Figure 3.

Comparison of the physical relationships between surface reflectance and spectral illumination (A) and the phenomenology of color contrast (B). (A) The cartoon at the top indicates the comparison being made between the return from surfaces under illumination with light whose power is uniformly distributed (right) and illumination of the same surface with chromatic light (left). Changes are shown in the correlation (1), variance in y (or bandwidth in x) (2), and overall intensity (3) of the spectral return from the target surface relative to the return generated by the same surface under an illuminant with uniformly distributed power at three different levels of variance [indicated by the light (broad bandwidth), medium, and dark (narrow bandwidth) gray lines] (see Fig. 2). The distance along the abscissa indicates the relative similarity of the power distribution of the illuminant and the reflectance efficiency function of the surface. The left ordinate shows the normalized change in the correlation, variance, and intensity of the returns (note that increasing variance in y represents a narrowing of the bandwidth of the spectral profile); the right ordinate in 1 also shows the direction in which the spectral return is shifted relative to the return from the same surface under neutral illumination (rightward indicates a shift toward longer wavelengths, and leftward, a shift toward shorter wavelengths). (B) The cartoon at the top shows an example of the test paradigm (see Experimental Methods). Graphs show average adjustments made by subjects in the hue (1), saturation (2), and brightness (3) of the target on the neutral background to match the color sensation elicited by the same target on a chromatic (but equiluminant) background. Each stimulus was presented at three levels of saturation [indicated, as in A, by light (low saturation), medium, and dark (high saturation) gray lines]. The distance along the abscissa indicates the relative similarity of the hue of the surround and the hue of the target. The ordinate shows the normalized change in the hue, saturation, and brightness of the target on the chromatic surround relative to the target on the achromatic surround. CW on the right ordinate in 1 represents shifts in hue that are clockwise in Munsell color space, whereas CCW represents hue shifts that are counterclockwise. The data are the average of the responses of the two authors and one naïve subject to all contrast stimuli; bars are standard errors.