Dark adaptation
In ocular physiology, adaptation is the ability of the eye to adjust to various levels of darkness and light.
The human eye can function from very dark to very bright levels of light; its sensing capabilities reach across nine orders of magnitude. This means that the brightest and the darkest light signal that the eye can sense are a factor of roughly 1,000,000,000 apart. However, in any given moment of time, the eye can only sense a contrast ratio of 1,000. What enables the wider reach is that the eye adapts its definition of what is black.
The eye takes approximately 20–30 minutes to fully adapt from bright sunlight to complete darkness and become 10,000 to 1,000,000 times more sensitive than at full daylight. In this process, the eye's perception of color changes as well (this is called the Purkinje effect). However, it takes approximately five minutes for the eye to adapt from darkness to bright sunlight. This is due to cones obtaining more sensitivity when first entering the dark for the first five minutes but the rods take over after five or more minutes.
Dark adaptation is far quicker and deeper in young people than the elderly.
A minor mechanism of adaptation is the pupillary light reflex, adjusting the amount of light that reaches the retina.
In response to varying ambient light levels, rods and cones of eye function both in isolation and in tandem to adjust the visual system. Changes in the sensitivity of rods and cones in the eye are the major contributors to dark adaptation.
Above a certain luminance level (about 0.03 cd/m2), the cone mechanism is involved in mediating vision; photopic vision. Below this level, the rod mechanism comes into play providing scotopic (night) vision. The range where two mechanisms are working together is called the mesopic range, as there is not an abrupt transition between the two mechanism. This adaptation forms the basis of the Duplicity Theory.
Rhodopsin, a biological pigment in the photoreceptors of the retina, immediately photobleaches in response to light.Visual Phototransduction starts with the isomerizing of the pigment chromophore from 11-cis to all-trans retinal. Then this pigment dissociates into into free opsin and all-trans retinal. Dark adaptation of both rods and cones requires the regeneration of the visual pigment from opsin and 11-cis retinal. Therefore, the time required for dark adaptation and pigment regeneration is largely determined by the local concentration of 11-cis retinal and the rate at which it is delivered to the opsin in the bleached rods. The decrease in calcium ion influx after channel closing causes phosphorylation of metarhodopsin II and speeds up the cis-retinal to trans-retinal inactivation. The phosphorylation of activated rhodopsin is mediated by recoverin. Rods are more sensitive to light and so take longer to fully adapt to the change in light. Rods, whose photopigments regenerate more slowly, do not reach their maximum sensitivity for about half an hour.Cones take approximately 9–10 minutes to adapt to the dark. Sensitivity to light is modulated by changes in intracellular calcium ions and cyclic guanosine monophosphate.
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