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Adaptation
Chromatic Adaptation
Light Adaptation
Dark Adaptation
Transient Adaptation
Transient Adaptation Factor (TAF)
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 its 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 different 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 (TAF) 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.
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