Lightness Constancy

Lightness Constancy is a theory proposed by psychology researcher Hans Wallach in 1948 that explored human visual perception of neutral colors (colors that have brightness but no hue; white, gray and black). This theory states that although these colors, when seen in isolation on a blank screen appear to emit light, when paired with a surrounding ring of different brightness, those items will no longer appear to emit light.

An example of this is how the moon, when high in a dark sky, appears to emit light. This type of visual illusion is believed to be the result of a mathematical ratio between the degrees of brightness that the eye and brain is capable of discriminating.