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In this scenario, I would like to focus specifically on humanoids (read: human shaped blinking eyes, protected by eye-lashes and evolved in a waterless environment and fitting human eyesockets, from the outside at least) with the desired effect of them having superior eyesight to humans - by having a better night vision but without worsening overall eyesight.

In this case, let's also disregard technicalities related to evolution (like "Why would they have both night and day vision?", or "That would not be favoured since it would be costly."). Should the technicality be insane though ("It works but now they need 50,000 kcal a day to function.") I'd appreciate a heads up.

My own research focused on the two main differences between humans and animals with a superior night vision: the tapetum lucidum and the photoreceptor cells. There is a TL;DR summary of the main conclusions below.

Tapetum lucidum

The obvious and flashy (;)) option is naturally the tapetum lucidum. It's a retroreflector lying behind the retina that causes light to pass the retina twice (there and back), which makes for a stronger stimulus for the photoreceptors and enhances light sensitivity. Dark environments appear lighter, since the quantity of detected light increases.

As was however stated on Wikipedia and in this question, this enhancement comes unfortunately at the expense of visual acuity since it causes images to appear blurry.

Rods and Cones

In the same question, it was suggested that one increases the number of rods.

Now, Rod cells are more sensitive to light but lack the color vision of cone cells and their response time to stimuli [para. 2] is slower - that means less details and lower change in image detection. Cone cells do what rod cells can't, but are far less sensitive to light.

Increasing the number of rod cells does not sound like a bad idea, but contrary to the statement in that answer, humans actually have more rod cells (100 mil), than cone cells (7 mil), which is why we are actually able to see something in the darkness to begin with.

TL;DR

Tapetum lucidum has blurriness and loss of acuity. Should the increase in the number of rods have an ample effect on the effectivity of the night vision, it might be at the expense of the cone cells, which would, in turn lead to color-blindness and loss of acuity and movement perception.

And now I am stumped. Is there a way to minimize the drawbacks? Is there another way to achieve better night vision? Thank you in advance for possible answers.

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    $\begingroup$ It would be immensely helpful to specify what specific improvement you'd expect. One f-stop? Two? Four? More than that and you run into inherent photon noise. Humans already have very good night vision considering that we are diurnal animals; whatever mechanism you select should definitely not interfere too much with our daytime vision, which is way more important than night vision. $\endgroup$ – AlexP Jun 19 '19 at 20:53
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    $\begingroup$ Are you physically limiting the creature to two eyes and stereoscopic vision? Would a literal 4-eyed creature with two distinct pairs of eyes, where each pair is tuned one for daylight, and the other pair is optimised for night vision ? Or are you wanting something that looks plausibly human ? $\endgroup$ – Criggie Jun 20 '19 at 4:12
  • $\begingroup$ If they evolved separately from us, then a similar body shape does not mandate a similar biology inside their eyes. $\endgroup$ – WGroleau Jun 20 '19 at 5:36
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    $\begingroup$ @Whelkaholism: I would be really surprised to see a quantitative study showing that house cats have all that much better night vision than humans; for starters, their eyes are physically much smaller, and in low light that really counts. As far as I know, they have about 2 f-stops advantage in low light compared to us (which is significant but not earth-shattering; similar to a good P&S camera compared to a smartphone), compensated by clearly inferior day vision -- their day vision acuity is significantly lower than ours and their color discrimination is between bad and very bad. $\endgroup$ – AlexP Jun 20 '19 at 11:08
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    $\begingroup$ @AlexP Fair enough! I guess cats also have acute hearing, smell and whiskers that makes their night-time performance overall much better than just eyesight alone $\endgroup$ – Whelkaholism Jun 20 '19 at 11:09

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Let's start with a dose of reality

I have "superior night vision" (hah), aka, super-light-sensitive eyes. As a teen I could read comfortably under a full moon and I can see comfortably at light levels that cause most people to trip over tree roots.

My highlight was as a kid when rangers turned off the lights in some cave tour in Montana. I've been through a lot of caves — most of which really are completely dark — but this one had phosphorescent lichen on the walls. When the lights went off, I could see the shadows of people as they shuffled and moved their arms between me and the light source. Nobody else could see it (they all thought I was lying or experiencing "spelunker illusion," the ability for your brain to superimpose a "shadow" of your own hand in total darkness because the brain knows where it is).

Can I see in complete darkness? Heck, no. Can I see an LED at some unreasonable distance in a completely dark room? Yeah, like a lighthouse. Smoke alarm LEDs keep me awake in hotel rooms, as does the light through the door peephole (stuffed with tissue), the light under the door (towels), and the light through useless hotel blinds (trash bags and blue painter's tape).

  1. I'd trade this ability for anything short of cancer in a heartbeat. The pain I experience during the day without dark sunglasses is excruciating. Even with sunglasses, I often walk around in full sunlight with one eye closed and the other squinted. Photo flashes feel like knives and holding my eyes open so the good doctor can look inside them is agony on a biblical scale.

  2. I do not have extra rods/cones. One ophthalmologist claimed I had less pigmentation on the retina than usual. Others have told me, "we don't know, everything looks normal, it's just the way you are." Whether or not this involves Tapetum lucidum, I do not know. The general consensus has been that the rods/cones are simply more sensitive (how is anybody's guess, I suppose we'll find out if I leave my corpse to science).

