Eigengrau

[[File:CrumeyFig3.pdf|thumb|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.]] [[File:Photon-noise.jpg|thumb|An example of noise observed in the dark]] [[File:Image noise in low light sample (Canon 100D) at ISO 6400 (detail).jpg|thumb|Another example of noise observed in the dark]] '''Eigengrau''' ([[German language|German]] for 'intrinsic gray'; {{IPA|de|ˈʔaɪ̯gŋ̍ˌgʁaʊ̯|pron|De-Eigengrau.ogg}}), also called '''Eigenlicht''' ([[Dutch language|Dutch]] and German for 'intrinsic light'), '''dark light''', or '''brain gray''', is the uniform dark [[gray]] background color that many people report seeing in the absence of [[light|visible light]].{{Cn|date=December 2025}}

==Background== The term ''Eigenlicht'' dates back to the nineteenth century,<ref>{{cite journal |last=Ladd |first=Trumbull |year=1894 |title=Direct control of the retinal field. |url=https://zenodo.org/record/1429102 |journal=Psychological Review |volume=1 |issue=4 |pages=351–55 |doi=10.1037/h0068980}}</ref> 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.<ref>{{cite journal |author=Hansen RM, Fulton AB |date=January 2000 |title=Background adaptation in children with a history of mild retinopathy of prematurity |journal=Invest. Ophthalmol. Vis. Sci. |volume=41 |issue=1 |pages=320–24 |pmid=10634637}}</ref><ref>{{Cite web |last=Davis |first=Ben |date=August 20, 2024 |title=There Are So Many Amazing Colors You Don't Even Know About! |url=https://news.artnet.com/art-world/amazing-colors-tyler-thrasher-2525875 |url-status=live |archive-url=https://web.archive.org/web/20250419174120/https://news.artnet.com/art-world/amazing-colors-tyler-thrasher-2525875 |archive-date=2025-04-19 |access-date=2025-11-12 |website=Artnet News |language=en-US}}</ref>

Eigengrau is perceived as lighter than a [[black]] object in normal lighting conditions because [[contrast (vision)|contrast]] is more important to the visual system than absolute brightness.<ref>{{cite journal |last=Wallach |first=Hans |year=1948 |title=Brightness Constancy and the Nature of Achromatic Colors |journal=Journal of Experimental Psychology |volume=38 |issue=3 |pages=310–24 |doi=10.1037/h0053804 |pmid=18865234 |via=APA PsychNet}}</ref> For example, the night sky looks darker than Eigengrau because of the contrast provided by the stars.

[[Contrast (vision)|Contrast]] threshold data, collected by Blackwell<ref>{{cite journal |author1=Blackwell |first=H. Richard |date=1946 |title=Contrast Thresholds of the Human Eye |url=https://doi.org/10.1364/JOSA.36.000624 |journal=Journal of the Optical Society of America |volume=36 |issue=11 |pages=624–643 |doi=10.1364/JOSA.36.000624 |pmid=20274431 |url-access=subscription}}</ref> and plotted by [[Andrew Crumey|Crumey]], shows Eigengrau occurring at adaptation [[luminance|luminances]] below approximately 10− 5 cd m−2 (25.08 mag arcsec−2).<ref>{{cite journal |last1=Crumey |first1=Andrew |date=2014 |title=Human contrast threshold and astronomical visibility |journal=Monthly Notices of the Royal Astronomical Society |volume=442 |issue=3 |pages=2600–2619 |arxiv=1405.4209 |doi=10.1093/mnras/stu992 |doi-access=free}}</ref> This is a limiting case of [[Ricco's law]]. == Cause == 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 the [[retina]] produces random events indistinguishable from those triggered by real [[photon]]s.<ref>{{cite book |last=Barlow |first=H.B. |author-link=Horace Barlow |title=Visual Psychophysics |publisher=Springer-Verlag |year=1972 |isbn=978-0-387-05146-8 |location=New York |chapter=Dark and Light Adaptation: Psychophysics.}}</ref><ref>{{cite book |last=Barlow |first=H.B. |author-link=Horace Barlow |url=https://archive.org/details/vertebratephotor0000unse |title=Vertebrate Photoreception |publisher=Academic Press |year=1977 |isbn=978-0-12-078950-4 |location=New York |chapter=Retinal and Central Factors in Human Vision Limited by Noise |url-access=registration}}</ref> Later experiments on [[rod cell]]s of [[cane toad]]s (''Rhinella marina'') showed that the frequency of these spontaneous events is strongly temperature-dependent, which implies that they are caused by the thermal [[isomerization]] of [[rhodopsin]].<ref>{{cite journal |last1=Baylor |first1=D.A. |last2=Matthews |first2=G |last3=Yau |first3=K.-W. |year=1980 |title=Two components of electrical dark noise in toad retinal rod outer segments |journal=Journal of Physiology |volume=309 |pages=591–621 |doi=10.1113/jphysiol.1980.sp013529 |pmc=1274605 |pmid=6788941}}</ref> 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 the [[half-life]] of a rhodopsin molecule is about 420 years.<ref>{{cite journal |last=Baylor |first=Denis A. |date=1 January 1987 |title=Photoreceptor Signals and Vision |url=http://www.iovs.org/cgi/reprint/28/1/34 |journal=Investigative Ophthalmology & Visual Science |volume=28 |issue=1 |pages=34–49 |pmid=3026986}}</ref> The indistinguishability of dark events from photon responses supports this explanation because rhodopsin is at the input of the [[transduction (physiology)|transduction]] chain. On the other hand, processes such as the spontaneous release of [[neurotransmitter]]s cannot be completely ruled out.<ref>{{cite journal |last1=Shapley |first1=Robert |last2=Enroth-Cugell |first2=Christina |year=1984 |title=Visual Adaptation and Retinal Gain Controls |journal=Progress in Retinal Research |volume=3 |pages=263–346 |doi=10.1016/0278-4327(84)90011-7}}</ref>

== See also == * [[Closed-eye hallucination]] * [[Impossible color]]

== References == {{reflist}}

[[Category:Color]] [[Category:Darkness]] [[Category:Shades of gray]] [[Category:Vision]]