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Incoming light reacts with the several types of cone cells in the eye. In humans, there are three types of cones sensitive to three different spectra, resulting in trichromatic color vision. Each individual cone contains pigments composed of opsin apoprotein, which is covalently linked to either 11-cis-hydroretinal or more rarely 11-cis-dehydroretina.

The opsins (photopigments) present in the L and M cones are encoded on the X chromosome. A very small percentage of women may have an extra type of color receptor because they have different alleles for the gene for the L opsin on each X chromosome. X chromosome inactivation means that only one opsin is expressed in each cone cell, and some women may therefore show a degree of tetrachromatic color vision. Tetrachromacy is also demonstrated among several species of birds, fish, amphibians, reptiles and insects.

Humans cannot see ultraviolet light directly because the lens of the eye blocks most light in the wavelength range of 300–400 nm.

So, besides above mentioned exceptions, is it possible due to mutations or genetic engineering to see UV or infrared. What kind of mutations should be made or are the changes to be made in the opsines to big to create 'alien' eyes?

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It might be easier to embed a microchip and a camera, though. – Wayne Werner Feb 23 at 21:12
I seem to remember that kids can generallybsee farther into the UV range than adults typically do, perhaps because their lenses are smaller... but don't take my word for it. – keshlam Feb 24 at 6:42
UV sensing is possible and some animals do have UV-sensitive opsins. IR cannot cause electronic transitions. They can only affect vibrational states. Therefore the mechanism of IR perception would be totally different from that of the UV-visible spectrum. – WYSIWYG Feb 24 at 6:50
@keshlam: afaik this is because the lens filters UV and the older you are the more it filters. People without a lens (or artifical lense) are indeed able to see UV, just not as a further colour, more like an overall white glow. – PlasmaHH Feb 24 at 9:47
You might be interested in tetrachromates ( there seem to be some humans that are able to it. – PlasmaHH Feb 24 at 9:47


Far-red vision (>700nm)

The ability for retinal-binding proteins to absorb far-red (between 700nm-850nm) light has been experimentally confirmed in this paper. While the authors did not attempt this in vivo in an animal model, they managed to use directed mutagenesis to induce a significant shift in the absorption peak of the chromophore of the retinal binding protein.

The authors show the shifts induced by these mutant proteins in the figure below. The mutants have significantly different absorption peaks, and assuming that the proteins do not lose their ability to transduce signals via optic nerves, they would allow vision in different wavelengths.

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Ultraviolet vision (300-400nm)

It is known that humans lacking a lens (aphakia) are able to see ultraviolet light. As noted in the question, the lens blocks out long-wave UV, and its removal or absence results in ultraviolet vision (although with low sharpness due to the lack of a lens).

However, aphakic patients report that the process has an unusual side effect: they can see ultraviolet light. It is not normally visible because the lens blocks it. Some artificial lenses are also transparent to UV with the same effect. The receptors in the eye for blue light can actually see ultraviolet better than blue. Military intelligence is said to have used this talent in the second world war, recruiting aphakic observers to watch the coastline for German U-boats signalling to agents on the shore with UV lamps.

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I am not too sure about IR absorption. IR does not have enough energy to cause electronic transitions. Moreover I could not find a mention of "IR" or "infrared" in the linked paper. The wavelengths mentioned there correspond to far red. – WYSIWYG Feb 24 at 6:45
Many animals that "sense" IR use thermoreceptors for it. The mechanism is totally different from that of opsins. – WYSIWYG Feb 24 at 6:48
@WYSIWYG I don't get what you mean by "doesn't have enough energy". There is an entire field dedicated to absorbances caused by electronic transitions in the infrared region. Furthermore, thermoreceptor-based IR sensing works for far longer wavelengths from 5-30µm. I edited the answer to say "near-infrared" instead, since the M11 mutant has significant absorbance over 700nm (the rightmost boundary of the graph). – March Ho Feb 24 at 7:19
IR spectroscopy measures vibrational transitions as clearly mentioned in the first line. I meant that IR does not cause electrons to be promoted to an excited state (that is what I mean by electronic transitions). They cause stretching and bending of bonds (which are vibrational transitions). What you are referring to is called far-red. – WYSIWYG Feb 24 at 7:34
@WYSIWYG Thanks for the corrections. Edited. – March Ho Feb 24 at 8:18

March Ho's answer is quite good. A few extra tidbits:

  • The population of humans contains DNA encoding for two substantially different M receptors. (differing by more than normal variation) See tetrachromacy in humans. (This doesn't produce IR vision except ...)
  • The population of humans contains DNA encoding for several different L receptors. Two well studied variants have very similar peak absorbances at $555\,\text{nm}$ and $559\,\text{nm}$. Although one variant has a slightly fatter "tail" out into the IR, this doesn't normally result in significant IR vision, except ...
  • People who work with intense near-IR lasers do report seeing a variety of "weird pink" from sufficiently intense lasers. This happens with scotopic adjusted eyes and seems to be a "hard to interpret" mixture of rod+L excitation. Also, some people report seeing the Wood Effect (intense IR scatter of sunlight from grass and leaves) when visible wavelengths are filtered out after scoptopic adjustment. One theory for IR vision in humans is that detection is via a two-photon process in the rods. Given that only intense sources are perceived, this isn't too far fetched. (... and is quite cool -- nonlinear optics in your own eye ...)
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