Seeing Black and White

How the human visual system determines the lightness of a surface, that is, its whiteness, blackness, or grayness, remains--like vision in general--a mystery. In fact, we have not even been able to create a machine that can determine, through an artificial vision system, whether an object is white, black, or gray. Although the photoreceptors in the eye are driven by light, the light reflected by a surface does not reveal its shade of gray. Depending upon the level of illumination, a surface...