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Eyes that constantly emanates light, either just the iris and pupil (resembling a tapetum lucidum reflection, but visible from any angle) or the whole eyeball.

Could a person still see or would they get blind? Or would they evolve and get more resistant photoreceptors? Or they wouldn't see anything because of the eyes' light?

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    $\begingroup$ The answer depends somewhat on the purpose of the emanated light. For illumination, so the person can see better? Or for signalling and identification? $\endgroup$ – A. I. Breveleri Jun 11 '16 at 4:09
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    $\begingroup$ @A.I.Breveleri Well, actually, this person with glowing eyes would also have a glowing body, since birth. So I'm asking if such a person could see with these glowing eyes. Such glow is because of a genetic mutation with a magical glowing energy in the world. $\endgroup$ – Yuuza Jun 11 '16 at 4:15
  • $\begingroup$ In my wanderings of the interwebs, I came across some freelance animal costumes that have glowing eyes in real life. I'm not qualified to comment on how well they work, but you can google "fursuit glowing eyes" or similar to see for yourself. $\endgroup$ – AaronD Jun 11 '16 at 6:55
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    $\begingroup$ To an alien who has eyes that are sensitive in the (far) infrared, every human would be "glowing", this is how night vision devices work. And obviously humans are able to see. As long as the glow has different wavelengths than your visible spectrum, you're fine. You could even make your eyes have some ultraviolet glow (which humans can't see but bees can), and have an environment full of fluorescent substances that light up in the visible spectrum when you shine your "eye flashlights" on them. $\endgroup$ – Guntram Blohm Jun 11 '16 at 9:25
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    $\begingroup$ I'd like to point out that we do glow in infrared, which would make ir vision useless. The pit viper can see its warm-blooded prey in ir because it is cold blooded. $\endgroup$ – JDługosz Jun 12 '16 at 13:55
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One way to give a glow without causing vision problems would be if that glow is monochromatic (just one frequency), and at the same time that person happens to be blind for exactly that frequency, without affecting vision of any other frequency. That way his seeing would only be minimally affected because most things we see have a very wide spectrum (Lasers being an obvious exception), and even if they don't, it's unlikely they would have exactly that frequency. And the glow itself would not affect the seeing because, well, that person doesn't see it.

An interesting side effect would be that the person would be completely unaware of the own glowing, unless someone told the person about it, or it is revealed e.g. in a video (the video camera would, of course, not be blind to that frequency, and the replay would then be in frequencies that person can see). Another possibility to detect the own glow is through fluorescence: Since usually the wavelength emitted by the fluorescent substance is larger, it would be visible to the person, who would then observe fluorescent substances to glow on approach.

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  • $\begingroup$ I was thinking of this too. Perhaps polarity could come into play as well $\endgroup$ – Jezzamon Jun 11 '16 at 8:14
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    $\begingroup$ @BrunoLopes: White is a combination of "all" colors, or more specifically, a combination of colors that stimulates your red, green and blue photoreceptors equally. So the whilte glow would blot out everything. $\endgroup$ – Guntram Blohm Jun 11 '16 at 9:20
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    $\begingroup$ It is possible for the light to be a mix of 3+ very specific wavelengths that appear white, and for the individual to be blind to all of them. $\endgroup$ – Someone Somewhere Jun 11 '16 at 12:09
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    $\begingroup$ @SomeoneSomewhere: If you choose your wavelengths well, you can even do it with two wavelengths. Assuming that the chromaticity diagram on this page is sufficiently accurate, you could mix white e.g. from the wavelengths 490 nm and 600 nm $\endgroup$ – celtschk Jun 11 '16 at 16:02
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    $\begingroup$ Now that I think about it, we humans do glow in infrared frequencies, and we are blind to these frequencies. $\endgroup$ – Emilio M Bumachar Jun 11 '16 at 19:37
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A light source inside the eyeball, bright enough to be visible to observers, would wash out any focused image on the retina. It's possible to see under such conditions, since the retina and visual cortex respond to changes in light level and not to sustained light, but it's hard to believe that such a person could see very well at all.

