colorimeter
Colorimeters are not spectrophotometers. Colorimeters are
tristimulus (three-filtered) devices that make use of red,
green, and blue filters that emulate the response of the
human eye to light and color. In some quality control applications,
these tools represent the lowest cost answer. Colorimeters
cannot compensate for metamerism (a shift in the appearance
of a sample due to the light used to illuminate the surface).
As colorimeters use a single type of light (such as incandescent
or pulsed xenon) and because they do not record the spectral
reflectance of the media, they cannot predict this shift.
Spectrophotometers can compensate for this shift, making
spectrophotometers a superior choice for accurate, repeatable
color measurement.
Sample Being Measured
top
Applications
Spectrophotometry's applications are seemingly boundless.
Colormatching measurements are made every day by those comparing
a reproduced object to a reference point. Spectrophotometry-assisted
color measurement can be useful in areas such as:
- Color testing of inks
- Color control of paints
- Control of printed colors on packaging material and
labels
- Color control of plastics and textiles throughout the
development and manufacturing process
- Finished products like printed cans, clothing, shoes,
automobile components, plastic components of all types
- Corporate logo standardization
Color and appearance instruments are used to measure the
properties of paints and coatings including color, gloss,
haze and transparency. Appearance is the manifestation of
the nature of objects and materials through visual attributes
such as size, shape, chroma, color, texture, glossiness,
haze, transparency, opacity, hue, luster, orange peel, translucency,
etc.
Color and appearance instruments generally fall into one
of four categories, colorimeters, densitometers, spectral
cameras, and spectrophotometers. Colorimeters measure color
using three or four filters that match human color receptors.
Colorimeters can show L, a, b or L*, a*, b* numbers but
can only measure in one light source. Densitometers measure
the density of ink films using one or more filters. Densitometers
do not give complete color information, but are useful for
specification and control of printed colors. Spectral cameras
provide measurements with full spectral and spatial information.
Spectrophotometers operate on the principle of reflected
light. Spectrophotometers measure individual wavelengths
and then calculate L, a, b or L*, a*, b* values from this
information. These color and appearance instruments can
measure in all standard illuminants.
To accomplish their readings, color and appearance instruments
many use any of a number of measurement scales. These include:
Hunter L, a, b: a color standard that was finalized
in 1958. ΔL=lightness, Δa=green and red and Δb=blue and yellow.
CIELAB: an international color standard adopted
in 1976. CIE is a tricolor system that is based on the fact
that any color can be matched by a suitable mix of the 3
primary colors. When a color is expressed in CIELAB, ΔL*
defines lightness, Δa* denotes the red/green value and
Δb* the yellow/blue value.
CIELCH: a color standard developed from CIELAB.
While CIELAB uses Cartesian coordinates to calculate a color
in a color space, CIELCH uses polar coordinates. This color
expression can be derived from CIELAB. The ΔL* defines lightness,
ΔC* specifies chroma and h° denotes hue angle, an angular
measurement. The L*C*h° expression offers an advantage
over CIELAB in that it's very easy to relate to the earlier
systems based on physical samples, like the Munsell Color
Scale.
Delta CIELAB and CIELCH: Assessment of color is
more than a numeric expression. Usually it's an assessment
of the color difference (delta) from a known standard. CIELAB
and CIELCH are used to compare the colors of two objects.
XYZ: the XYZ space allows colors to be expressed
as a mixture of the three tristimulus values X, Y, and Z.
The term tristimulus comes from the fact that color perception
results from the retina of the eye responding to three types
of stimuli. After experimentation, the CIE set up a hypothetical
set of primaries, XYZ, that correspond to the way the eye's
retina behaves.
Yxy: Yxy space expresses the XYZ values in terms
of x and y chromaticity coordinates, somewhat analogous
to the hue and saturation coordinates of HSV space.
CIELAB Tolerancing
When tolerancing with CIELAB, you must choose a difference
limit for ΔL* (lightness), Δa* (red/green), and Δb* (yellow/blue).
These limits create a rectangular tolerance box around the
standard.
When comparing this tolerance box with the visually accepted
ellipsoid, some problems emerge. A box-shaped tolerance
around the ellipsoid can give good numbers for unacceptable
color. If the tolerance box is made small enough to fit
within the ellipsoid, it is possible to get bad numbers
for visually acceptable color.
CIELCH Tolerancing
CIELCH users must choose a difference limit for ΔL* (lightness),
ΔC* (chroma) and ΔH* (hue). This creates a wedge-shaped
box around the standard. Since CIELCH is a polar-coordinate
system, the tolerance box can be rotated in orientation
to the hue angle. When this tolerance is compared with the
ellipsoid, we can see that it more closely matches human
perception. This reduces the amount of
disagreement between the observer and the instrumental values
CMC Tolerancing
CMC is not a color space but rather a tolerancing system.
