Editing Chromatic adaptation (section)

{{Short description|Human ability to adjust to changes in illumination}}
'''Chromatic adaptation''' is the human visual system’s ability to adjust to changes in illumination in order to preserve the appearance of object colors. It is responsible for the stable appearance of object colors despite the wide variation of light which might be reflected from an object and observed by our eyes. A '''chromatic adaptation transform''' ('''CAT''') function emulates this important aspect of color perception in [[color appearance model]]s.

An object may be viewed under various conditions. For example, it may be illuminated by sunlight, the light of a fire, or a harsh electric light. In all of these situations, human vision perceives that the object has the same color: a red apple always appears red, whether viewed during the day or at night (if the red apple is illuminated as rods in our eyes do not see red). On the other hand, a camera with no adjustment for light may register the apple as having varying color. This feature of the visual system is called chromatic adaptation, or [[color constancy]]; when the correction occurs in a camera it is referred to as [[white balance]].

Though the human visual system generally does maintain constant perceived color under different lighting, there are situations where the relative brightness of two different stimuli will appear reversed at different [[illuminance]] levels. For example, the bright yellow petals of flowers will appear dark compared to the green leaves in dim light while the opposite is true during the day. This is known as the [[Purkinje effect]], and arises because the peak sensitivity of the human eye shifts toward the blue end of the spectrum at lower light levels.