Color Constancy

Color constancy is the tendency of the human visual system to perceive an object as having a stable, consistent color across substantially different lighting conditions, so that, for example, a green apple continues to look green whether it is viewed at midday under mostly white sunlight or at sunset under strongly reddish light. The effect is achieved because the eye's cone cells detect overlapping ranges of light wavelengths reflected from an object, and the visual system then estimates the composition of the illuminating light and effectively discounts its influence, extracting the object's underlying reflectance properties, which are what primarily determine the color a person actually perceives. An early account of the underlying process was offered centuries ago by the scholar Ibn al-Haytham, who recognized that light reflected from an object is modified by the object's own color and that the visual system must separate these two contributing factors. Modern research has identified specialized double-opponent cells within the visual cortex that compute local ratios of activity among the different cone types, with the cortical area known as V4 considered particularly important for producing color constancy. The ability to perceive stable object colors despite changing illumination supports reliable object recognition and consistent behavior across a visual environment that is, in physical terms, constantly changing in its lighting.