dark adaptation

n.
The physical and chemical adjustments of the eye, including dilation of the pupil and increased activity of rods in the retina, that make vision possible in relative darkness.

dark-adapt dark'-a·dapt' (därk'ə-dăpt') v.
dark-adapted dark'-a·dapt'ed (därk'ə-dăp'tĭd) adj.

In the eye, the visual pigment rhodopsin is formed by reaction between vitamin A aldehyde and the protein opsin, and is bleached by exposure to light, stimulating a nerve impulse (this is the basis of vision). At an early stage of vitamin A deficiency it takes considerably longer than normal to adapt to seeing in dim light after exposure to bright light, because of the limitation of the amount of rhodopsin that can be reformed. Measuring the time taken to adapt to dim light (the dark adaptation time) thus provides a marker of early vitamin A deficiency. More severe vitamin A deficiency results in night blindness, and eventually complete blindness.

An increase in the sensitivity of the eye to light when a person remains in darkness or in low illumination.

n.

The adjustment of the eye under reduced illumination, in which sensitivity to light is greatly increased. Also called scotopic adaptation.

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The noun has one meaning:

Meaning #1: the process of adjusting the eyes to low levels of illumination; cones adapt first; rods continue to adapt for up to four hours

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