This question is similar to "How would the human eye need to change in order to see in low light without any changes to physical appearance?", including the apsect of change to the human eye, only the more challenging common fantasy ability most well known in Dungeon & Dragon's darkvision: the ability to see in pitch dark in shades of gray (or more realistically, very near pitch dark?).

Now in the D&D version, such sight is equivalent to seeing in pitch black as if in dim light. While an answer that accommodates that level of clarity is fine, I'm willing to accept some level of sight where it is not that clear, simply more like shadow against slightly darker shadow in a short range.

I know more pupil dilation, as well as additional changes to the eye (mainly retinal change, tapetum lucidum [though this would cause issues for looking like a normal eye in some situations]) can all help in low-light, but is there anything more or different that can be realistically added to the eye's capability that would give true (nearly) dark vision, that still has the eye visibly look like a normal human (or elf, dwarf, etc.)? And is there actually enough "light" in an (apparently) pitch black cave to make this feasible?

NOTE: I'm not interested in infravision (seeing in the infrared spectrum), nor in a magic solution.

  • $\begingroup$ What kind of cave are you thinking of? The kind you see in the movies with big large cavernous openings, or the kind you find in spelunking, where there's basically 0 photons after the first 2 rooms? $\endgroup$ – Cort Ammon Sep 23 '18 at 2:31
  • $\begingroup$ @CortAmmon Either, but more the spelunking kind (but that relates to my light availability aspect of the question) $\endgroup$ – ScottS Sep 23 '18 at 3:53
  • $\begingroup$ Are you strictly searching for "vision", or are you open to other senses? $\endgroup$ – L.Dutch Sep 23 '18 at 6:00
  • $\begingroup$ @L.Dutch just vision $\endgroup$ – ScottS Sep 23 '18 at 6:11
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    $\begingroup$ The human eye is very sensitive to light, with the threshold of detection for a dark-adapted eye being about ten photons falling on a rod cell in 0.1 seconds. There simply isn't much room to make it even more sensitive. Remember that the problem with images taken in very low light is that light itself it made of individual photons, and in very low light there are simply very few photons and they are spaced wide apart. Look up photon shot noise. $\endgroup$ – AlexP Sep 23 '18 at 19:27

Tl/Dr: There are thermal limits if you get too much better than what we can see. They make it impossible to see better, even with the fanciest of equipment.

You may underestimate just how unbelievably pitch dark a cave is.

Fortunately, we can do math.

One of the first things a spelunker is taught to do in a cave is to turn out their light. You do this to learn just how dark a cave is. You do this to learn to appreciate why you need extra light sources. They're not optional.

Consider this structure in a cave. You have an opening 2 feet by 2 feet wide. Large enough to squeeze through. If you're a real spelunker, you won't even put the word "squeeze" there... its a huge opening! It leads into a room that is 10 feet wide, and there's another 2 foot by 2 foot opening on the other side. You'll find that roughly three thousandths of the light from your opening will reach the opening on the other side. Combine that with the reality that rocks absorb a lot of the light that hits them, and perhaps a thousandth of the light goes into the room beyond.

Outside, a nice sunlit space emits around 1000 cd/m^2. So one room in, you're lit by 1cd/m^2. It's roughly lit like a late sunset or sunrise.

Now go two rooms in. Now there's 1mcd/m^2. This is roughly moonlight in intensity.

Go three rooms in. Now there's 1ucd/m^2. At this point, we're at the absolute limits of human vision.

Now go four. 1ncd/m^2. Cats are going to have trouble seeing here.

If I may skip ahead a few rooms, at 7 rooms we run into a fascinating problem. Everything emits blackbody radiation do to thermal excitement. 7 rooms in, it turns out that the blackbody radiation from everything is comparable to the intensity of light that's made it through those 7 passages.

What does this mean? Not only is everything glowing evenly, but your own eyevalls are emitting this radiation too. You literally wont be able to see through your own lens because the lens will be glowing almost as bright as the scene itself.

