Color constancy is a visual phenomenon wherein colors of objects remain relatively the same under changing illumination. A red apple gives the same red appearance under either a fluorescent or incandescent lamp, although the lights that the apple reflects to the eye are quite different. The light reflected from an object into the human visual pathway is proportional to the product of the illumination incident on the surface and the invariant spectral reflectance properties of the object. The fact that humans possess approximate color constancy indicates that our visual system does attempt to recover a description of the invariant spectral reflectance properties of the object. Often, color constancy and chromatic adaptation are used interchangeably. However, Brill and West seem to suggest otherwise. Using Hunt's demonstration: a yellow cushion looks yellow when the whole scene is covered by a blue filter, but the cushion looks green when it is covered by the blue filter cut to the exact shape of the cushion. They stated that the yellowness of the cushion in the first instance is seen immediately despite the bluish cast of the whole slide; then the bluish cast diminishes after several seconds of chromatic adaptation. The difference in time dependence suggests that chromatic adaptation may be a different phenomenon from color constancy.
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