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from Order of the Stick

Darkvision, or seeing in the dark. But how does it work? In my world, I've been envisioning darkvision as eyes that emit a color of light that most humanoids can't see. Are there any problems inherent in this?

Details
  • It works by emitting light from the eyes in a spectrum humans can't see: for the purposes of this question, let's say low UV light.
  • I'd like it to be able to go about 30-60ft, although it can dim as it goes.
  • Color vision doesn't matter.
  • I don't mind if the FOV is shrunk
  • I don't mind if people's darkvision interacts
  • Yes this is for D&D
  • Let me know if you need more info.

Thanks!

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    $\begingroup$ This seems to be akin to how people imagined vision worked before science, the opposite of how vision actually works. Where does the sensory-input part come in? $\endgroup$ Jun 15 at 11:14
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    $\begingroup$ Not clear how having the eyes saturated with their own light would help vision in general, let alone in the dark. There is a reason why photo cameras don't place the flash lamp inside the lens. $\endgroup$
    – AlexP
    Jun 15 at 12:36
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    $\begingroup$ I mean the reality is "it's magic". Darkvision is not explainable through any conventional physics. Darkvision is what used to be infravision, but infravision was raising too many questions, so the D&D writers just kinda waved their hands and said "you know what, fine, you can just see in the dark, it's darkvision, so there". The explanation is the same as "why can skeletons be animated" and "why can my character create this ball of fire out of nothing": it's magic. There really isn't meant to be a real world explanation. $\endgroup$
    – JamieB
    Jun 15 at 20:04
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    $\begingroup$ Your eyes receive to see. Anything else can emit. $\endgroup$
    – DKNguyen
    Jun 15 at 21:28
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    $\begingroup$ If this is for D&D, I recommend posting it on the RPG site as you may get different answers there (particularly when it comes to problems interacting with other game elements). $\endgroup$
    – bta
    Jun 17 at 16:16

6 Answers 6

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There are already animals that do what you want, in the sound domain: bats. Let's just steal shamelessly.

More flash for your buck

Constantly producing a distinctive light source bright enough to see by is energetically wasteful, dangerous (eventually at least one predator will figure it out!) and chaotic when multiple of your species are using it at once. Instead, your species has the ability to discharge a very bright, very sudden flash that they alone can see. The most efficient way to do that is to produce a very narrow wavelength, flash a lot of it in a short time, and have a correspondingly narrow sensor, in this case specialised rods in their retinas. Compare with bats, whose ears are specifically shaped to focus the wavelength of their ultrasound. Bats have to deal with the Doppler effect, but you probably don't.

Stroboscopic vision

It takes a brief pause for the flash to be repeated, because the flashing organ needs to recharge. This gives darkvision a peculiar quality compared to regular vision, because it lacks the continuity that creates the sense of movement. The effect is akin to stroboscopic disco lights. This also means that they can avoid producing their distinctive light if they're trying to lay low; or they can use it like a morse code to communicate with others of their species.

Blinding yourself with light coming out of your eyeballs

Obvious problem, as pointed out by others. Bats aren't a great help here, because sound is slow enough that they can disengage their ear bones while screaming and re-engage them for the echo - good luck with that at c. But you don't have to filter in the time domain: you can filter in the rotational domain. Your flashing organ produces light that is perfectly polarised. Your wavelength-specific rods are sterically aligned to collect every plane of light except the one you flash out. Light reflected off natural objects is almost always randomised in polarisation, although smooth reflective surfaces will presumably look completely dark at certain angles.

