Disclaimer: Infrared is maybe the wrong term, I took it to mean anything that we do not currently perceive with a longer wavelength than what we see as red. I don't think in the following they refer to "seeing any light with long wavelengths".

I encountered a fourth-hand claim of army experiments to give humans infrared vision a while ago. He quotes in full from a textbook (Sekuler, R., and Blake, R. (1994). Perception (3rd ed.). Springfield, Ill.: Thomas.) pp. 62-63:

The following story dramatizes how photopigments determine what one can see. During World War II, the United States Navy wanted its sailors to be able to see infrared signal lights that would be invisible to the enemy. Normally, it is impossible to see infrared radiation because, as pointed out earlier, the wavelengths are too long for human photopigments. In order for humans to see infrared, the spectral sensitivity of some human photopigment would have to be changed. Vision scientists knew that retinal, the derivative of vitamin A, was part of every photopigment molecule and that various forms of vitamin A existed. If the retina could be encouraged to use some alternative form of vitamin A in its manufacture of photopigments, the spectral sensitivity of those photopigments would be abnormal, perhaps extending into infrared radiation. Human volunteers were fed diets rich in an alternative form of vitamin A but deficient in the usual form. Over several months, the volunteers' vision changed, giving them greater sensitivity to light of longer wavelengths. Though the experiment seemed to be working, it was aborted. The development of the "snooperscope," an electronic device for seeing infrared radiation, made continuation of the experiment unnecessary (Rubin and Walls, 1969). Still, the experiment demonstrates that photopigments select what one can see; changing those photopigments would change one's vision.

The second-hand source is cited 4 times according to Google Scholar, so apparently some people had access to it (or were unscrupulous about scraping references from another textbook...). But in the one citing paper I have access to, they cite it for a different claim.

On a Wikipedia talk page about infrared vision, this Snopes report was considered relevant. While the claim is much more crude and about Britain's airforce instead of the US navy, it does have some similarities.

Lundquis states himself he couldn't get his hands on the second-hand source, so we don't know the authors of the first study. I tried finding it via Google too (a while ago) and didn't succeed. I don't know how navy experiments are published (delayed maybe?!), so that was a probable hindrance.

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    I am having the same trouble finding anything beyond what you have already reported. I will say that the US Military (as well as others all over the world) have engaged in dubious experiments of questionable validity for a long time. Many are borne out of desperation, or a misunderstanding of how things really work. These experiments are generally stopped after a while, but there is always an advocate for strangeness when attempting to be better then someone else. – Larian LeQuella Jun 12 '11 at 16:18
  • You might be interested in this. Using optical filters to block out everything but near-infrared: amasci.com/amateur/irgoggl.html – endolith May 29 '18 at 15:38

This question is very much related to this question, Does eating carrots improve your eyesight? Although this specifically gets at attempting to see into the infrared spectrum. I will get to two main things:

First of all, Vitamin A is instrumental in healthy eyes, which in essence will help your eyesight.

However, there are physiological limits that nature has placed on us. The vitreous humour of the eye actually absorbs IR radiation, so even if we had the photochemical ability to detect it, most wouldn't reach the retina. Then, if it reached the retina, and was detected, we don't have the neural structure to interpret that data. As if all this wasn't bad enough, if we had all the required mechanisms in place, we'd end up blinding ourselves with our own body heat!

The second point is that the US DoD has invested money in many very questionable research projects. For instance, remote viewing is one of the better known ones, or the Stargate Project (yeah, they actually used that name!). Most of this stems from wanting to explore every possible advantage you can possibly get. And no matter how unlikely an idea is, there will be an advocate for it.

Apparently, this guy has given the issue some thought as well. While checking trackbacks on some of my quoted references, I found this site, which has an expanded discussion and additional references.

First, immediately in front of the photoreceptors in the eye of all vertebrates is a few centimetres of water-based vitreous and aqueous fluid. Infra-red radiation is much lower in energy than visual light (remember the energy of a photon is inversely proportional to its wavelength), and water is very good at absorbing infra-red light. Therefore the eye would not actually be transparent for infra-red light, so may absorb a substantial fraction of the photons (van der Berg, 1997) before it hits the retina. It’s might not be so bad, though, because at the low energy of 10000nm, the light may even be too weak to be absorbed by water.

