Visual Sensitivities to Combined Chromaticity and Luminance Differences*

Color discrimination for the general case of combined chromaticity and luminance differences has been investigated by analysis of the errors of visual trichromatic colorimetry. Formulas based on those devised by Silberstein have been used to determine the coefficients and axes of ellipsoids in color space from the data. Methods for transforming the results to other coordinate systems are presented and used to give the results in terms of both the ICI tristimulus X, Y, Z values and the ICI x, y, chromaticity values. Most of the results are nearly symmetrical above and below the chromaticity (constant luminance) cross section of each ellipsoid. Those cross sections are comparable with results previously published, and the agreement is satisfactory. The influence on color discrimination of changing the level of luminance has been studied for three colors. The ratio between the standard of deviation of each primary and its amount seems to be a function of only the luminance contributed by that primary to the color mixture.