Threshold increment versus background luminance for various target diameters (in arcmin). Data from tables 4 and 8 of Blackwell (1946), plotted in Crumey (2014). The flat curves at low light indicate Eigengrau.An example of noise observed in the darkAn example of noise observed in the dark #2
Eigengrau (German for "intrinsic gray";pronounced[ˈʔaɪ̯gŋ̍ˌgʁaʊ̯]ⓘ), also calledEigenlicht (Dutch and German for "intrinsic light"),dark light, orbrain gray, is the uniform darkgray background color that many people report seeing in the absence oflight. The termEigenlicht dates back to the nineteenth century,[1] and has rarely been used in recent scientific publications. Common scientific terms for the phenomenon include "visual noise" or "background adaptation". These terms arise due to the perception of an ever-changing field of tiny black and white dots seen in the phenomenon.[2][3]
Eigengrau is perceived as lighter than ablack object in normal lighting conditions, becausecontrast is more important to the visual system than absolute brightness.[4] For example, the night sky looks darker than Eigengrau because of the contrast provided by the stars.
Contrast threshold data, collected by Blackwell[5] and plotted byCrumey, shows Eigengrau occurring at adaptationluminances below approximately 10− 5 cd m−2 (25.08 mag arcsec−2).[6] This is a limiting case ofRicco's law.
Researchers noticed as early as 1860 that the shape of intensity-sensitivity curves could be explained by assuming that an intrinsic source of noise in theretina produces random events indistinguishable from those triggered by realphotons.[7][8] Later experiments onrod cells ofcane toads (Rhinella marina) showed that the frequency of these spontaneous events is strongly temperature-dependent, which implies that they are caused by the thermalisomerization ofrhodopsin.[9] In human rod cells, these events occur about once every 100 seconds on average, which, taking into account the number of rhodopsin molecules in a rod cell, implies that thehalf-life of a rhodopsin molecule is about 420 years.[10] The indistinguishability of dark events from photon responses supports this explanation, because rhodopsin is at the input of thetransduction chain. On the other hand, processes such as the spontaneous release ofneurotransmitters cannot be completely ruled out.[11]
^Hansen RM, Fulton AB (January 2000). "Background adaptation in children with a history of mild retinopathy of prematurity".Invest. Ophthalmol. Vis. Sci.41 (1):320–24.PMID10634637.
^Wallach, Hans (1948). "Brightness Constancy and the Nature of Achromatic Colors".Journal of Experimental Psychology.38 (3):310–24.doi:10.1037/h0053804.PMID18865234.
^Shapley, Robert; Enroth-Cugell, Christina (1984). "Visual Adaptation and Retinal Gain Controls".Progress in Retinal Research.3:263–346.doi:10.1016/0278-4327(84)90011-7.
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