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Human eye adaptation to light

The adaptation process has three parts: (1) a rapid phase, which is necessarily electrical in origin and is probably a readjustment of cellular or neural activity; (2) a mechanical readjustment of pupil size, which takes a certain period of time; and (3) a slow phase, which determines the duration of the entire adaptation process and is presumably due to the time required to stabilize the ratio of the photopigment in the photoreceptor cells to the photopigment that has changed and has not changed after the change in brightness.

A dark-adapted observer is one who has spent enough time in darkness for his eyes to be optimally adapted to function in darkness. "Light adaptation" is a loose term that generally refers to the process by which the eye adapts to the brightness of the visual field that is sufficient to ensure daylight vision. Adaptation to lower brightness takes longer than the corresponding adaptation to higher brightness. In particular, adaptation to low brightness, if it involves the transition from cone-based to rod-based vision, takes longer.