Abstract
We can perceive two modes of color appearance, i.e., the surface and the illuminant color mode. The real surface can appear either in the surface or in the illuminant mode according to the environmental conditions regardless of the physical property of the surface. In this study we investiged boundary stimulus conditions between the two modes. The aim of this study was to test whether transition of the two modes occurred when the stimulus was equal in lightness or lumnance to that of the ‘optimal color’ of the same chromaticity. In the experiment the observer adjusted the luminance of the test stimulus so that the stimulus just started to appear in the illuminant color mode. The chromaticity coordinates of the stimulus were chosen from eight points equally separated between a red (0.600, 0.330) and a green (0.285, 0.595) in the CIE 1931 (x, y) chromaticity diagram. We found that the luminances of the newly defined optimal-colors. This pseudo-optimal color had the spectral reflectance curve with a less gradient. It is suggested that the visual system might utilize the pseudo-optimal color when we perceive the surface.