From AMS Glossary
The process by which the human eye's sensitivity increases in response to marked luminance decreases.
If the average luminance on the retina decreases suddenly from daytime photopic levels, a light source's just-detectable threshold luminance (alternatively, its threshold illuminance at the eye) decreases steadily for three to four minutes. This occurs because the eye's cones become much more sensitive during this period, although their sensitivity does not increase afterward. Note that cones are the only photoreceptors found in foveal (or central) vision, but that they coexist with rods outside the fovea. After about seven to ten minutes in very low-light scotopic conditions (0.1 lux or less), the rods become more sensitive than the cones, and visual sensitivity once again increases, reaching a maximum at about 20–30 minutes. The fully dark-adapted eye is nearly 105 times more sensitive than it is at daytime light levels, and the maximum rod sensitivity is about 103 times the maximum cone sensitivity. Although dark-adapted observers have a much lower threshold illuminance for detection, this is offset by the greatly increased threshold contrast in scotopic conditions. Mesopic vision occurs when both rods and cones contribute to vision at light levels between entirely photopic and scotopic conditions. Cones dominate photopic vision while only rods contribute to scotopic vision. Color vision is not possible in scotopic conditions, and the peak spectral sensitivity of scotopic vision occurs at shorter wavelengths than it does for photopic vision. Thus to the dark-adapted eye, blues and greens tend to look brighter than reds (the Purkinje shift).