So this says we have a very small region to work with. Rooms 5 and 6 are bright enough to avoid this blackbody issue, but too dark for a human eye to see. There's a mere order of 6 in brightness between those. That's the target for your darker sight.

Cats can see roughly 6x better than us. If I can handwave that up to 1 order of magnitude, that leaves 5 orders of magnitude of darksight which goes beyond what cats can see but stays above thermal limits.

You could probably get another order of magnitude via better processing. In such dark environments, you'd be moving slow. If you could get your eyes to slow down as well (avoiding saccades), you could probably increase your integration time (a.k.a. eyes on target time). That could probably give you another order of magnitude.

You could get another order of magnitude by dropping the resolution of your senses. When in dark environments like that, you really don't need good vision. Indeed, cats are known to have 20/100 vision or worse. Accepting even worse vision would permit you to capture more "photons per pixel," (though the human eye doesn't operate in pixels).

So that leaves you with 3 order of magnitude. 1,000 times darker than what you could see with cat eyes and long integration times is that thermal limit.

At that point, we're starting to turn to magic. Its rare for an organic body to push to within 3 orders of magnitude of such a fundamental limit. More likely it will evolve echolocation abilities, which will be much more effective in those settings. Indeed those who are practiced at echolocation can walk around perfectly without seeing anything at all. I've seen videos of blind people riding bikes, relying on nothing but sound to tell them where to go.

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    $\begingroup$ These problems can be solved if your creature emits some sort of radiation and detects it. For example, it can both emit and see in infrared. $\endgroup$ – vsz Sep 23 '18 at 10:32
  • $\begingroup$ I didn't think my post was too long to not bother reading (unless your TLDR refers to the links posted to show research) :-). Something I don't understand about your answer is the blackbody radiation relevance, as the link notes: "Black-body radiation is the thermal electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment," but the human body temp would not typically be equal to the temp of the surroundings in most cases. Or is your reference to the eye "glowing" equally with respect to visible spectrum only (effectively blackbody visibly)? $\endgroup$ – ScottS Sep 23 '18 at 15:22
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    $\begingroup$ @vsz If the creature is emitting the radiation, then it's no longer dark sight any more than I can say I have darksight because I can carry a flashlight. $\endgroup$ – Cort Ammon Sep 23 '18 at 17:55
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    $\begingroup$ Way to break 100k, Cort! They should give you a hat or a badge or something for breaking the 6-digit rep barrier. $\endgroup$ – JBH Sep 24 '18 at 18:24
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    $\begingroup$ @JBH Thanks =D We'll have to see what the StackExchange overlords have in store for me =) Until then, I have a bright shiney number! $\endgroup$ – Cort Ammon Sep 24 '18 at 18:32

Let me tell you about realistic dark vision

My eyes are ultra sensitive to light. I can see clearly into shadows most people see as black. When I was younger, I could comfortably read a book by a full moon. Noon-day sun is INCREADIBLY PAINFUL. I've never owned a pair of sunglasses that were dark enough, but I have had folks making the glasses for me that didn't believe I could see through them. I spend most clear days squinting at the ground. Realistic darkvision stinks.

But, why are my eyes ultra sensitive? There have been facts and supposition from many doctors.

  • Low pigmentation on the back of the eye. This causes extra reflection for the rods and cones.

  • Unusually sensitive rods and cones.

  • A brain disorder that over-estimates the input transmitted from the rods and cones.

  • Too much pupil dialation (not in my case, though, during full daylight my pupils are a pinhole).

And then one doctor simply shrugged his shoulders and said, "I dunno, it's just the way you are..."

Frankly, any or more of the suggestions I just gave you (including, "I dunno, it's just the way you are...") are perfectly valid reasons for darkvision. What really matters is the consequences. You can't have realistic darkvision without realistic pain during full daylight (at least not that I've ever heard of).