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    $\begingroup$ Great solution! You can probably extend that by having tunable wavelengths you emit and see - creatures would see only wavelength of their choosing. So humans in party would opt to align frequencies to see further or have everyone see what others see - if you want interaction between darkvisions. But goblin would have another frequency and wouldn't be detected. Then you can have "fast darkvision" where some creatures can quickly change frequencies to detect others with darkvision, and "slow darkvision" where you cannot do that. $\endgroup$ Jun 16 at 7:16
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    $\begingroup$ Also, your eyeballs do not need polarization (though it is a great solution for the problem). You can simply have great AR coating for the emitted light, so there would be no reflection on the eye-air interface. Simply have 1D stack of layers where you can tune height and you can get perfect AR for any individual wavelength (this stack could have arbitrarily high reflectance of any other wavelength, so others wouldn't be able to blind you) $\endgroup$ Jun 16 at 7:27
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    $\begingroup$ There is a name for light-based sonar: Lidar wiki, but this shows it better. Now what is also interesting with lidar, is that it can be used to calculate depth (or how far away an object is) by measuring the time it takes for light to go out, bounce, and come back. Throw in lightspeed, and you get how far away something is. Effectively granting depth perception with just 1 eye. Measuring travel time also makes other lidars nearly useless. $\endgroup$
    – vinzzz001
    Jun 16 at 9:15
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    $\begingroup$ @ZizyArcher AR would definitely also work, although I'm not aware of biological materials that have those properties. My bias for polarisation sensitive vision is that it allows me to cite the greatest research paper of all times, Dung Beetles Use the Milky Way for Orientation $\endgroup$
    – Ottie
    Jun 16 at 11:36
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    $\begingroup$ @vinzzz001 you've actually out-batted me, I had not considered the echo/lidar angle! I assumed the timescales involved here would be far too short for any biological process to measure, but perhaps some clever interference-based detection could work? $\endgroup$
    – Ottie
    Jun 16 at 11:41
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I see one big problem with eyes emitting "darklight" (my shortcut definition for "light that can be seen by species gifted with darkvision"). Or any device emitting darklight. But let me explain it :

Your sight is dependent of others, too

That's functionally the main problem of this approach at darkvision. Regardless of physics or biology, you'll face situations that don't really match your criterias nor the common principle of darkvision like in Dungeons and Dragons.

Indeed, if you emit light and receive the light in return, and others emit light, too, at some point one will interact with each other, often unwillingly. If you're unsure to understand, take two flashlights at night, a buddy and test yourself, keeping the flashlight targetting where you see. Here are some simple nasty consequences you can check out.

What are (some) of the consequences from this?

  • If light is dimmed up over distance, adding more sources of light will extend your sight range. In other words, the more darksighted people you are, the better you'll see (at long range).
  • You'll be able to see where others see : Let's say a goblin is ambushing you around a tunnel's corner and both of you are darksighted. He's waiting the moment he sees you. Problem : What he sees is what he emits. Light bounces in all direction, so you'll see the ground he sees, too. The deduction is easy to make from then :).
  • You'll become blinded if there are too many darksighted people looking at the same thing. Indeed, if each person is emitting light enough to see, any new person will add more light to it, to the point it will become too lit up to see it or at least see its shadows and shape clearly.
  • You'll have an harder time looking at others darksighted people eyes. If you look directly at a flashlight, you'll be dazzled. The same thing could very well happen here.

Other biological consequences

  • You'll have to deal with an hard-time sleeping : If your eyes themselves are constantly emitting light, closing them will not stop the emission (your eyes don't stop seeing). It's like closing your eyes to take a nap while the sun's out, albeit it is directly coming from within your eyelid.
  • If you add these very specific light receptors, you'll probably have to remove others. It's a bit like what night predators do by reducing their color vision for a more focused and efficient black and white one, but this time you're likely to add a new type that's overlapping an existing one, functionally speaking. This means that your dwarves, elves and the such will lose some sight a way or another to implement these new cones. And the same happens for the light emitters, if they are inside the eye and not around it.
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    $\begingroup$ You could remove most of these consequences from having multiple sources by making each person have a very small band of unique wavelengths they emit/detect, making it rare to meet somebody on the same band. Maybe even make it so that a combination of wavelengths is only visible to an eye if it returns in the very same combination. As for the biological consequences, maybe add an off function to closed eyelids? Like, say that the light-organelle only works when it is being pulled on by the same muscle that keeps your eye open. $\endgroup$
    – vinzzz001
    Jun 15 at 12:29
  • $\begingroup$ @vinzzz001 If there's not enough people limiting wavelength could be a way to mitigate this but I doubt it would be enough : Let's say that each can see about one mm wavelength of the UVs, all across the UV range (400-150nm). You can have 250 persons with a unique light, before they overlap on each others. Also adding this much kind of ultra-accurate sight/emission add a biological and evolutive burden : Why would you 250 different kind of phenotypes for the same race, when they are not predators to each other? It's especially true for social species which have a very incentive to share sight. $\endgroup$
    – Tortliena
    Jun 15 at 12:39
  • $\begingroup$ I don't need the light not to interact; that's one of the reasons wanted it to work like this, actually. Thanks for the input! $\endgroup$ Jun 15 at 19:16
  • $\begingroup$ @Firedestroyer That's why you changed your question :p. While I understand and won't mind, it's seldom seen as nice to edit a question which invalidates big chocolate chips of answers :). $\endgroup$
    – Tortliena
    Jun 15 at 20:32
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Eyes are receivers

Eyes are receivers of light. The light is focused on retina where receptors sense the light and send the information to brain through optic nerve.

