Effect of luminance on color perception of protanopes.

Small-field color-naming performance of two protanopes over a 4-log luminance range was impoverished in comparison with that of normal trichromats, and was more strongly affected by changes in luminance. At 200 cd/m2 responses to mid-spectral lights were dominated by 'yellow'; with lowering luminance, 'green' and 'red' were increasingly used. In the color spaces derived from these data the first two dimensions for trichromats are red-green and yellow-blue: those of the protanopes appear to be brightness and 'red-blue'. In the protanopes' color space the greater separation of stimuli at 0.2 cd/m2 suggests that with low luminance their color discrimination improves.