Nonlinear changes in visual sensitivity balance under changes in the chromaticity of ambient illuminant

Introduction. A linear transformation of cone responses with a diagonal matrix is known to be the simplest and good approximation to describe the shift in color appearance under changes in the chromaticity of illuminant. However, how the diagonal components are defined is not clearly understood. The present study investigates how the coefficients change in a real environment. Methods. Subjects were asked to stay in an illuminated booth for at least 15 minutes, and then, asked to adjust the apparent color of a color patch in the front wall to be achromatic. The illuminant chromaticities were D65, blue, orange, purple and green. The stimulus was presented by a CRT display placed behind a small hole on the wall, and the light from CRT display was independent from the room illuminant. The luminance level of the stimulus was varied among 5 levels, which provides with five lightness levels. The observer also conducted HFP with the same stimulus to see which stage of the visual system introduces the changes in color shift under illuminant color changes. To evaluate the shift in sensitivity balance at each cone class, the observers also made achromatic adjustments under cone plateau. Results and discussions. As the lightness level of the stimulus increased, the achromatic point shifted from illuminant chromaticity to white, which was measured under cone plateau and was different between observers. The result was evaluated with an extension of relative M-cone weight by Ahn and MacLeod(1994), and the achromatic loci were fit with a linear function on a log-luminance-contrast axis. Also, the behavior of relative M-cone weights for achromatic adjustments and HFP settings were different, which suggests the nonlinear characteristics originate in post-receptoral stages in the visual system.