P.S., for the record, how I see at night is usually in false color. My brain substitutes color for the objects it recognizes. Dirt is usally brown. Roads are blue. Fields without trees are grayish white. Trees (think "forested") are dark green. The colors aren't magic... My brain knows perfectly well what I'm looking at, and it's doing its best to help me out.

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    $\begingroup$ For your last paragraph, I completely get it (I get the rest, too, but this is particular to the last paragraph). When I was a kid (some many years ago), we only had a black and white TV set. The mind tricks you into thinking you're seeing color for the parts which are there. Grass becomes green, sky becomes blue, etc. Yes, I can see a downside to your malady, but it seems as though you've learned to function with it. Great answer, btw. $\endgroup$ – Pᴀᴜʟsᴛᴇʀ2 Sep 23 '18 at 12:36
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    $\begingroup$ Sounds like a useful adaptation to add for Dark Vision would be some sort of nictitating membrane that filters light out during the day (perhaps a polarising membrane, and the lens of the eye has a different polarisation? Add muscles that allow the lens to rotate, and it could increase/decrease the amount of light that gets through for finer control than just "on" or "off") $\endgroup$ – Chronocidal Sep 23 '18 at 17:14
  • $\begingroup$ Basically a secondary eyelid that is, semi transparent $\endgroup$ – Garret Gang Sep 23 '18 at 18:38
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    $\begingroup$ @Chronocidal, I'd give my proverbial left eye to have a set! $\endgroup$ – JBH Sep 23 '18 at 22:12
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    $\begingroup$ @ScottS, I can only tell you what doctors have told me over the years. As a young teen, I was once on a cave tour where (as usual) they shut off the lights to show people what "total darkness" was. I've been in those caves, this wasn't one of them. It had phosphorescent lichen on the walls. I couldn't see in 3D, but I could see everyone's shadows against the walls well enough to move around them. It freaked people out. So, considering they're the best guesses of experts, if they're actually part of the issue, they must be enough to help in near-pitch black conditions. $\endgroup$ – JBH Sep 24 '18 at 18:17

Shortest and simplest answer is to sacrifice colour vision.

In the retina, there are two primary types of photo-receptors; rods and cones. Rods are very sensitive because they respond to light across our entire visual spectrum, which is why they won't help us with colour vision but will help us see at night.

Cones, on the other hand are less sensitive because they respond to specific light frequencies, meaning that when the signals from these photo-receptors are processed by the brain, we can perceive colour through a mix of different proportions of signals. But, we need more light to see with these because not all the light is absorbed by every cone receptor.

This is why most nocturnal animals are in fact colourblind; they have a heavy population of rod receptors in the eye, and less cones. The species from which humans evolved had this model but as we became more active during the day, cones were more heavily distributed, especially in the middle of the retina, because being able to differentiate colours gives an advantage at detecting the camouflage of predators.

It's also why our ability to detect movement out of our peripheral vision is so much better than in the middle of our eye; because the edges of our vision is still mostly handled by rod receptors. This is why on a dark night you'll detect movement, turn to look at it, and won't be able to see a thing.

So, to increase night vision, replace the cones with rods, make them more dense, and you'll see a lot better at night. Mind you, it could be quite blinding through the day without being able to contract the iris to make a much smaller pupil than we have through the day now, so that is another factor. This is in part why cats have the iris shapes they do; it allows them to reduce the amount of light that gets into their eyes during the day more effectively. but, in theory at least, you should still be able to get a similar result with a strong circular iris.

  • $\begingroup$ While I appreciate some of the information in this answer, I can see from another answer here that it is really environmental limits (lack of photons to detect) that cause an issue with realistic darkvision, no matter how dense the rods in one's eyes (though denser would allow for much better vision, capturing more of the fewer photons as light became less and less). $\endgroup$ – ScottS Sep 24 '18 at 17:41

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