Color-discrimination data are compared with the predictions of a generalized fluctuation theory for visual threshold behavior. Our observations for the tritanopic component of vision at low luminances are in good agreement with the expectations from this theory. We measured just-noticeable differences of hue with equiluminous square-wave test objects, which were modulated only in chromaticity. A chromaticity-contrast sensitivity function was introduced for the description of these results, in analogy of the luminance-contrast sensitivity function. Observations were made for different spatial frequencies at four reference wavelengths and at several luminance levels. The results do not show an attenuation of the low frequencies such as appears in the luminance-threshold contrast modulation. We infer from this that spatial interactions are different in the chromaticness and brightness channels of the visual system. Furthermore a decrease of the luminance level causes an increase of the neural integrative interaction of the color signals. We divided the measured chromaticity-contrast sensitivity function into an optical and a nervous component. A calculation for the optical part is given.
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