Colour Constancy, Categories, and Spectral Sensitivity
Colour constancy is a multi-stage process whose receptoral and post-receptoral components have been separated by electrophysiological and behavioral studies. A psychophysical study was conducted to determine contributions of the visual processing stage which extracts information about colour categories, and to relate the results to the spectral sensitivity of the chromatic opponent system. Changes in colour appearance with illumination were investigated by using free-viewing and a successive matching method for 40 Munsell hues (saturations: 8 and 4). Subjects were adapted to standard illuminant C and briefly inspected a test chip under a variable illuminant A (tungsten) or g (green). Using criteria similar to those for unique hues, subjects specified ‘typical hues’ under illuminant C: red, yellow, green, blue, and violet (yellow should not contain red or green, green should not contain yellow or blue, etc). For each hue and saturation a dominant wavelength was computed. The degree of colour constancy was then established for all subjects (examined in three series of experiments), all Munsell chips and illuminations, as a one-dimensional Brunswick ratio, BR. The BR was determined by using u‘ v’ coordinates as related to the ratio of perceptual to physical colour changes: BR=1-perceptual/physical. BR was highest for typical (not intermediate) hues and lowest for hues resembling variable illuminants. The BR as a function of dominant wavelength resembles the spectral sensitivity function of chromatic opponency. These findings are interpreted as contributions of high-order opponent interactions to colour constancy.