Realistic darkvision?

Question

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.

Answer 1

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.

Answer 2

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,¹ 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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_{¹ Addendum 2022: since writing this answer I have found a pair of sunglasses dark enough that I don't feel pain in full daylight... they're a pair of welding goggles. I look a little steampunk walking through town in my Fedora (a stylish way to keep light out of my eyes) and a pair of welding goggles, but hey - if you can't live life outside the box, what are you doing here on Worldbuilding? Am I right?}

Answer 3

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.