Dark adaptation

From Glossary of Meteorology
Revision as of 17:48, 26 January 2012 by imported>Perlwikibot (Created page with " {{TermHeader}} {{TermSearch}} <div class="termentry"> <div class="term"> == dark adaptation == </div> <div class="definition"><div class="short_definition">The process...")
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)

dark adaptation

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).

Copyright 2022 American Meteorological Society (AMS). For permission to reuse any portion of this work, please contact permissions@ametsoc.org. Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act (17 U.S. Code § 107) or that satisfies the conditions specified in Section 108 of the U.S.Copyright Act (17 USC § 108) does not require AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a website or in a searchable database, or other uses of this material, except as exempted by the above statement, require written permission or a license from AMS. Additional details are provided in the AMS Copyright Policy statement.