If I'm writing a sci-fi story and I wanted to include humans with modified eyesight that see into the ultraviolet and infrared bands of the electromagnetic spectrum, what would I have to change about my character's eyes?
What would things look like to that character?
If they only see infrared or ultraviolet light, what would things in the visible electromagnetic spectrum look like?
This a complicated answer because perception is created at multiple points in the optic chain starting with the lens (which is slightly colored and therefore actively filters out UV and purples) to the optic nerves (which are sensitive to three main peaks of the visible EM spectrum) and finally to the brain that perceives and translates the nerve impulses into something we recognize, as sentient beings, as colors.
So the answer to the question has as much to do with the biology of the being's organs as it does the neuroscience of the brain. If we're talking about just the human eye then we'd have to make changes to the biology and optic nerve responses.
If you want the human eye to perceive the wavelengths below the visible EM spectrum, then you'll need to reassign at least one of the cones to generate a response stimulus at those wavelengths, and you'll have to define how broad you want the response to be. Then, to perceive EM radiation above the visible spectrum, you'll have to assign another cone to be sensitive to that range and define how sensitive it is (the smaller the wavelength range it detects, the more sensitive it is, but the less it sees overall).
Here's another image that shows the (non-normalized human cone sensitivity to EM radiation wavelengths):
So, you might have to slide the blue cones to UV and the red and green to IR. Or, you could put all of them in UV, or in IR, and still be able to define a set of false colors generated by the human eye's red-green-blue (RGB) cone receptor nerves that live entirely in the IR or UV bands of the EM spectrum. The resulting image, recorded by the eye, would still look full color, but since the response ranges will be wildly different, there's no telling what the final image would look like.
It's entirely possible that this is what the human eye would see, when looking at our own sun, if the cone receptors were sensitive to wavelengths in the UV and X-Ray ranges:
Here's one of the sun using ONLY UV sensitivity. This seems to favor the blue cone receptor dominating the visible sensitivity:
So, the reason butterflies are interesting to this discussion is that their optics support seeing seven to ten unique color bands, where we old humans see only three (even though they overlap at points). If you're discussing changes to the human eye, you may want to consider adding in a few more cone receptors that are sensitive at different wavelengths. Why?...
Have a look at this image, you'll see just how sensitive the human eyes are to very narrow ranges of the entire electromagnetic spectrum. The entire UV and IR bands, combined, are approximately 10x the size of the narrow visible light band.
So, in order to see the entire IR spectrum, or the entire UV spectrum, and still have enough sensitivity to minor changes in the response of wavelength fluctuations, you'll need eyes that have more cones.Like the pieris rapae:
Imagine all those cones spread out over the IR range? Might need a few more.
Your eyes will not register a response, and therefore won't fire a nerve, until they are excited by the frequency of EM they are tuned to. So, if your hypothetical eye is sensitive to IR and UV, but not the visible light in between, then those areas that emit ONLY visible light will appear black; they will not have excited the receptors in the eyes and therefore no signal will be sent to the brain. The concept is similar to our eyes now. If we close our eyes, or turn off the lights, then everything appears black. Now, do this in a room with an IR light (emitter), it's still the same effect. Your eyes won't see anything since they are not programmed to excite a response when bombarded by that wavelength. Yes, the EM radiation still hits your eyes, but the receptors don't register it, so you're brain doesn't see it.
There are a number of issues here.
Resolution
Ok, so let's say you create a new kind of cone that's sensitive to UV. Where are you going to put it? The retina is already jam-packed with cones, so you need to remove other cones to fit the new cones. So your guys can see UV, but their sensitivity to one or more other color channels gets worse.
Now, if these are cybernetics or similar, you could potentially miniaturize the new cones and old cones and solve this issue. Maybe even give your guys better resolution than normal.
Another issue is that IR bands aren't physically capable of high-resolution imaging relative to visible light. The deeper into IR, the worse the image gets. On the other side, UV gets better resolution to an extent.
Usable Light
Sunlight generates a good deal of near IR, but it falls off exponentially, so you're down to 12% (ish) by the 1500 nm range.
You can use longer wavelengths, but you'd need to make the cones very sensitive in that band. Maybe you could center the cone's sensitivity in deep IR with exponential falloff away from the peak. The exponential gain in sunlight towards near IR would be offset by exponential falloff in cone sensitivity, potentially giving you good low-light vision across a broad spectrum.
