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Try and recall the last time you went from a very bright area to a dim one. Perhaps you were outside on a bright sunny day just after a fresh snow fall. Then you walk indoors. Recall how difficult it was to see for several minutes just after you went indoors? This phenomenon is not so difficult to explain. It is called dark adaptation.
The time course of dark adaptation is well known. It is a two branched function; one for the cone receptors and the other for the rods.
As was discussed elsewhere, the receptors contain photopigments in their outer segments. When light is absorbed by these photopigments they undergo certain changes which stops them from helping to send visual signals to the brain. These changes are reversed in darkness.
Our visual system is most sensitive when the photopigments have not absorbed any light for about 30 minutes. Under these conditions we say that the photopigments are fully regenerated. When the rod photopigments are exposed to light they undergo a process called bleaching. It is called bleaching because the photopigment color actually become almost transparent. In the dark when they regenerate and regain their pigmentation again.
In the rod receptors the unbleached photopigments appears purple and is sometimes called visual purple. The technical name for the rod photopigment is rhodopsin.
The cone receptors (most of which are in the fovea) also have outer segments which contain photopigments. The photopigments in the cones also bleach when exposed to light. There are three classes of cone photopigments. Each class is photochemically a little different than the other and therefore their spectral absorbencies are different.
If you click on to receptor absorbencies you can see how the four (3 cone and 1 rod) types of receptors differ.
By clicking on microspectrophotometry you can get a brief idea of how the spectral absorbance information of the receptor photopigments is obtained.

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