Editing Eye (section)

===Colour perception===
{{main|Colour vision}}
"Colour vision is the faculty of the organism to distinguish lights of different spectral qualities."<ref name=Ali&Klyne1985p161>{{harvnb|Ali|Klyne|1985|page=161}}</ref> All organisms are restricted to a small range of electromagnetic spectrum; this varies from creature to creature, but is mainly between wavelengths of 400 and 700&nbsp;nm.<ref name=Fernald1982>{{Cite book
 | year=1982
 | title=The Senses
 | page=[https://archive.org/details/senses0000barl/page/98 98]
 | isbn=978-0-521-24474-9
 | url=https://archive.org/details/senses0000barl
 | url-access=registration
 | publisher=Cambridge University Press
 | location=Cambridge
 |author1=Barlow, Horace Basil  |author2=Mollon, J.D.
}}</ref> This is a rather small section of the electromagnetic spectrum, probably reflecting the submarine evolution of the organ: water blocks out all but two small windows of the EM spectrum, and there has been no evolutionary pressure among land animals to broaden this range.<ref name=Fernald1997>{{Cite journal
 | author=Fernald, Russell D.
 | year=1997
 | title=The Evolution of Eyes
 | journal=Brain, Behavior and Evolution
 | volume=50
 | issue=4
 | pages=253–259
 | doi=10.1159/000113339
 | url=http://www.stanford.edu/group/fernaldlab/pubs/1997%20Fernald.pdf
 | pmid=9310200
 | access-date=2008-09-16
 | archive-date=2012-11-20
 | archive-url=https://web.archive.org/web/20121120200307/http://www.stanford.edu/group/fernaldlab/pubs/1997
 | url-status=live
 }}</ref>

The most sensitive pigment, [[rhodopsin]], has a peak response at 500&nbsp;nm.<ref name=Goldsmith1990/> Small changes to the genes coding for this protein can tweak the peak response by a few nm; pigments in the lens can also filter incoming light, changing the peak response.<ref name="Frentiu2008">{{Cite journal |author=Frentiu, Francesca D. |author2=Adriana D. Briscoe |year=2008 |title=A butterfly eye's view of birds |journal=BioEssays |volume=30 |issue=11–12 |pages=1151–1162 |doi=10.1002/bies.20828 |pmid=18937365 |s2cid=34409725}}</ref> Many organisms are unable to discriminate between colours, seeing instead in shades of grey; colour vision necessitates a range of pigment cells which are primarily sensitive to smaller ranges of the spectrum. In primates, geckos, and other organisms, these take the form of [[cone cell]]s, from which the more sensitive [[rod cell]]s evolved.<ref name=Goldsmith1990>{{Cite journal
 | author=Goldsmith, T.H.
 | year=1990
 | title=Optimization, Constraint, and History in the Evolution of Eyes
 | journal=[[The Quarterly Review of Biology]]
 | volume=65
 | issue=3
 | pages=281–322
 | doi=10.1086/416840
 | jstor=2832368
 | pmid=2146698| s2cid=24535762
 }}</ref> Even if organisms are physically capable of discriminating different colours, this does not necessarily mean that they can perceive the different colours; only with behavioural tests can this be deduced.<ref name=Frentiu2008/>

Most organisms with colour vision can detect ultraviolet light. This high energy light can be damaging to receptor cells. With a few exceptions (snakes, placental mammals), most organisms avoid these effects by having absorbent oil droplets around their cone cells. The alternative, developed by organisms that had lost these oil droplets in the course of evolution, is to make the lens impervious to UV light—this precludes the possibility of any UV light being detected, as it does not even reach the retina.<ref name=Goldsmith1990/>