Earth has animals that have a photoreceptor in the near UV that obviously provides them some sort of benefit. (Given how biological material often glows under UV this might be of hunting benefit.)

Earth also has animals that have infrared sensitivity, although my impression is that it isn't in the eye.

I'm curious how far this can usefully be extended (Superman's x-ray vision can't work, there's no meaningful source of x-rays), both from a biological standpoint and from a standpoint of frequencies that would convey useful information. Things like the cornea not passing UV isn't relevant--that's a biological flaw, not an actual limit. I cited two examples showing that there is both the biological possibility and useful information to be gained beyond both ends of what we can sense--although whether the near IR is biologically possible for a warm-blooded creature isn't established. How far does this extend?

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    $\begingroup$ When I see a question asking about the benefit of a particular adaptation, I always ask myself "what environment is it a beneficial adaptation to?" Superman, in the 1950's and ever since his birth on rough paper and in the cinema since, there has been precious little X-Ray energy about, unless someone was standing between him and the naked core of a reactor or between a dentist's flash tube and him. A nuclear-age superpower. Is this set on Earth? If not what's the radiation around, besides the visible and infra-red and a little bit of UV? $\endgroup$ Commented May 18, 2021 at 23:48
  • $\begingroup$ Material physics dictates that the same mechanism is most optimized for a limited range of wavelength, which is why we have lamps and antennas. Do you actually care if it's the eyes or whether it is just detection? $\endgroup$
    – DKNguyen
    Commented May 18, 2021 at 23:54
  • $\begingroup$ The closest thing to comicbook-style X-ray vision that exists IRL is probably terahertz radiation (en.wikipedia.org/wiki/Terahertz_radiation); but I don't know if there's any plausible way for a biological organism to produce or detect it. $\endgroup$ Commented May 19, 2021 at 1:11
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    $\begingroup$ VTC:Needs Clarity. As written, the question is meaningless. How wide is possible? It's your world, as wide as you want it. How wide in real life? Only what you see... that's it. Humanity certainly has no ability to change it other than artificially. Keep in mind, there's how wide the spectrum is and how resolved the spectrum is. Butterflies see about the same spectral width we do - but they have 15 receptors to our 3 and can see a great many shades of, e.g., blue than we can. So, what are you really asking? What rule of your world are you asking for help to define? $\endgroup$
    – JBH
    Commented May 19, 2021 at 1:44
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    $\begingroup$ @JBH Now I remember, in the early animations, rays emanated from his eyes, it never occurred to me that that particular ancient fallacy was being perpetuated, but now you point it out, yes..... $\endgroup$ Commented May 19, 2021 at 2:45

3 Answers 3


For visual wavelength

enter image description here

  • Transmittance through the human cornea as a function of wavelength. Source

A bit of closer look at UV bands

enter image description here

  • Ultraviolet action spectra. (Cullen, 2002). These are spectra of for threshold ultraviolet damage to the corneal epithelium for different species as derived by different researchers. Note the similarity and overlap of the curves. The green curve is for damage to human conjunctival epithelium. UV-A is far less hazardous to the cornea than UV-B and UV-C. (Figure created by Ms. Erin Chaney, US Army Center for Health Promotion and Preventive Medicine. Aberdeen Proving Ground, MD 21010-5433 USA.) Source

Sooo, how wide is more or less from 320nm to 2000nm something, and is defined by transparency of mammal cornea.

U can put hundreds of pigments in there and have unprecidented view of everything, rainbow included, and next bottleneck are the brains I guess.

As for xray and shtuf, no idea.

