The problem of being white: testing the highest luminance rule.

It is a common understanding that white serves as an “anchor” for the visual system for lightness scaling purposes. By lightness we are referring to surface color perception in the achromatic domain. The importance of “surface white” is stated also in the literature about luminosity perception, where it is often claimed that in order for a region of the visual field to appear as self-luminous, its luminance must be somewhat higher than the luminance of a surface perceived as white under the same conditions of illumination. Implicit in this assumption is that the visual system is able to determine what is to be seen as white instead of luminous, glowing or light gray. A “highest luminance rule”, eventually corrected by an area factor, seems so far to be the best candidate. This approach has been applied to several lines of data with apparent success. However, below we will describe two experiments, one concerning lightness and the other perceived luminosity, that show the severe limitations of the highest luminance rule hypothesis.