CMC tolerancing is based on CIELCH and provides better agreement
between visual assessment and measured color difference.
CMC tolerancing was developed by the Colour Measurement
Committee of the Society of Dyers and Colourists in Great
Britain and became public domain in 1988. The CMC calculation
mathematically defines an ellipsoid around the standard
color with semi-axis corresponding to hue, chroma and lightness.
The ellipsoid represents the volume of acceptable color
and automatically varies in size and shape depending on
the position of the color in color space.
Visual Color and Tolerancing
Poor color memory, eye fatigue, color blindness and viewing
conditions can all affect the human eye's ability to distinguish
color differences. In addition to those limitations, the
eye does not detect differences in hue (red, yellow, green,
blue, etc.), chroma (saturation) or lightness equally. In
fact, the average observer will see hue differences first,
chroma differences second and lightness differences last.
Visual acceptability is best represented by an ellipsoid.
As a result, our tolerance for an acceptable color match
consists of a three-dimensional boundary with varying limits
for lightness, hue and chroma, and must agree with visual
assessment. CIELAB and CIELCH can be used to create those
boundaries. Additional tolerancing formulas, known as CMC
and CIE94, produce ellipsoidal tolerances.
CIE94 Tolerancing
In 1994 the CIE released a new tolerance method called CIE94.
Like CMC, the CIE94 tolerancing method also produces an
ellipsoid. The user has control of the lightness (kL) to
chroma (Kc) ratio, as well as the commercial factor (cf).
These settings affect the size and shape of the ellipsoid
in a manner similar to how the l:c and cf settings affect
CMC. However, while CMC is targeted for use in the textile
industry, CIE94 is targeted for use in the paint and coatings
industry.You should consider the type of surface being measured
when choosing between these two tolerances. If the surface
is textured or irregular, CMC may be the best fit. If the
surface is smooth and regular, CIE94 may be the best choice.
Visual Assessment vs. Instrumental
Though no color tolerancing system is perfect, the CMC and
CIE94 equations best represent color differences as our
eyes see them.
Choosing the Right Tolerance
When deciding which color difference calculation to use,
consider the following five rules (Billmeyer 1970 and 1979):
1. Select a single method of calculation and use it consistently.
2. Always specify exactly how the calculations are made.
3. Never attempt to convert between color differences calculated
by different equations through the use of average factors.
4. Use calculated color differences only as a first approximation
in settingtolerances, until they can be confirmed by visual
judgments.
5. Always remember that nobody accepts or rejects color
because of numbers - it is the way it looks that counts.
White and Yellow Indices
Certain industries, such as paint, textiles and paper manufacturing,
evaluate their materials and products based on standards
of whiteness. Typically, this whiteness index is a preference
rating for how white a material should appear, be it photographic
and printing paper or plastics.
In some instances, a manufacturer may want to judge the
yellowness or tint of a material. This is done to determine
how much that object's color departs from a preferred white
toward a bluish tint.
The effect of whiteness or yellowness can be significant,
for example, when printing inks or dyes on paper. A blue
ink printed on a highly-rated white stock will look different
than the same ink printed on newsprint or another low-rated
stock. The American Standards Test Methods (ASTM) has defined
whiteness and yellowness indices. The E313 whiteness index
is usedfor measuring near-white, opaque materials such as
paper, paint and plastic. In fact, this index can be used
for any material whose color appears white. The ASTM's E313
yellowness index is used to determine the
degree to which a sample's color shifts away from an ideal
white. The D1925 yellowness index is
used for measuring plastics.
top
X-Rite - Optronik's focus: Non-Contact Color Measurement
Teleflash
The X-Rite TeleFlash system provides online color measurement
and evaluation of color deviation to the running production
line and non-contact lab applications. TeleFlash can accurately
measure the color of products that are textured, finely
patterned or glossy, such as extruded vinyl, bulk goods,
coil coatings, synthetic films, paints (wet and dry), textiles,
carpeting, granules, food pigments, paper, powders, glass,
ceramics, metal, minerals and plaster.
TeleFlash offers a measuring distance of up to five feet,
tolerating small variations in the measuring distance from
system to sample. The system's thermochromism compensation
allows for color measurement without the time usually required
for cooling and stabilizing.
More specifics: Teleflash product description.
For other color measurement solutions, please visit the
X-Rite corporate website xrite.com.
|
