Editing Night vision (section)

==Biological night vision==
{{further|Adaptation (eye)#Accelerating dark adaptation|Scotopic vision}}

All [[photoreceptor cell]]s in the vertebrate eye contain molecules of [[Photoreceptor protein#Photoreceptors in animals|photoreceptor protein]] which is a combination of the protein [[photopsin]] in [[Cone cell|color vision cells]], [[rhodopsin]] in [[Rod cell|night vision cells]], and [[retinal]] (a small photoreceptor molecule). Retinal undergoes an irreversible change in shape when it absorbs light; this change causes an alteration in the shape of the protein which surrounds the retinal, and that alteration then induces the physiological process which results in vision.

The retinal must diffuse from the vision cell, out of the eye, and circulate via the blood to the liver where it is regenerated. In bright light conditions, most of the retinal is not in the photoreceptors, but is outside of the eye. It takes about 45 minutes of dark for ''all'' of the photoreceptor proteins to be recharged with active retinal, but most of the night vision [[Adaptation (eye)|adaptation]] occurs within the first five minutes in the dark.<ref name="Sensory">"Sensory Reception: Human Vision: Structure and function of the Human Eye" vol. 27, p. 179 Encyclopædia Britannica, 1987</ref> Adaptation results in maximum sensitivity to light. In dark conditions only the rod cells have enough sensitivity to respond and to trigger vision.
[[Image:Cone-response-en.svg|right|thumb|410px|Normalised [[absorption spectrum|absorption spectra]] of the three human photopsins and of human rhodopsin (dashed). Drawn after Bowmaker and Dartnall (1980).<ref>{{cite journal |last1=Bowmaker |first1=J K |last2=Dartnall |first2=H J |title=Visual pigments of rods and cones in a human retina. |journal=The Journal of Physiology |date=1 January 1980 |volume=298 |issue=1 |pages=501–511 |doi=10.1113/jphysiol.1980.sp013097 |pmid=7359434|pmc=1279132 }}</ref>]]

Rhodopsin in the human rods is insensitive to the longer red [[wavelengths]], so traditionally many people use red light to help preserve night vision. Red light only slowly depletes the rhodopsin stores in the rods, and instead is viewed by the red sensitive [[cone cell]]s.{{Citation needed|date=August 2019}}

Another theory posits that since stars typically emit light with shorter wavelengths, the light from stars will be in the blue-green color spectrum. Therefore, using red light to navigate would not desensitize the receptors used to detect star light.<ref>{{Cite journal |last1= Luria |first1= S.M. |last2= Kobus |first2= D.A. |publication-date= 26 April 1985 |date= April 1985 |title= Immediate Visibility after Red and White Adaptation |publisher= Naval Submarine Medical Research Laboratory |location= Submarine Base, Groton, CT |url= http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA156271&Location=U2&doc=GetTRDoc.pdf |journal=  |access-date= 25 March 2012 |archive-date= 1 December 2012 |archive-url= https://web.archive.org/web/20121201034224/http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA156271&Location=U2&doc=GetTRDoc.pdf |url-status= dead }}</ref><ref>{{Cite journal| last1 = Luria| first1 = S. M.| last2 = Kobus| first2 = D. A.| publication-date = 3 July 1984| date=July 1984| title = THE RELATIVE EFFECTIVENESS OF RED AND WHITE LIGHT FOR SUBSEQUENT DARK-ADAPTATION| publisher = Naval Submarine Medical Research Laboratory| location = Submarine Base, Groton, CT |url= http://www.torpedo.nrl.navy.mil/tu/ps/pdf/pdf_loader?dsn=8969829}}{{dead link|date=April 2025|bot=medic}}{{cbignore|bot=medic}}</ref>

Many animals have a tissue layer called the ''[[tapetum lucidum]]'' in the back of the eye that reflects light back through the [[retina]], increasing the amount of light available for it to capture, but reducing the sharpness of the focus of the image. This is found in many [[nocturnal]] animals and some [[deep sea]] animals, and is the cause of eyeshine. Humans, and monkeys, lack a ''tapetum lucidum''.<ref>{{cite journal|url=https://makezine.com/projects/make-35/how-to-make-and-use-retroreflectors/ |title=How to Make and Use Retroreflectors |journal=Make |date=2013-10-03 |author=Forrest M. Mims III| access-date=2017-10-21}}</ref><ref>J. van de Kraats and D. van Norren: "Directional and nondirectional spectral reflection from the human fovea" J.Biomed. Optics, 13, 024010, 2008</ref>
[[File:Human eye in dim light.jpg|thumb|300x300px|The pupil of the eye dilates in the dark to enhance night vision. Shown here is a pupil of an adult naturally dilated to 9 mm in diameter in [[mesopic]] light levels. The average human eye is not able to dilate to this extent without the use of mydriatics.]]
Nocturnal mammals have rods with unique properties that make enhanced night vision possible. The nuclear pattern of their rods changes shortly after birth to become inverted. In contrast to conventional rods, inverted rods have [[heterochromatin]] in the center of their nuclei and [[euchromatin]] and other transcription factors along the border. In addition, the outer layer of cells in the retina (the [[outer nuclear layer]]) in nocturnal mammals is thick due to the millions of rods present to process the lower light intensities. The anatomy of this layer in nocturnal mammals is such that the rod nuclei, from individual cells, are physically stacked such that light will pass through eight to ten nuclei before reaching the photoreceptor portion of the cells. Rather than being scattered, the light is passed to each nucleus individually, by a strong lensing effect due to the nuclear inversion, passing out of the stack of nuclei, and into the stack of ten photorecepting [[rod cell|outer segments]]. The net effect of this anatomical change is to multiply the light sensitivity of the retina by a factor of eight to ten with no loss of focus.<ref>{{cite journal | author = Solovei, I. |author2=Kreysing, M. |author3=Lanctôt, C. |author4=Kösem, S. |author5=Peichl, L. |author6=Cremer, T. | date = April 16, 2009| title = Nuclear Architecture of Rod Photoreceptor Cells Adapts to Vision in Mammalian Evolution  | journal = Cell | volume = 137 | issue = 2 | pages = 945–953 | url = http://www.science-direct.com/science?_ob=ArticleURL&_udi=B6WSN-4W3325G-S&_user=10&_coverDate=04%2F17%2F2009&_rdoc=24&_fmt=high&_orig=browse&_srch=doc-info(%23toc%237051%232009%23998629997%231050051%23FLA%23display%23Volume)&_cdi=7051&_sort=d&_docanchor=&_ct=27&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=3dd47ebb589ebdd767f957eea220aa01  | doi = 10.1016/j.cell.2009.01.052 | pmid = 19379699|display-authors=etal| doi-access = free }}</ref>

[[Pupillary response|Pupillary dilation]] is a biological process that contributes a relatively minor amount to night vision. In humans, the irises can adjust the size of the pupil from 2 mm in bright light, to as large as 8 mm in dark conditions, but this varies by individual and age, with age causing the maximal pupil diameter to decrease. However, some humans are capable of dilating their pupils to over 9 mm in diameter in the dark, giving them better night vision capabilities.{{Citation needed|date=June 2024}}