Individuals do not evolve; only populations do.

A light source on the outside of the eyeball, that did not extend close enough to the pupil to be visible, would not directly wash out retinal images. However the glow would be reflected by any dust or smoke in the air and would noticeably interfere with good vision, especially in low light conditions.

So if the mutation involved all the tissue in the person's body, then the inside of the eyeball would glow, and it would definitely result in a disadvantaged or impaired vision. But if the mutation and consequent glow were confined to the skin, then it would plausibly make the whites of the eyes glow, but only on the outside of the eyeball. This would probably ruin the person's night vision but otherwise would just be a slight handicap.

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  • $\begingroup$ Good. So it appears that the only problem with glowing eyes is the night vision. Maybe the body could respond by dimming the light amount when night arrives? $\endgroup$ – Yuuza Jun 11 '16 at 5:22
  • $\begingroup$ @BrunoLopes - no, not the only problem, but probably the biggest one. It has to get really, really dim to be helpful. $\endgroup$ – iAdjunct Jun 11 '16 at 5:29
  • $\begingroup$ @iAdjunct Hmm... So the problem seems to be the glowing eyes. What about some kind of tapetum lucidum reflection that is always visible? I think that would be less troublesome, as no light is emitted, just reflected. This way, the ambient light is reflected, making the eyes to appear to be glowing. Although the sun light would probably damage the retina. $\endgroup$ – Yuuza Jun 11 '16 at 5:44
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The question isn't really whether or not they'll go blind, it'd whether or not the signal to noise ratios will be high enough.

If somebody is 20 ft away shining a 100 Lumens flashlight with a beam width of 20 degrees (which is somewhat small) and your pupil has a diameter of 4 mm (which it may at average ambient lighting), 346 micro-Lumens will enter your eye.

If a firefly were 2.8 cm away from your eye, it would be just as bright.

If your eye produced even the same amount of light internally, you'd never be able to see that kind of a flashlight. You wouldn't haven an issue during the day, but you would be ridiculously easy prey at night because you wouldn't be able to see anything.

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    $\begingroup$ And, of course, your eyes are glowing so every other creature around can see them! I suggest sleeping in a safe place at night... $\endgroup$ – Matthew Jun 11 '16 at 11:06
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Just make the eye white glow. That area of the eye is already opaque enough to block so much bright daylight that we see just fine during the day, so some surface bioluminescence shouldn't penetrate it much if at all.

You would still have reflections to deal with, hampering low light far vision, possibly quite a lot, but something like a torch should still be just fine (bright enough) for illumination in the dark. And close vision wouldn't really ever be in the dark anyway, their own light should be usable for reading distance.

If you want pupils to glow too, consider just reflective retinas for that.


Bigger problem would be tip of the nose! They'd either need very flat nose so they wouldn't see it, or they'd need to cover it! Though simple dirt/mud should work for that quite well even,in a no-tech situation.

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A continuous glow would not matter much at all in decent lighting, though it could matter in very dark situations.

Our eye doesn't actually see exactly what, say, a CMOS camera might see. We have all sorts of interesting processing built right into our eyes and brain. One of these is lateral inhibition. If one part of the retina is firing, it suppresses nearby areas, thus at the optic nerve level, we see something resembling the edges of the scene. This is converted into what we think we see later in the vision process. If you had a uniform glow, this would rapidly get tuned out, and the person would see just fine.

The one place I don't think this would work is at night. At night, our eyes undergo changes known as the Purkinje effect. This change to night vision does things such as shifting our vision away from the cones and into the rods. The rods rely on a chemical known as rhodopsin, which is maintained in an incredibly intricate balance such that a rod can be triggered by a mere 5 photons! However, to do this, the system has to go through a very long calibration phase, about 45 minutes, where it tunes all of the rods which were overstimulated during the day. (This is why, when someone shines a light on you at night, you are blinded for a quite a long period. You wreck the delicate balance in the rods, and they have to calibrate again). In low light, that glow would generate far far more photons than the rest of the scene, and would make for a much harder calibration.

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