According to Readings on Color, colorimetry is defined as “The science of measuring color and color appearance. Classical colorimetry deals primarily with color matches rather than with color appearance as such. The main focus of colorimetry has been the development of methods for predicting perceptual matches on the basis of physical measurements.”
- 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.
The SLS-9400 is a tri-stimulus colorimeter from Gamma Scientific that provides laboratory grade accuracy with dual LCD/CRT measurement capability.
The SLS-9400 colorimeter calibrates LCD, LED, plasma, projection displays and video walls. The colorimeter measures color accuracy, white balance and uniformity of white balance. It is easy to operate, portable and affordable.
Colorimetry Concepts in Display Test and Measurement
Though color perception is inherently subjective, it is possible to make objective measurements of the color of a source or object as perceived by a standard human observer. Various standard human observers are defined in the discipline of Colorimetry. According to the standard model, the perceived color of a given spot can be reduced to a three-dimensional value. The three dimensions of color can be described in different ways; in perceptual terms, perhaps the most straightforward approach is to describe color in terms of brightness, hue, and purity, or saturation.
A two-dimensional description of color, which corresponds to the combination of hue and purity, omitting the third dimension of brightness. The luminance (brightness) and chromaticity of a spot on a display, taken together, provide a complete description of its color.
Gamut mapping (RGB plotting)
All colors produced by a display are created by some combination of three primary colors: Red, Green, and Blue. In fact, each display color can be described in terms of the amount of R, G, and B primaries present. (This description is alternative, but equivalent, to a description in terms of brightness, hue, and purity.) If the chromaticity coordinates of the three primaries are plotted in a chromaticity diagram, the triangle enclosed by these points represents the full range of colors reproducible by the display. This range is the display’s color gamut.
Correlated Color Temperature (CCT)
This metric is used to describe the color of a white light (such as a display backlight) by comparing its chromaticity to that of an idealized incandescent source, known as a black body. The color of an incandescent source (which glows due to heat) depends upon its temperature; lower temperature sources are more red or yellow; higher temperature sources are more blue. The CCT of a white light is the temperature of the black body which most closely matches its chromaticity.
White Balance, Gray Balance
After characterizing the chromaticity and gamma functions for each of a display’s three color channels (R, G, B), it is possible to calculate the amount of R, G, and B required to reproduce any color within the display’s color gamut. A particularly important feature is the location of the white point: The chromaticity of a specified white light, to which the observer is assumed to be adapted. Light sources with different CCT have slightly different chromaticities, but once the location of the desired white point is specified, the proportion of R, G, and B primaries required to reproduce it is known as the display’s white balance. The chromaticity of a neutral gray is the same as that of the white point, but since the gamma functions of a display are typically non-linear, a different gray balance may be required to reproduce the same chromaticity at a lower luminance level.
Dominant Wavelength & Purity
These values, taken together, represent an alternative description of chromaticity. Dominant wavelength corresponds to hue, while purity corresponds to saturation. The relationship between wavelength and hue can be understood in terms of the colors of the visible spectrum, as observed in the rainbow: Shorter wavelengths correspond to violet and blue hues; medium wavelengths to greens and yellow; longer wavelengths to orange and red hues. In the chromaticity diagram (figure 3), hues change in an arc about the central white point, moving clockwise from violet through blue, cyan, green, yellow, orange, and red. Purity increases as chromaticity moves from the central white point to the outer limit of the spectrum locus: The horseshoe-shaped curve representing the chromaticities of pure spectral light. Thus, for example, an intensely-saturated red would be plotted near the edge of the diagram, while a pink color of the same hue (or dominant wavelength) would fall near the center.