The Dimensionality of Color Vision

Color vision has been frequently studied in laboratories by asking observers to judge whether pairs of stimuli—typically viewed as the respective halves of a small, illuminated circle (Fig. 2)—appear the same (''match'') or are different. If the two halves are identical in wavelength and intensity content, they of course match. Differences in color appearance may be introduced by changing the intensity or wavelength content of one half or by changing both these features. A change in the intensity of the light in one half of the circle yields a mainly achromatic difference between the two; one side becomes brighter than the other. Changes in the wavelength content of one half may yield a complex of change, in hue and saturation principally but also possibly in brightness. If the observer is then allowed to adjust the relative intensity so as to remove any brightness difference, the two sides will have a pure chromatic difference. It is from this basis that color vision is often studied.

Special cases of this viewing arrangement are those in which the two sides of the small circle differ in wavelength content but have identical color appearances. Pairs of stimuli that differ physically but appear the same are said to be metameric. The occurrence of such metameric matches defines a fundamental feature of human color vision—its limited dimensionality. To better understand this feature, consider color matches obtained when separate lights having fixed spectral content are superimposed on one half of the matching field (Fig. 2). Each of these separate lights is called a primary (P), and the task given to a viewer is to adjust the relative intensity (e) of each of the primaries so as to make that half of the field appear identical to the other half (in turn, traditionally called the test light). The test light can be composed of any fixed combination of wavelengths and intensities. The important result is that most human observers can match the appearance of all test lights by simply adjusting the relative intensities of only three primary lights. Some matches are possible with only one or two primaries, and more than three will work, but three is the minimum needed to complete all matches. Humans are thus said to have trichromatic color vision. There are some qualifications to this conclusion; (i) there are restrictions on the wavelengths that can be employed as primaries, and (ii) for some test lights one of the primaries must be added to the side containing the test light. In any case, the ability of humans to complete matches using only three independent variables means that any color can be represented in a three-dimensional space where the location in that space is specified by the proportions of the three primaries required to capture the appearance of the color. Such spaces are highly useful in the many practical uses of color (e.g., the specification of colored

Figure 2 Schematic representation of the color matching test. In this test a subject views a small circular field. The two halves of the field are independently illuminated from different light sources. As shown here, a light of some fixed wavelength and intensity content (a test light) is projected onto the left half of the field. Lights from three separate sources are superimposed on the left. These are primary lights (P1-P3) that differ in wavelength. The task of the subject is to adjust the intensities of the three lights (e1-e3) until the two halves of the field appear identical. Illustrated is a trichromatic match where combinations of three primary lights are required to complete the match.

Figure 2 Schematic representation of the color matching test. In this test a subject views a small circular field. The two halves of the field are independently illuminated from different light sources. As shown here, a light of some fixed wavelength and intensity content (a test light) is projected onto the left half of the field. Lights from three separate sources are superimposed on the left. These are primary lights (P1-P3) that differ in wavelength. The task of the subject is to adjust the intensities of the three lights (e1-e3) until the two halves of the field appear identical. Illustrated is a trichromatic match where combinations of three primary lights are required to complete the match.

signal lights or of the color of a commercial logo) and so have been intensively evaluated.

The mixing of lights reveals another surprising feature of color perception. Many pairs of lights can be added together to yield an achromatic percept. Such pairs are called complementary colors and their occurrence is counterintuitive in that the perceived colors associated with the two wavelengths viewed separately utterly disappear when they are mixed together. In the color solid described previously, complementary colors are placed on opposite sides of the hue circle such that a line drawn between them passes through the location of white. The facts of color mixing make clear that the visual system is not a simple wavelength analyzer. From the viewpoint of the biology of color vision, the fact that people are trichromatic has long been taken to predict the nature of the transformations occurring in the visual system.

Indeed, more than two centuries ago it was hypothesized that the fundamental facts of human color perception imply that there must be three kinds of physiological mechanisms in the eye responsible for processing those aspects of light that lead to color. This turned out to be an inspired prediction.

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