The color specimens developed by the OSA Committee on Uniform Color Scales are intended to provide a uniform sampling of the three‐dimensional domain of realizable surface colors. The research of this article confirms the distinction between basic and nonbasic color terms and defines the locations of the eleven basic surface colors within the OSA space through systematic, monolexemic color naming of 424 samples of the OSA set seen against a gray background. It is concluded that the coordinate system of the OSA color space is well suited to the specification of color order: The locations of the basic colors are reasonably arranged and can be conveniently and precisely visualized.
Categorical color perception has previously been tested using a naming method, and the data from 27 subjects so examined in several experiments have been combined to yield a “categorical color difference index” (CCDI) that can be computed for any pair of colors in the OSA set of 424 samples. A new experiment is performed in which categorical color perception is encouraged without the explicit use of names by allowing 10 seconds to elapse between the presentation of two stimuli before they are judged same or different. All of the colors being compared are either identical or nearest neighbors in the orange region of the OSA space. Nearest neighbors, which are separated by 2 OSA units, clearly differ when presented simultaneously; with the delay, errors (“same” responses) sometimes occur. These errors decrease as CCDI increases, suggesting that categorization occurs when colors must be remembered. Response time, on the other hand, is independent of CCDI and therefore may reflect color differences based upon discrimination, rather than identification.
Many formulas f o r predicting perceived color differencesrequire that three terms be squaredprior to the extraction of the square root of their sums. Two of these terms relate to chromatic differences measured along tritan and redgreen axes. The root-rnean-square calculational procedure predicts that the relative directions of simultaneous chromaticity changes along these principal axes should not affect the perceived color difference. Experimental data are presented which strongly disconfirm this prediction.
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