Color Contrast

(1) In colorimetry, color contrast is the characteristic difference between two chromaticities x, y and x + Δx, y + Δy. (Descriptions in this article conform to the xyz colorimetric system of the International Commission on Illumination (see, Figure 3]; a similar approach may be used for most trichromatic color measurement systems.) The difference between the two chromaticities can be specified by the shortest distance between the points of the given chromaticities on a two-dimensional color triangle: . The minimum chromaticity difference Δl_min that is perceptible to the human eye is called the threshold contrast. The chromaticity difference Δl, which is expressed in terms of the minimum number of threshold contrasts, is called the color contrast n between the chromaticities x, y and x + Δx, y + Δy. The proviso concerning minima is necessary because the number of threshold contrasts in the transition from one point to another on the chromaticity triangle depends on the path taken, so that .the line connecting the two chromaticity points usually does not correspond to the minimum number of threshold contrasts.

The attempt to construct a chromaticity triangle such that a straight line would correspond to the minimum number of threshold contrasts and equal linear segments would correspond to equal numbers of threshold contrasts has led to the plotting of “equal-contrast,” or uniform, chromaticity diagrams. However, only an approximate solution to this problem has as yet been found.

(2) In physiological optics and in theories of color vision, color contrast is the change in the perception of any color caused by the simultaneous or prior effect on the eye of radiation of a different color, called the inducing color (simultaneous or sequential color contrasts, respectively). Simultaneous color contrast occurs when the color-sensitive photoreceptors of the retina are affected by the background color or colors that are adjacent to and surround the color being studied. Sequential color contrast involves a short period of time between the effect on the photoreceptors of the inducing color and the color being tested. As the result of color contrast, the perception of color can change with respect to brightness, hue, and saturation. These changes always increase differences between colors. Thus, a gray field appears darker on a white background and lighter on a black background; a gray field on a red background acquires a greenish tinge, whereas on a yellow background it becomes bluish.

The phenomena of both types of color contrast are closely related to the photochemical changes in the relationships between the spectral sensitivity of selective light receptors of the retina and the interaction of stimuli in the visual centers of the brain.