Walsh & Hoyt: Normal Color Perception

The perceptions of light by normal human observers can be characterized using a three-dimensional ""color space."" The three axes of this space are an achromatic dimension and two chromatic axes, which can be related to the activity of the three retinal cones. Relative differences in the activity of the three retinal cones give rise to chromatic differences. Thus, along the deutan chromatic axis, the activity of the S-cone (maximally sensitive for Short wavelength light) remains constant, but the activity of the M-cone (Medium wavelength) changes with respect to that of the L-cone (Long wavelength). Along the tritan axis, the ratio of M-cone to L-cone activity remains constant, but S-cone activity varies relative to the sum of M- and L-cone activity. Variations along these two chromatic dimensions cause differences in hue, which is related to the perceived dominant wavelength (i.e., red, yellow, blue), and differences in saturation, which is related to the purity of the spectral composition, with paler, less saturated shades representing greater mixture of white light with the dominant hue (e.g., pink, rose, red). Differences in brightness (""luminance"" for light sources, ""reflectance"" for objects that reflect light) occur when light changes along the third dimension, the achromatic axis. Along this axis, the activity of all three cones changes together, such that the ratio of one cones activity relative to another remains constant. Conventional color diagrams such as the CIE (Committee Internationale de lEclairage) chart depict the colors within a plane of the two chromatic dimensions, while omitting brightness. The CIE chart is a roughly polar diagram with hue represented by the angle of the vector from the central point representing white (equal mixture of all hues) and saturation represented by the vector length.