If you go to far IR (8-15 µm, 8000 to 15000 nm), you'd be able to see objects at room temperature, although that does include your own eyeball which could get awkward. You'd likely need some kind of specialty cooling system to keep the lens, retina, etc. cooler than whatever you're trying to look at. Not sure how viable such a cooling system is with biology, although it's perfectly plausible with cybernetics.
At the other end, note that far UV is absorbed by the atmosphere and is damaging at a molecular level, so there's a physical limitation on how deep you can go into UV (200-300 nm).
Color Space
There are a couple ways to go about this. First, you can replace the existing red/blue cones with new cones that have a broader response curve. So now an object that reflects IR becomes red, or more red than before. Same thing applies to UV objects looking blue or violet (depending on which cones you alter for UV response). You could potentially replace just one cone (say green) with a really wide response to both ends, but I'm not sure it would be beneficial.
Second, you can add new cones in the new ranges. This gives you much better control over which spectrum you can cover, and probably gives you better light absorption (most materials are crap at absorbing a huge range of wavelengths for photoelectric effects, although multi-junction cells might help here).
Now, there are two sub-options here. The simplest way is to attach these new cones to existing nerve outputs. So you'd see IR as red, UV as blue, like before. (Or IR as blue and UV as green, or whatever floats your boat. Again, not sure there's any particular reason to do it, but you might find that it helps night vision or something.)
The other option is to generate new nerve signals. This also requires rewiring the brain to accept these new signals. Obviously, it's possible, but I have no idea how hard it would be, or if it could reasonably be done on an adult.
If it worked though, the person would have a vastly increased color space. The difference between red and IR would be blatantly obvious to these people, along with the difference between blue/violet and UV. There would also be a difference to them between green, and green with IR, or green with UV, or green with IR and UV.
There would be 1 null color ("true" black), 5 primary colors (one for each cone), 10 secondary colors (each combination of two cones), 10 tertiary colors (each combination of three cones, which is also the combination of absences of cones), 5 quaternary colors (each combination of four cones, or absence of 1 cone), and 1 everything color ("true" white). Plus all the trillions of intermediate colors. I took the liberty of naming them and coming up with tentative pronunciations. Those aren't ANSI standard naming conventions or anything.
The specifics of what real-world objects translate to what pentachromatic colors depends a lot on exactly what response curves you use. Also, it's possible to move the existing cones so your guys' "red" wouldn't correspond to normal red.
For example, you could have IR equate to far-ish IR, Red equate to near IR, Green equate to red/green, Blue equate to green/blue, and UV equate to UV. This gives you a really broad spectral range, but you lose a lot of human color response. To normal people, you'd seem red-green color blind.
To expand on JDlugosz's answer about perception of the colour wheel - with the VERY important assumption that these modified humans are born with enhanced eyesight - I think your modified humans will lose the ability to perceive the colour pink.
The colour pink is what your mind calls the joining of both ends of the visible spectrum. That's why pink is between red and violet on the colour wheel. If you were to take rectangular spectrum in Andrew's very fine answer, and wrapped it end to end, pink would be at that join.
If you were to take the enhanced spectrum, and wrap it end to end, you would get a new colour at the point where infra-red and ultra-violet meet.
I call this colour transpink.
What happens to the old pink? I don't know for sure, but I imagine it'd be like the colour you seen when you mix two shades of a colour together. Maybe pink becomes green, in an interesting form of colourblindness.
When looking at a rainbow, your people will see extra colours above and below red and violet. These will be infra-red and ultra-violet. The midday's sky, will - if I remember my Rayleigh Scattering well enough - be a violet hue. And sunsets will have wicked amounts of infra-red in them.
To someone with enhanced vision, sunscreen is facepaint. I'm not sure how sunscreen works precisely - it either absorbs UV, or reflects it. Either way, it'll change the 'colour' of people's faces as a non-sunscreened face will reflect a portion of the UV spectrum. Sunscreen will change that.
There already are people who can see part of the UV-spetrum. These wavelength are normally excluded by the lens, not due to lacking sensitivity of the cones. If the lens is absent (and replaced by an artificial lens) UV-light becomes visible.
Here is a pretty detailed description of somebody who underwent cataract surgery and now is able to see a wider spectrum of light than usual.
Especially relevant to the question are the pictures that are supposed to simulate how certain things now look for this guy.
Lets go with the idea of modifying the pigment in the cone cells in the eye. This has some grounding in today's technology. With some gene therapy, they've been able to reverse colorblindness in monkeys (article in national geographic).