Second, neither of the two photo-active molecules in vertebrates photoreceptors, 11-cis-retinal (A1) or 11-cis-3,4-dehydroretinal (A2), would not stable if it was absorbing at this wavelength. In order to activate these pigments, energy must be supplied from either the light or from the thermal energy (Ala-Laurila et al, 2004). The lower the wavelength of light, the less energy the photons have, so the greater the thermal energy must contribute to the activation energy. The thermal energy, however, can activate the photoreceptor by itself – one of the reasons why cold-blooded creatures are theoretically more sensitive to single photons (because a single photon even is indistinguishable from random thermal activation, and cold creatures will have less thermal activity) (Aho et al, 1988). If we were trying to set up a molecule that would activate from an infra-red photon, even one with a much shorter wavelength than 10 microns, we’d have a molecule that would activate very easily due to thermal noise. In other words, we’d have a lot of ‘static’ in a thermal visual system – so much so, that I doubt we’d be able to even see anything.

The third problem is the neural circuitry that would be required to process another colour. The retina has colour opponent ganglion cells, which feed into the colour-sensitive parvocellular layers of the lateral geniculate nucleus, which in turn stimulate sets of neurons called blob cells in the the visual cortex (Dacey, D.M., 1996). These systems are responsible for processing colour differences (for example, the difference between green and blue). There is a rare syndrome known as cerebral achromatopsia, where a person has a loss of colour vision (see the world in shades of gray) despite fully function cone photoreception, because of lesions to the parts of the brain responsible for this colour discrimination (Barbur et al, 1994). That said, it may be an undamaged brain will be able to adapt to accommodate any changes to colour-related inputs, even during adulthood (Neitz et al, 2002).

The last problem and perhaps the greatest problem with allowing humans to visualise thermal emissions, is that the eye itself it an emission source. The eye is covered in warm blood vessels, each emitting at the same wavelength as the other humans who we would like to detect with our thermal vision. Thermal signals from behind the photoreceptors could perhaps be shielded, allowing the photoreceptor cells to only detect signals from directly in front of it, but even the aqueous and vitreous humours and corneal cells are emitting thermal signals. The internal signal would still likely be greater than that reaching the photoreceptors from external emissions sources (like another person) . Pythons and pit vipers are cold-blooded, so they don’t have this problem. But to a warm-blooded human, it would be like trying to look out into the night from inside a brightly-lit room.

Given this write-up, I think I would venture to say that the results may have been part of tactical deception as indicated by Snopes.

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    This is very relevant (and cool information), but the story ("extend the spectrum a little downwards") was more plausible than what you talk about ("give humans heat vision"). There is the possibility of human tetrachomats and the brain interpreting completely foreign stimuli, namely sonar, as spatial info. In the navy experiment it would have been enough to extend it a little beyond normal vision. Is that still so implausible? The guy you cite hasn't considered alternative forms of retinal, right? – Ruben Jun 12 '11 at 17:05
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    For what I meant infrared was maybe the wrong term, I meant anything we do not currently perceive that has lower wavelength than what we perceive as red. Do you have a recommendation on how to reword without using this tortuous sentence? – Ruben Jun 12 '11 at 17:07
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    @Ruben anything below red is infrared. Now, there are some very rare reports of people having sensitivity beyond what is generally normal, however, it is not that far into the IR spectrum. What people generally MISTAKE for IR is light sensitivity (i.e. being able to see with very little light in the normal spectrum). That is more along the lines of preserving rhodopsin in the eye. But that's a different mechanism all together. – Larian LeQuella Jun 12 '11 at 17:43
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    For philosophy.SE I just dug up something that is more commonly known (not to me though ^^) and better sourced: Aphakic patients see some ultraviolet light. Again, the story repeats: "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." Maybe this is a common trope, but maybe one of all those stories is actually true. – Ruben Jun 13 '11 at 11:56
  • I have heard that artificial corneas give people a small extra range of UV sensitivity as receptors (don't know whether colour or b/w ones) are sensitive to UV but the normal cornea blocks everything below about 400nm. – matt_black Nov 5 '11 at 21:29

There was work done after WWII on this issue. Millard and McCann fed high levels of vitamin A2 to eight medical students at the University of Rochester School of Medicine; the control group of seven medical students did not receive vitamin A2. Students in the experimental group had a 30% lowering of the threshold for red light, a 25% improvement of red in respect to blue vision, and an increase in sensitivity to red light shown by spectral sensitivity curves. The paper does not mention the alleged military work. Reference: Effect of Vitamin A2 on the Red and Blue Threshold of Fully Dark Adapted Vision, Ernest B. Millard, JR., and William S. McCann. 1949 MAY 01: 807-810

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