Adaptation
Chromatic Adaptation
Light Adaptation
Dark Adaptation
Transient Adaptation
Transient Adaptation Factor
Adaptive Color Shift

(Term of physiology)

Adaptation
1. The process by which the visual system changes its sensitivity, depending on the luminances prevailing in the visual field. The system becomes accustomed to processing higher or lower light levels in it's environment than it was exposed to before. In a quick first step, some change is acheived by increasing or reducing the iris opening (in photographic terms: the aperture), which directly increases or reduces the amount of light that can enter the eye. In a second step, the receptive cells on the retina of the eye change their actual sensitivity. The latter is a slower process, so that it may take a few minutes until the visual system is fully adjusted to the new situation.

Since there are several types of receptive cells in the eye, which are sensitive to differend bands in the visible spectrum, the adaptation also manages the "white balance" of the eye, by chromatic adaptation. If the new lighing situation has a different color temperature, eg. there is an increased amount of red light light relative to the total amount of light, then the cells responsible for sensing red light will reduce their sensitivity relative to the sensitivity of the other cells. As a result, a white surface will again appear white to the observer after a certain time, allthough it reflects a proportionally increased amount of red light.

A very obvious example of (quantitative) adaptation can be observed by a person walking from full sunshine into a building. The environment in the building will appear almost pitch black at first. A few minutes later, the person can again distinguish details (eg. read text from a piece of paper). But by then, viewing out of the window will have become uncomfortable, since the proportionally very high luminance levels outside will cause strong glare.

2. A specific state of eye sensitivity resulting from this process.

Transient Adaptation is a special case, where the human eye has to adapt from low to high light levels and back in short intervals. This happens when the visual environment has very high contrasts, eg. a computer monitor (< 200 cd/m²) and a sunlit wall outside a window (> 5'000 cd/m²) can be seen next to each other without turning the head. Excessive transient adaptation soon results in eye fatigue.

The Transient Adaptation Factor defines the relative amount by which the equivalent contrast is reduced due to readaptation from one luminous background to another.

Light Adaptation is the special case when the visual system becomes adapted to luminances of more than about 3.4 cd/m².

Dark Adaptation is the special case when the visual system becomes adapted to luminances of less than about 0.034 cd/m².

Adaptive Color Shift is the change in the perceived object color caused by the change of the state of chromatic adaptation.