Abstract
Purpose: Color constancy is not perfect so that color appearance of a surface gradually changes as an illumination changes. In the most previous studies color appearance of a color chip under a test illumination was matched to a color under a standard white illumination with an asymmetric color matching method. In those experiments color difference between a color in perfect color constancy and that actually obtained was utilized as an index for color constancy. However, color difference does not tell us how the test color appearance categorically changes. In the present study we carried out categorical color naming for color chips under different illuminants in order to reveal categorical color constancy. Methods: A LCD projector illuminated a test color chip (5deg x 5deg) chosen from the 424 samples in the OSA uniform color scales. The observer named the test color using only a basic color name in the Berlin-Kay 11 basic color names. The LCD projector (3000K, 6500K, and 25000K) could illuminate the whole area (50deg × 40deg), which included the test color chip and the gray surround (whole illumination condition). It could also separately illuminate the color chip with the test color temperature and the surround with the standard white (6500K) (spot illumination condition). This spot illumination made it possible to measure color appearance change of the test color chip caused by spectral component change of the test illuminant without adaptation or surround effects. Results: We found that the categorical color regions of the 11 basic colors were remarkably similar under the whole illumination conditions of 3000K, 6500K, and 25000K. In the spot illumination condition the categorical color regions significantly shifted according to the illuminants. Despite of these shifts categorical names remained the same for color chips in the focal color regions. Conclusions: Our color vision possessed robust categorical color constancy and categorical color constancy holed best in focal color regions.