  • $\begingroup$ That's a limit in the human cornea, not an inherent biological limit as some species can see in the near UV. $\endgroup$ Commented May 19, 2021 at 23:01
  • $\begingroup$ @LorenPechtel True. People with replaced lens also can see uv. Was a bit in the rush with this one. But op didn't provided any relevent data on his creatures, so I assumed ones typical - extend humans and humans alike. Assuming just random all possible creatures makes it quite pointless, because the anwer is - whole EM spectrum - what we an detectwith our technological means, there can be a creature which "sees" it. X rays included. Will it or wont it have some benefits, and which they are - are function of environment. But yes, not the best answer on my side. $\endgroup$
    – MolbOrg
    Commented May 19, 2021 at 23:50
  • $\begingroup$ I'm the OP, and you're missing the point--it's not the whole EM spectrum. L.Dutch makes a good case for an upper limit based on illumination that probably hits before you run out of biological ability to do it. (You will hit a biological limit when you can no longer focus.) $\endgroup$ Commented May 20, 2021 at 4:19
  • $\begingroup$ @LorenPechtel U know Wilson cloud chamber, there is not always a need to focus to see. Another one Scintillation Detectors. Vision, ability to see is quite avague category, not all animals see well, but we do not say they do not see. If u do not set limits on creatures, gods of chaos only know what it can be, but people here were asking about planet size creatures it can be armed well with detectors. Wilson camera is small, organ of size of eyes of whale can have such setup, for navigtion on some different planet. Life is quite rich on assortment of things it can do, if it has incentive. $\endgroup$
    – MolbOrg
    Commented May 21, 2021 at 9:48
  • $\begingroup$ @LorenPechtel in general I get sense of your intent to fish for some smart and fancy ways to see, because u can't figure it on your own. If that so, then it may be useful to approach it from the position I suggest - whatever we can detect with our technological means, such detection can be recreated with biological ones. It can be better or worse in quality, it may have some size restrictions for creatures, as yardstick take the size of appropriate devices if u not sure. As for what useful stuff can be detected that's quite broad question, which depends on environment as well $\endgroup$
    – MolbOrg
    Commented May 21, 2021 at 10:01

To complement MolBorg's answer, you also need to take into account the transmission coefficient of the medium standing between the eye and the seen object, in most cases the atmosphere.

This is the transmission coefficient of the atmosphere

enter image description here

As you can see we human have somehow already managed to extend our spectral sensitivity in the spectral windows where the atmosphere is transparent.

In general it's pointless to develop a sensitivity for a frequency to which the medium is opaque (again, that's why we send x-ray telescopes into space).

  • $\begingroup$ At first glance this defines the useful upper limit--but the birds and the bees (and maybe some others, those are just the ones I know) have a sensor in the near UV. UVA and UVB do get through the atmosphere enough to harm us, I do agree that UVC and above will provide no useful information. $\endgroup$ Commented May 19, 2021 at 23:00

There is actually a rare condition called Tetrachromacy caused by a mutation of genes in the retina that let a small number of people see into the UV spectrum. Attached is a link to a BBC article about one such person.

BBC -Tetrachromacy

As to your question about how useful this would be the answer really depends on your point of view. According the the article the main effect is that the people concerned see a large range of colors in objects that normally don't appear to be very colorful to people with normal vision.

This is because the objects concerned don't reflect much color in the normal spectrum but do in the UV. An example given in the article was a gray/dull colored pebble pathway that to the person with Tetrachromacy sparked like multi-colored jewels.

So the chief advantage would I guess be that you would see more color/beauty in the mundane world. Which I would argue falls under the heading of 'Nice to have' rather than Useful to have. This would also explain BTW why the mutation is not more common, it doesn't bestow any particular evolutionary advantage to a carrier of the mutation but at the same time it doesn't seem to be disadvantageous either so there's no real evolutionary pressure selecting for or against it - at least in humans.

On a societal level I think it would be a 'cool' (good adaptation) because the world would be more beautiful/colorful for everyone but that seems to be about the limit of its impact.

Lastly as a frame challenge - I would suggest mutations/adaptations to the eye that gave us better night or distance vision would be more useful than one making different spectrum of light 'visible'. That said seeming into the IR range would be difficult. As I understand it the size of the human iris would have to increase considerably in order to capture enough IR photons to be useful. (Someone can correct me if I wrong on that last bit!)


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