The Mystery of Tetrachromacy

You aren't paying attention, by "inputs" I meant "inputs from retina to the nervous system", not "inputs to the eye".

Most metamerism can be trivially explained by different combinations of light stimulating cones in the same way. But it looks like red+blue and violet are different and there is just no reason for them to be perceived the same, except that apparently there is also no reason to distinguish between them.

The sensitivity of the red cones appears to go up near violet (http://www.yorku.ca/eye/specsens.htm)

Wikipedia claims that is because of "a second resonancy of the red-sensitive cone cells." (https://en.m.wikipedia.org/wiki/Violet_light)

The natural human cornea is opaque to shorter-wavelength UV, but there's no reason it couldn't be replaced with something transparent to UV-A at least. This is supposedly an unintended side-effect of some cornea replacement surgery[1].

UV may have the potential to be damaging, but plenty of animals see UV-A safely - for example, bees[2] and mantis shrimp[3] can perceive UV to 300nm. Some birds (whose eyes are much more like ours, of course) can also perceive UV[4][5].

If we wanted some ready-made genes, I vote for borrowing from the European starling, which has receptors that sit in our enormous gap between blue/violet and green as well as UV-A [6].

Near infrared could be very useful, because (in combination with red, green, etc.) it can be used to easily distinguish vegetation, and it scatters less in the atmosphere, so it could allow clear vision at much farther distances. Most materials that are transparent to red/green/blue are also transparent to NIR, so as far as I know, the rest of the eye shouldn't need to be modified.

Most people can apparently see a little bit into the NIR if they wear NIR-bandpass goggles, but it's extremely dim and doesn't have a perceived colour that's distinct from the rest of the visible spectrum, so that's not really what the GP is suggesting. They're suggesting wiring up a true NIR-specific receptor in the eye which would provide a fourth, fifth, or sixth primary colour (in addition to red, green, blue, and optionally the in-between-red-and-green that tetrachromats see as well as a hypothetical UV-A receptor).

I can't find a cite at the moment, but some birds have NIR patterning on their feathers (I have photographed this myself at a zoo), and the last time I looked into it, there was some speculation that tropical birds in particular had some NIR sensitivity (in addition to UV-A) and had evolved spectrally-complex colouration as a result.

Thermal infrared is a much longer wavelength, and I'm not aware of a good material for lenses/eyes that is transparent to both. It's possible to have an uncooled sensor that detects it, but I agree that it's a much larger leap than UV-A or NIR.

[1] http://www.itworld.com/article/2732596/consumer-tech-science...

[2] https://www.agriculture.purdue.edu/agcomm/newscolumns/archiv...

[3] http://www.iflscience.com/plants-and-animals/mantis-shrimp-s...

[4] http://www.livescience.com/26994-how-birds-uv-vision.html

[5] http://www.webexhibits.org/causesofcolor/17B.html

The reason I commented is that the rest of your comment was excellent, and very informative. (I did not know our cornea and lens are UV opaque. And then the subsequent follow-ups to your point, explaining that it's more subtle than that by nl and blincoln, were also very interesting.) Your knowledge of the subject and willingness to explain it indicates to me that you're valuable to the HN community. So I felt it was worthwhile to point out the one bit of your comment which doesn't work well on HN, since your account is new.

If it's okay, I think we should set the non-vertabrates aside for the discussion since their optical systems are so different.

As to the safety of UV-A, I don't think you're correct. A common element of all the comparative species selected is that their life span in the wild is far shorter than our own, and there isn't going to be selection pressure happening before they would accumulate UV damage. UV damage to the eye is kind of like yellowing of plastics, it accumulates over time. As is pointed out in your first linked article, aphakia patients are advised to wear blue-blocking glasses to protect their vision. My first thought as to how bad could it be was retinoblastoma—I am not aware of any studies looking at the incidence rate of Rb in aphakia patients and PubMed doesn't show anything either, but I don't think it's an unreasonable thought.

Aside from the ionization problem, UV light also has a much higher propensity towards scatter. Floaters are bad enough for some in visible light! Of course, we're talking futuristic gene augmentation, so it's not unreasonable to suggest we'd vacuum the eyes out every now and then, so maybe floaters will not be a problem :) We still have a problem with the much higher dispersion of UV light relative to visible, given the materials we have to work with in the eye; that, plus scatter, means we're probably going to have very fuzzy UV vision. Replace the whole lens system and maybe you get better UV-vis vision, but we still haven't overcome the safety problem. Maybe instead of vacuuming out the eyes, we just swap them out for a new autologous gene-tweaked set every couple years.

NIR would seem to be more realistic than UV (certainly considering safety), but here, the quantum efficiency of the pigments would seem to be a limiting factor. As you point out, we can already actually see a little into NIR (IIRC, in scotopic with something like 10-6 QE at 750nm relative to 555nm). At first glance, the chemical structure of human receptor pigments doesn't look to be super favorable for modification to getting something that would have a good absorption cross-section in NIR. Compare the structure of vitamin A or retinal with a strongly NIR-active chromophore, there's a lot of steps bridging the two and the structure as a whole is hostile towards selective modifications, not to speak of the rings where the modifications need to happen.

As to thermal IR, I think we'd need a metamaterial for something that would work in vis and in LWIR, and that's a handwavey statement on my part. It would definitely be of interest to the military!

Overall, I think it's more plausible to augment our vision by wiring a camera directly into our visual cortex. Biology is too hard.