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 one thousand. 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 ten thousand to one million 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 to bright sunlight from darkness. 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.
Ambient Light Response 
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
Dark Adaptation 
Rhodopsin, a biological pigment in the photoreceptors of the retina immediately photobleaches in response to light. 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.
The sensitivity of the rod pathway improves considerably within 5–10 minutes in the dark. Color testing has been used to determine the time at which rod mechanism takes over, when the rod mechanism takes over colored spots appear colorless as only cone pathways encode color.
Four factors affect dark adaptation:
- Intensity and Duration of the pre-adapting light
By increasing the levels of pre-adapting luminances, the duration of cone mechanism dominance extends, while the rod mechanism switch over is more delayed. In addition the absolute threshold takes longer to reach. The opposite is true for decreasing the levels of pre-adapting luminances.
- Size and Location of the Retina
- Wavelength of the Threshold Light
Varying the wavelengths of stimuli also effect the dark adaptation curve. Long wavelengths, such as extreme red, create the absence of a distinct rod/cone break as the rod and cones cells have similar sensitives to light of long wavelengths. Conversely at short wavelengths the rod/cone break is more prominent because the rod cells are much more sensitive than cones once the rods have dark adapted.
- Rhodopsin Regeneration
Inhibition by neurons also affects activation in synapses. Together with the bleaching of a rod or cone pigment, merging of signals on ganglion cells are inhibited, reducing convergence. Alpha adaptation, i.e. rapid sensitivity fluctuations, is powered by nerve control. The merging of signals by virtue of the diffuse ganglion cells, as well as horizontal and amacrine cells, allow a cumulative effect. Thus that area of stimulation is inversely proportional to intensity of light, a strong stimulus of 100 rods equivalent to a weak stimulus of 1,000 rods.
In sufficiently bright light, convergence is low, but during dark adaptation, convergence of rod signals boost. This is not due to structural changes, but by a possible shutdown of inhibition that stops convergence of messages in bright light. If only one eye is open, the closed eye must adapt separately upon reopening to match the already adapted eye.
Light Adaptation 
With light adaptation, the eye has to quickly adapt to the background illumination to be able to distinguish objects in this background. The process for light adaptation occurs over a period of five minutes.
Increment Threshold 
Using increment threshold experiments, light adaptation can be measured clinically. In an increment threshold experiment, a test stimulus is presented on a background of a certain luminance, the stimulus is increased until the detection threshold is reached against the background. A monophasic or biphasic threshold versus intensity TVI curve is obtained through this method for both cones and rods.
- 1. Dark Light
The threshold in this portion of the TVI curve is determined by the dark/light level. Sensitivity is limited by neural noise. The background field is relatively low and does not significantly affect threshold.
- 2. Square Root Law
This part of the curve is limited by quantal fluctuation in the background. The visual system is usually compared with a theoretical construct called the ideal light detector. To detect the stimulus, the stimulus must sufficiently exceed the fluctuations of the background (noise).
- 3. Weber's Law
- 4. Saturation
At saturation, the rod system becomes unable to detect the stimulus. This section of the curve occurs for the cone mechanism under high background levels.
Insufficiency of adaptation most commonly presents as insufficient adaptation to dark environment, called night blindness or nyctalopia. The opposite problem, known as hemeralopia, that is, inability to see clearly in bright light, is much rarer.
The fovea is blind to dim light (due to its cone-only array) and the rods are more sensitive, so a dim star on a moonless night must be viewed from the side, so it stimulates the rods. This is not due to pupil width since an artificial fixed-width pupil gives the same results.
See also 
- Neural adaptation
- Purkinje effect
- Photopic vision
- Mesopic vision
- Scotopic vision
- Accommodation (eye)
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