Cie Color Model - Xerox DocuColor 40 Reference Manual

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1-2 Desktop Color Primer
Like the sun, most light sources we encounter in our daily environment emit a mixture
of many light wavelengths, although the particular distribution of wavelengths can
vary considerably. Light from a tungsten light bulb, for example, contains much less
blue light than sunlight. Tungsten light appears white to the human eye which, up to a
point, can adjust to the different light sources. However, color objects appear different
under tungsten light than they do under sunlight because of the different spectral
makeup of the two light sources.
The mixture of light wavelengths emitted by a light source is reflected selectively by
different objects. Different mixtures of reflected light appear as different colors. Some
of these mixtures appear as relatively saturated colors, but most appear to us as grays or
impure hues of a color.

CIE color model

In the 1930s, the Commission Internationale de l'Eclairage (CIE) defined a standard
color space, a way of defining colors in mathematical terms, to help in the
communication of color information. This color space is based on research on the
nature of color perception. The CIE chromaticity diagram (plate 2) is a two-
dimensional model of color vision. The arc around the top of the horseshoe
encompasses the pure, or spectral, colors from blue-violet to red. Although the CIE
chromaticity diagram is not perceptually uniform—some areas of the diagram seem to
compress color differences relative to others—it is a good tool for illustrating some
interesting aspects of color vision.
By mixing any two spectral colors in different proportions, we can create all the colors
found on the straight line drawn between them in the diagram. It is possible to create
the same gray by mixing blue-green and red light or by mixing yellow-green and blue-
violet light. This is possible because of a phenomenon peculiar to color vision called
metamerism. The eye does not distinguish individual wavelengths of light. Therefore,
different combinations of spectral light can produce the same perceived color.
Purple colors, which do not exist in the spectrum of pure light, are found at the
bottom of the diagram. Purples are mixtures of red and blue light—the opposite ends
of the spectrum.