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Is the regeneration of visual purple fairly linear over the two hour period?

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The fully dark-adapted eye, in which photopigment regeneration is complete, restores retinal sensitivity to its maximal level. Rods and cones differ markedly, however, in their rate of dark adaptation. Cones attain maximum sensitivity in 5-7 minutes, while rods require 30-45 minutes or longer of absolute darkness to attain maximum sensitivity after exposure to bright light.

The cones have a faster rate of photochemical regeneration because they function in greater light than the rods. The cones, however, do not achieve the same level of sensitivity as the rods. The rods slowly adapt to dim illumination, but eventually achieve a much greater sensitivity than the cones. Depending on the preadaptation to light, dark adaptation is about 80% complete within 30 minutes, but it may take hours, or even days, to acquire total dark adaptation.

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Daytime exposure to ordinary sunlight can produce temporary but cumulative aftereffects on dark adaptation and night vision. Both civilian and military studies have documented significantly diminished rod performance after prolonged sunlight exposure at, for example, the beach or ski slope. Two or three hours of bright sunlight exposure has been shown to delay the onset of rod dark adaptation by 10 minutes or more, and to decrease the final threshold, so that full night vision sensitivity could not be reached for hours. After 10 consecutive days of sunlight exposure, the losses in night vision were reported to cause a 50 % loss in visual acuity, visibility range, and contrast discrimination. Repeated daily exposures to sunlight prolong the time to reach normal scotopic sensitivity, so that eventually normal rod sensitivity may not be reached.

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edz

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Edited by EdZ (08/10/06 10:08 AM)

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