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point, can adjust to the different light sources. However, color objects appear different 
under tungsten light than they do in 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.

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.