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Takayuki Fukuya, Keiji Uchikawa; The transition luminance between the surface-color and the illuminant-color modes may reveal the illuminant represented in the visual system. Journal of Vision 2007;7(9):239. doi: 10.1167/7.9.239.
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Color appearances may be classified into two modes, the surface-color mode and the illuminant-color mode. The mode of color appearance is determined by the luminance of the stimulus. It has been reported that the transition luminance between the surface-color and the illuminant-color modes depends on the chromaticity of the stimulus. When an illuminant is fixed real surfaces having the same chromaticity should have a luminance limit (the luminance of the optimal color) because spectral components of a surface reflectance range from 0 to 1. If the luminance of a color surface exceeds this luminance limit it should no more appear in the surface-color mode but in the illuminant-color mode. We hypothesize that the visual system somehow knows the luminance limit of a color surface and uses it to determine the transition luminance between the two modes. To confirm this hypothesis we compared the transition luminance and the luminance limit of various color surfaces under various color illuminants (D65, 3000K, 25000K, (x′, y′) = (0.545, 0.341), (0.269, 0.562), (0.164, 0.076)). The transition luminance was measured in experiments in which the observer could adjust luminance of the test stimulus for a chromaticity fixed. The luminance limit was calculated under the same color illuminants. It was found that, under a white illuminant, the transition luminance and the luminance limit showed a similar variation across the chromaticity. However, under chromatic illuminants, they were found to be quite different. When chromatic illuminants, desaturated than the real illuminants, were used for the calculation the luminance limit became quite similar to the transition luminance. The present results suggest that the visual system might use the luminance limit of a surface under a desaturated illuminant to determine the transition luminance.
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