Colorimetry:
colorimetry is defined as the science of measuring color and color appearance.
Or
Colorimetry, measurement of the wavelength and the intensity of electromagnetic radiation in
the visible region of the spectrum.
It is used extensively for identification and determination of concentrations of substances that
absorb light.
Colorimetry Applications:
The science of colorimetry is used to quantify the response of the human visual system and
match human color perception for applications in a variety of industries.
Display Manufacturing: Quality control for industrial production lines and incoming inspection
of display glass. Display calibration for LED, LCD, plasma, projection, DLP, CRT and LCOS
displays
Broadcasting: Measuring and calibrating video walls for color accuracy, uniformity of
brightness and white balance.
Graphic Design and Computer Animation: Professionals who rely on color accuracy and
precision color measurement benefit from understanding colorimetry.
Spectral Power Distribution (SPD):
In radiometry, photometry, and color science, a spectral power distribution (SPD) measurement
describes the power per unit area per unit wavelength of an illumination (radiant exitance). More
generally, the term spectral power distribution can refer to the concentration, as a function of
wavelength, of any radiometric or photometric quantity (e.g. radiant energy, radiant flux, radiant
intensity, radiance, irradiance, radiant exitance, radiosity, luminance, luminous flux, luminous
intensity, illuminance, luminous emittance).
It is also a pictorial representation of the radiant power emitted by a light source at each
wavelength or band of wavelengths in the visible region of the electromagnetic spectrum (360 to
770 nanometers).
Light may be precisely characterized by giving the power of the light at each wavelength in the
visible spectrum. The resulting spectral power distribution (SPD) contains all the basic physical
data about the light and serves as the starting point for quantitative analyses of color. The SPD
can be measured by a spectrophotometer. From the SPD both the luminance and the chromaticity
of a color may be derived to precisely describe the color in the CIE (International Commission on
Illumination (usually abbreviated CIE for its French name, Commission internationale de l'éclairage))
system. Other systems of color measurement can also be related to the SPD. These systems have
been successful in predicting color perception from the SPD, but it is not possible to proceed in
the opposite direction. That is, the SPD cannot be predicted from the characteristics of the color
as perceived by the human eye.
The SPD of light from an illuminated surface is the product of the percent reflectance of the
surface and the SPD of the light which falls on the surface.
Lamp manufacturers publish SPD curves of specific light sources. The spectral make-up of a
light source affects its ability to render colors naturally.
Figure 1. Spectral Power Distribution curve. Illuminant A represents an incandescent tungsten
filament lamp, D50 represents daylight, D65 corresponds roughly to the average midday light in
Western Europe / Northern Europe, F1 represents fluorescent daylight illuminants. The
International Commission on Illumination (usually abbreviated CIE for its French name) is the
body responsible for publishing all of the well-known standard illuminants.
Additive color mixture:
Additive color mixture is a method to create color by mixing a number of different light colors,
with shades of red, green, and blue being the most common primary colors used in additive color
system.
There are two ways to mix colors. If we mix light, we call it additive mixture. When we mix
inks, dyes or pigments, that is subtractive mixture. There is really no difference in the way the
colors behave in the two circumstances. Only the mechanics of the processes are different.
Additive color mixture begins with the absence of light (black), and adds colors of light together
to form new colors. The illustration shows how this kind of mixture might be done with three
slide projectors.
Red, green and blue are typically used as the additive primary colors. In pairs, they combine to
create cyan, magenta and yellow. When all three of the additive primary colors are added
together, in approximately equal intensities, they produce white light.
Additive mixture is used in theatrical lighting, and in computer monitors and TV screens.
(Examine your computer screen or TV with a very strong magnifying lens, and you'll see that the
image consists of a matrix of red, green, and blue dots, or pixels.)
The red color we use for additive color mixture is composed of the light from one-third of the
spectrum. The green color is another third of the spectrum. And the blue color is the remaining
third.
Where two colors overlap, or are added together, their combined light accounts for two-thirds of
the spectrum. (1/3 + 1/3 = 2/3) Those combinations always produce the colors cyan, magenta,
and yellow. Where all three colors overlap, the entire spectrum is present, so we see white light.
It's that simple.
This demonstration of additive color mixture lays the foundation to the understanding
complementary colors, subtractive color mixture, and primary colors.
Figure 2. Additive color mixing: adding red to green yields yellow; adding all three primary
colors together yields white.
If we combine these three primary colors in different combinations, we find that:
Red + Green = Yellow
Red + Blue = Purple (called magenta)
Green + Blue= Cyan (a dark blue)
Red + Green + Blue= White
Substituting each of the first three relationships into the last one, it is also true that
Yellow + Blue= White
Red + Cyan= White
Green + Purple= White
Chromaticity Coordinates:
Chromaticity is an objective specification of the quality of a color regardless of its luminance.
The chromaticity coordinates of a colour are the three tristimulus values, symbolized by x, y, and
z, representing the proportions of the three additive primaries that when mixed match the given
colour. Also called chromaticness.
Spectrophotometry:
Spectrophotometry is the quantitative measurement of the reflection or transmission properties of
a material as a function of wavelength.
Spectrophotometry uses photometers, known as spectrophotometers, that can measure a light
beam's intensity as a function of its color (wavelength). Important features of spectrophotometers
are spectral bandwidth (the range of colors it can transmit through the test sample), the
percentage of sample-transmission, the logarithmic range of sample-absorption, and sometimes a
percentage of reflectance measurement.
A spectrophotometer is commonly used for the measurement of transmittance or reflectance of
solutions, transparent or opaque solids, such as polished glass, or gases. Although many
biochemicals are colored, as in, they absorb visible light and therefore can be measured by
colorimetric procedures, even colorless biochemicals can often be converted to colored
compounds suitable for chromogenic color-forming reactions to yield compounds suitable for
colorimetric analysis. However they can also be designed to measure the diffusivity on any of the
listed light ranges that usually cover around 200 nm - 2500 nm using different controls and
calibrations. Within these ranges of light, calibrations are needed on the machine using standards
that vary in type depending on the wavelength of the photometric determination.
Spectrophotometer:
Figure 3: Basic structure of spectrophotometers
Figure 3 illustrates the basic structure of spectrophotometers. It consists of a light source, a
collimator, a monochromator, a wavelength selector, a cuvette for sample solution, a
photoelectric detector, and a digital display or a meter. Detailed mechanism is described below.
A spectrophotometer, in general, consists of two devices; a spectrometer and a photometer. A
spectrometer is a device that produces, typically disperses and measures light. A photometer
indicates the photoelectric detector that measures the intensity of light.
Spectrometer: It produces a desired range of wavelength of light. First a collimator (lens)
transmits a straight beam of light (photons) that passes through a monochromator (prism) to split
it into several component wavelengths (spectrum). Then a wavelength selector (slit) transmits
only the desired wavelengths, as shown in Figure 3.
Photometer: After the desired range of wavelength of light passes through the solution of a
sample in cuvette, the photometer detects the amount of photons that is absorbed and then sends
a signal to a galvanometer or a digital display, as illustrated in Figure 3.
Dominant wavelength:
The wavelength of monochromatic light that would give the same visual sensation if combined
in a suitable proportion with an achromatic light.
