Lightness constancy is the ability to perceive black, gray, and white surfaces correctly regardless of illumination level. Lightness is the perceptual equivalent to the physical shade of gray of a surface, technically known as reflectance. A white surface reflects about 90% of the light it receives whereas black reflects only about 3%. Thus, a white paper will reflect 30 times as much light as a black paper lying next to it in the same illumination. This reflected light enters the eye when we look at the papers, and the light-sensitive rods and cones are stimulated proportionately. This seems to explain our perception of black and white, except that the intensity of illumination is seldom uniform in our world. Illumination intensity can vary by a factor ...