Editing Photoreceptor cell (section)

== Photosensitivity ==
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[[File:1416 Color Sensitivity.svg|thumb|300px|right|Normalized human photoreceptor absorbances for different wavelengths of light<ref>{{cite journal|author1=Bowmaker J.K.  |author2=Dartnall H.J.A. |name-list-style=amp |pmc=1279132 |title=Visual pigments of rods and cones in a human retina|journal=J. Physiol.|pmid=7359434|volume=298|pages=501–511|year=1980|doi=10.1113/jphysiol.1980.sp013097}}</ref>]]
Each photoreceptor absorbs light according to its [[spectral sensitivity]] (absorptance), which is determined by the [[photoreceptor protein]]s expressed in that cell. Humans have three classes of cones (L, M, S) that each differ in spectral sensitivity and 'prefer' photons of different [[wavelengths]] (see graph). For example, the peak wavelength of the S-cone's spectral sensitivity is approximately 420&nbsp;nm (nanometers, a measure of wavelength), so it is more likely to absorb a photon at 420&nbsp;nm than at any other wavelength. Light of a longer wavelength can also produce the same response from an S-cone, but it would have to be brighter to do so.

In accordance with the [[principle of univariance]], a photoreceptor's output signal is proportional only to the number of photons absorbed. The photoreceptors can not measure the wavelength of light that it absorbs and therefore does not detect color on its own. Rather, it is the ratios of responses of the three types of cone cells that can estimate wavelength, and therefore enable [[color vision]].