So, change the pigment in the cone cells to make it so that blue sees into the near UV, and red sees into the near IR. Could possibly find those genes in humming birds for the UV pigment. IR is harder because the wavelength is longer and the energy is lower. Turns out that goldfish can see UV and do a bit better than us in red. IR is hard for eyes because the light has such a long wavelength and lower energy.
If you look at an old panchromatic film advertisement, red colors are just harder to make sensitive film for. This is also why IR film is harder too. Lets go with the "we can overcome that".
Since I brought up film, this raises an important point. And this one isn't as easily overcome.
The vision of everything other than green is going to be blurry. Or alternatively, everything other than one part of the vision spectrum is going to be blurry.
With film, you have to adjust the focus for IR light.
That red dot is where the IR is focused. The red line is where visible light is focused. If you are looking at something that is 7 meters away, something that is 10 meters away will be blurry in IR. This is because of the chromatic aberration that you get with a simple lens (such as the ones in our eyes).
An example of this:
Top one is focused correctly, bottom one, the blue is out of focus. Everything will look like this.
This is because different wavelengths focus to different distances:
If you look at the human spectral sensitivity, you will see that we try to account for this:
Red and green are close together in wavelength and sensitivity, and we focus our eyes for them. As noted in hyper physics, there are some funny things that our brain and eyes do with blue light:
The "blue" cones are identified by the peak of their light response curve at about 445 nm. They are unique among the cones in that they constitute only about 2% of the total number and are found outside the fovea centralis where the green and red cones are concentrated. Although they are much more light sensitive than the green and red cones, it is not enough to overcome their disadvantage in numbers. However, the blue sensitivity of our final visual perception is comparable to that of red and green, suggesting that there is a somewhat selective "blue amplifier" somewhere in the visual processing in the brain.
The visual perception of intensely blue objects is less distinct than the perception of objects of red and green. This reduced acuity is attributed to two effects. First, the blue cones are outside the fovea, where the close-packed cones give the greatest resolution. All of our most distinct vision comes from focusing the light on the fovea. Second, the refractive index for blue light is enough different from red and green that when they are in focus, the blue is slightly out of focus (chromatic aberration). For an "off the wall" example of this defocusing effect on blue light, try viewing a hologram with a mercury vapor lamp. You will get three images with the dominant green, orange and blue lines of mercury, but the blue image looks less focused than the other two.
And so, while you may be able to see in those other parts of the spectrum, it won't be focused at all and may be hard to use outside of a blur around the green objects (that remain within the traditionally visible spectrum of light).
The specific response curve shape of each cone type and the nature of the processing done downstream lead to the perception of different colors and which colors blend into each other. We have a color wheel with purple completing the circle: with different sensors this might not be the case. There may be no "ring" but clear ends, or two different rings!
Depending on the processing, you might perceive two simultanious colors rather than their chord as a distinct color.
Adding (for example) a UV sense without messing up the existing eye mechanism, you might see uv as a distinct and separate overlay, while purple still works the same, and uv doesn't mix to form chords with other colors. Since we are discussing modification of human vision (as opposed to alien vision) that might be the case.
Are you allowed nonlinear optic contact lenses? (Or glasses, in the early days of this technology?)
They would up-convert or down-convert optical wavelengths according to applied voltage and possibly a "local oscillator" frequency (from an UV or IR LED) just like frequency band converters in amateur radio, and turn off when you wanted to see the natural spectrum.
It's a known technology in the physics lab, and while that doesn't prove we could package it (even in bulky glasses) today, makes it a hypothetical possibility.
I'd start with potassium niobate coated sunglasses to cover the near-infra-red band, mixing transmitted image with an internal IR LED to produce blue image... you'll get UV coverage in the next generation.
There's one way I can think of:
There is a form of red green color blindness which happens because of higher perceived spectral overlap between red and green. In this case, the two tend to bleed together. Hypothetically, if you could correct for this, red and green would appear as distinct colors, since there would be more gaping between the two spectrums.
Assuming that works, let's assume you could over-correct for that problem, and cause people to see partially into the infrared, like some animals can. This would account for infrared.
http://enchroma.com/technology/
For ultraviolet, you could alter the cones in the eye so that rather than perceiving all of the blue range, they would pick up about half of the blue range, and a small chunk of ultraviolet. This would be an extremely flawed solution, but it would be a comparatively simple modification.
