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When two colored samples are to be compared visually, the size of the perceived lightness difference will in general depend upon the spatial proximity of the samples. This is taken into account in the Judd formula for color difference [ D. B. Judd, Am. J. Psychol. 52, 418 ( 1939).] which includes a proximity factor. This factor weights measured reflectance differences so that the perceived lightness differences predicted by the formula vary with the spatial proximity of the samples. A few tentative values have been assigned by Judd for various separations. A continuous, empirical expression for the proximity factor has been found as a function of angular separation of the samples under specified viewing conditions. A divided visual field was used, with variably separable halves. The halves could be interchanged without the knowledge of the observer, who was required to identify the darker half many times at each of several values of separation. The observer’s difficulty of identification increased with increasing dividing linewidth and was measured statistically in terms of errors of identification. From the error data and certain assumptions, a relationship between proximity factor and angular separation was deduced. A few tests were made in order to study possible effects of dividing linewidth on chromaticity discrimination. Under the particular test conditions which we used, no effect was observed.
© 1961 Optical Society of America
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