Glowing Eyes

Thinking about the traditional fantasy or science fiction species/monster with glowing eye my first speculation is that it would be useless. Glowing eyes are not seen anywhere in nature as far as I know and that is often a very good indication of what is useful (though not only). Though many nocturnal animals (and ones in other low light environments) have eyes with a tapetum lucidum, a bright reflective layer behind the retina that improves low light vision by reflecting it back through the retina. This layer reflecting some light back out of the eye is responsible for the impression of glowing eyes. I can also add a bit of personal experience to my book learning. I once had my contacts glow in while I was in a room lit with an UV 'blacklight'. It made my vision fuzzy and reduced my ability to see less illuminated objects, but I could see my contacts glow when I looked in a mirror, with some difficulty.

I suspect if I was constantly had my lens/eyes glowing I would not notice it as a glow after a time. Instead my brain would compensate and I would not notice the glow or objects of the same color hidden in the glow. So my first thought is that any species emitting light from its eyes would be color blind in a section of spectrum near that part and I could not see any advantage to it.

However this is assuming that the glow from the eyes is in the visual spectrum of the species emitting the light. If instead a species gave off ultraviolet light from its eyes and saw in the normal visual spectrum it would cause otherwise invisible things to become visible to it. So my second though about the subject is that maybe some real world species gives off ultraviolet light and uses that along with eyes sensitive in the blue/green region to see otherwise invisible things as a forensic investigator with an ultraviolet light might.

But ultraviolet is very energetic (relatively). So it might not be possible for anything to bioluminesce in the ultraviolet spectrum. I've not heard of anything that does, though it is possible that it may not be as well investigated by researchers. So what then if a theoretical species had eyes that glowed blue and used the re-emitting of light in longer wavelengths such as green, yellow, or red?

But why even have an eye glow? This is the basic problem that objectively a designer would not want to have a detection device like and eye also emitting what it is intended to detect. It would be much better if using blue or ultraviolet light to find otherwise hidden prey to have the light emitting organ separate. At most you would want it around the aperture of the eye like a ring flash on a camera. That at least could give the classic horror/fantasy look of the glowing eye with a black dot of the pupil in it. If the light emission was under the voluntary control of the animal then it would also be useful for signaling.

So my bottom line is that I think it unlikely to have a creature (or robot for that matter) having glowing eyes. But if a sorcerer or engineer had his heart set on it it could be practically designed. With some drawbacks.