dark adaptation 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.

dark adaptation The changes that need to take place in the eye when an animal moves from a brightly lit environment to a relatively dark one to enable objects to be seen clearly. On moving to a darker environment, the pupils dilate and rhodopsin – the pigment in the rod cells that is broken down in bright light – is regenerated from its constituents.

dark adaptation (dark) n. the changes that take place in the retina and pupil of the eye enabling vision in very dim light. See rod. Compare light adaptation.