Color perception
From Encyclopædia
color perception is the process of distinguishing varying ways in which points or homogeneous patches of
light appear to a subject. The appearance may be described in terms of hue (red, orange, green, blue, yellow, and
violet being the spectral
colors); saturation (from pure, through
pastels or brown, to unsaturated, hueless grays); and lightness or brightness (blacks, through grays, to white).Other things being equal, the hue of an isolated patch varies with the wavelength of the
light at the eye; the lightness varies with the intensity of the
light's energy; and the saturation varies with the purity of any wavelength mixture. If all the wavelengths are represented, the mixture is achromatic--black, grays, or white (see
color).Wavelength and hue are not in one-to-one correspondence, however. The same hue can be evoked by very different wavelengths if the mixture of wavelengths in a single patch is suitably selected, or if the context is suitably arranged.
color-mixture research measures the viewer's ability to tell whether two adjacent patches of
color are the same or different--a
discrimination decision similar to those studied in PSYCHOPHYSICS. The normal viewer can match any
color by using a mixture of three wavelengths. Viewers who have defective
color vision, or
color blindness, may require abnormal proportions of the three wavelengths (anomalous trichromatism); may use only two wavelengths to match all others (dichromatism), confusing reds and greens with each other (deuteranopia and protanopia) or confusing yellows,
blues, and grays (tritanopia); or may match any wavelength with any other (monochromatism).Even the normal viewer has normal
color vision only in the
Central part of the retina; in the other parts of the retina the predominant photoreceptors are rods, which are sensitive to weak
light but which are all alike in their response to wavelength--that is, they are monochromats. Rods predominate in peripheral vision.The Young-Helmholtz theory of
color (see
HELMHOLTZ, Hermann) posits three different classes of cones in the
Central retina, which yield sensations of blue, green, and red from mixtures of which all other
colors would presumably be perceived. Direct physiological measurement has found different kinds of cones, with peak sensitivities at wavelengths of about 450 nanometers (reddish blue), 530 nm (green), and 570 nm (greenish yellow), respectively. (A nanometer is 1 X (10 to the power of - 9) meter.) Because
light at these wavelengths does not look pure, and because one cannot predict from the appearance of one such wavelength what a mixture of two or more will look like, the opponent-process theory, originally proposed by Ewald Hering and revised (1974) by Hurvich and Jameson, has gained widespread support. This theory states that the output of the cones is received by
cells that occur in sets of three kinds of opponent-process pairs. One responds with the sensation red or with its complementary, green, depending on how it is stimulated; one responds with blue or yellow; one responds with white or black. All
colors would be mixed from these.
cells that are consistent with this theory have been found in the visual systems of fish and
mammals.Context also affects
color perception. In the phenomenon of simultaneous contrast, the hue that is the complementary of the hue that surrounds a patch is added to the appearance of the patch. This phenomenon is of great practical and theoretical importance.Julian HochbergBibliography: Birren, F.,
color perception in Art (1986); Davidoff, Jules, Cognition through
color (1991); Evans, Ralph M., The
perception of
color (1974); Hochberg, Julian,
perception, 2d ed. (1978); Mollon, John, and Sharpe, Ted, eds., Colour Vision: Physiology and Psychophysics (1983).
