Adaptation is the ability of the eye to cope with different orders of luminance. Adaptation is necessary because the range of luminance our eyes have to cope with is so huge. The eye is capable of adapting to luminances as high as 1,000,000cd/m² and as low as 0.000,000,1cd/m². Once adapted, the eye can cope with a luminance range of 1:1,000, although this will decrease drastically with age.
It takes some time for the eye to fully adjust to the luminous environment. This can easily experienced when moving from a well-lit to a dark room or when switching off the light. The adaptation from light to dark takes considerably longer than the reverse, and may take half an hour or more to complete.
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Adaptation
is not immediate
(Images courtesy of Marlin Lighting)