Bioluminescence

To emit light by living organisms, bioluminescence is required. A different organ is used for bioluminescence. There is a mention of UV bioluminescence here.

Dark-vision using UV rays

For dark-vision using UV rays, following things are to be considered:

  • An organ (not eye) that emits UV rays by UV bioluminescence.
  • A receptor to sense UV rays. As told here,

Some fish have eyes that capture and perceive ultraviolet wavelengths

Dark-vision using infra-red rays

There is another way of dark vision used by snakes. As told here,

Snakes can 'see' in the dark thanks to protein channels that are activated by heat from the bodies of their prey.

Vipers, pythons and boas have holes on their faces called pit organs, which contain a membrane that can detect infrared radiation from warm bodies up to one meter away. At night, the pit organs allow snakes to 'see' an image of their predator or prey — as an infrared camera does — giving them a unique extra sense.

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  • $\begingroup$ I was going to answer, but I think adding to this one is better. Humans are actually already bioluminescent within visible light. It is too dim for us to perceive, but if the function of darkvision is to just crank light sensitivity to 1000%, then simple existing turns you into your own walking glow stick. You could maybe balance this more by making creatures that have darkvision naturally a bit more bioluminescent as well. Here's a quick source, although there are definitely better articles: sciencealert.com/… $\endgroup$ Jun 15 at 19:39
  • $\begingroup$ @TitaniumTurtle That is interesting. Maybe someday, we could make a night vision device using photo-multiplier. $\endgroup$
    – imtaar
    Jun 16 at 8:45
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Imho having the light source in the eye is the main issue. It's hard to imagine how the light source in the eye wouldn't outshine the light reflected from surfaces several meters away.

A quick fix would be to move the light source out of the eyes. That would mean in your image, the light cones would not originate from the eyes. My favorite place would be on the forehead (like the mythical third eye). When that "third eye" is opened the light source in it starts to emit the UV light.

Alternatively, to keep the light source in the eyes: The left eye emits the frequency the right eye can detect and vice versa.

Of course, within the setting of D&D with all its magic, gods and fantastical creatures it would be perfectly fine to assume there are materials available that have optical properties to solve this primary issue.


Just an additional thought: Fog and dust in the air will impair that kind of vision heavily. Just like with a flashlight, nearby fog/dust will reflect the light and outshine light reflected from further away, thereby limiting the visible distance.

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  • $\begingroup$ This is really a necessity. Emitters and detectors can't be in line with each other or one will block the other. $\endgroup$
    – bta
    Jun 17 at 16:06
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Backlit eyelids, Reflective corneas

The eyes do not emit light. Rows of bioluminescent cells on the back of the eyelids do it instead.

Creatures who use this kind of darkvision usually have a silvery sheen to their corneas. It's hard to spot but doable. Some disguised dark elves were caught this way.

These species control these cells unconsciously. They do not emit "dark light" when the eyes are closed, they turn off then back on fast when they blink, or when there's enough light to see by, (i.e. in broad daylight).

With some training, they can learn how to turn it on and off consciously. It's like wiggling one's ears, some dark elves say. But by learning how to turn it off, they can avoid being seen by other darkvision-enabled creatures.

Many predators in the dark learned that this light means food. Sometimes food that fights back, but that's life in the underworld for you.

In a group, only one of them need to have their eyelids lit. They can also use blink-pattern codes to communicate over long distances.

When in the dark, these cells activate, casting light on the cornea, who reflects the light in a broad cone, allowing vision.

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Other animals may be able to see the light

  • Some animals can see parts of the electromagnetic spectrum that humans cannot. For example, bees can see into the ultraviolet range a bit.

  • Many insects are attracted to bright lights

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Thus,

  • BEES!

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