Introduction. Shifts in the visual sensitivity introduce shifts in color appearance, which is often experienced under changes in illuminant chromaticity. A few previous studies suggested that the shift in color appearance could be explained by applying a set of fixed coefficient to each cone response, regardless of lightness level. The present study tested whether a set of fixed coefficients is applicable to a wide range of lightness. Methods. The observer made achromatic settings in a real room with variable-chromaticity illuminant on the ceiling. A matching stimulus was presented on a CRT screen, placed behind a small hole (5cm × 5cm) in a gray-painted wall. The illuminant chromaticity changed from white (D65) to blue, orange, green, and purple. The observer adapted to the illuminant for at least 5 minutes and then started the achromatic settings for five luminance levels of stimulus. Observer repeated the settings for five times in a random order. The results were assessed with relative M-cone weight (Ahn and MacLeod, 1994). Results. The achromatic point shifted monotonically toward D65, as the stimulus intensity increased. The shifts in achromatic point were fitted nicely with linear regressions on a log-cone-contrast vs. log-luminance-contrast plane. After a careful analysis for the three cone classes, systematic shifts were also found in the slopes and intersections of the fitted lines. Discussions. The previous reports, including ours (Kuriki, et al., 2000), suggested that a set of fixed coefficient is sufficient to estimate the shift in color appearance under a change in illuminant chromaticity. However, this was an over simplification. After a precise examination for L and M cones, they show slight but consistent change in the degree of nonlinearity (slope in a log-log plot), as well as S cones. This nonlinearity may explain the appearance of Land's two-color projection, which appears trichromatic, even though it is composed of dichromatic lights.
ASJC Scopus subject areas
- Sensory Systems