  3. I have a latent memory of some whacko who claimed the issue might be my brain, not my eyes, in that my brain was freaking out over what would otherwise be normal stimulus. It's an old memory, and not super clear, which leads me to believe it might have simply been a conversation with a friend rather than an official conversation with a doctor, but it is worth remembering that between the pupil and the visual center of the brain there is a LOT of cool juju that can be "sensitive" to light. However, for the record, this is the least plausible solution IMO.

My eyesight is great for midnight steal-the-flag, but I've yet to find a practical use for the ability thanks to the consequences. And that's important, if you want any realism at all you need to deal with the consequences of ultra-sensitive eyes while you create them.

(Before I go on, please note that some butterflies have five times the number of cones humans do (also see here). It doesn't make them more "brightness sensitive," it makes them more "color sensitive," which might suggest they have the same number of cones in total, but not the same number of each type. Might be worth looking into.)

Pupils

Insofar as I know biologically, pupils are your only logical choice. Whether it's Willk's extra-large pupils during the night or a less practical ultra-small pupil during the day. The problem with a day pupil is that, logically, there's only so small a hole the muscles can produce before the mass of the muscles themselves get in the way of closing the hole. It's much simpler to open the pupil wider at night. I upvoted Willk's answer. So should you.

Alternatively, you have all kinds o' critters (goats, frogs, cats...) that have non-circular pupils. The shape of the eye need not control the shape of the pupil, giving you tremendous control over the use of this solution.

Third eyelid

My cat has a wonderful eyelid that protects her eyes against wind and impact (aka, a "nictitating membrane"). Make that sucker semi-transparent and use it during the day to block daytime light.

Fanciful solutions

  • Like a car engine disabling some of its pistons to improve fuel economy when unnecessary, engineer the eye-nerve-brain interface to disable rods/cones when not needed.

  • The transparent outer layer of the eye can self-darken like Photochromic lenses (sunglasses and welder's helmets).

  • The transparent outer layer of the eye is columnated, like many fluorescent light panels (you know, the inch-thick panels in squares designed to block the glare from acute angles), but this comes with another price: limited peripheral vision.

  • It simply doesn't hurt. The brain is able to withstand the entire spectrum of brightness without experiencing pain. Said another way, just write your story and don't worry about what the occasional "informed" reader thinks.

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    $\begingroup$ The "It simply doesn't hurt" is something that evolution can achieve. If the starting point is eyes that are painfully sensitive to daylight, there will be similarly strong evolutionary pressures supporting three kinds of changes: Be active at night instead of the day; reduce the light-sensitivity; reduce the pain response from daylight vision. If the change(s) that reduce the pain response happen first, voila! $\endgroup$ – Jasper Jun 20 '19 at 1:07
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    $\begingroup$ @Jasper isn't pain the response of evolution to potential health hazards? Is there any proof the eye would not be damaged if only the pain sensation is fixed? $\endgroup$ – Patrick Trentin Jun 20 '19 at 6:48
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    $\begingroup$ Not sure why it is the "least plausible solution" (especially as when the impossible is removed...) The retina is sensitive enough to detect single photons - those are later filtered out by the brain during image processing. Leave anyone in complete darkness for long enough and chances are they will become similarly sensitive as you. In fact, this is why people trapped for days or weeks by cave-ins, when rescued, have their eyes covered. $\endgroup$ – Eth Jun 20 '19 at 9:07
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    $\begingroup$ @Paul : However, in this case, the trick is that the OP's experience is of unclear etiology. Thus it isn't necessarily the case that the eyes here are receiving more light and so becoming damaged. If the issue is in the brain , then it seems what'd be going on is that the brain is triggering the pain response because it "thinks" the light is much brighter than it actually is, which would be appropriate were the system normal and the eyes actually receiving that much light. $\endgroup$ – The_Sympathizer Jun 20 '19 at 12:30
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    $\begingroup$ @ChatterOne The pupil response can be extremely fast, so it's probably not that. You need about 20 min of darkness to completely switch to night-vision mode, but a flash of light will mostly reset it (protip: do not drive with headlight to an amateur astronomy site). The cave-in rescues are probably due to much longer adaptations to darkness, that also take longer to switch back to light mode. $\endgroup$ – Eth Jun 20 '19 at 13:20
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Giant pupils.

tarsier

Humans are limited in how big our pupils can get because somewhere along the line, evolution selected individuals where the iris was small enough that you could see the whites of our eyes. Probably that gives some sort of cultural benefit as one can see where a person is looking, and maybe you will later feel fondness towards a baby you saw looking at you and take care of it. But the downside is that the pupil can only be as big as the iris. Small iris means smaller maximum pupil and less light admitted at maximum dilation.

Your humanoids have eyes that are all iris, like this tarsier, or cats, or anything but humans and maybe dogs (hmm...). Their pupils can get so big that the eye becomes all pupil, admitting more light and so improving vision in low light. Plus they will look cool.

Infrared vision

pit viper

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 metre 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. https://www.nature.com/news/2010/100314/full/news.2010.122.html

Under the right circumstances, accurately perceiving infrared radiation (perhaps via a non-eye organ, like snakes) could be even better than just seeing warm bodies. Under conditions of changing atmospheric temperature, objects in the environment change their own temperatures at different rates according to thermal's mass and composition. Even our very blunt distant temperature sense (perceiving air temperature differences) can tell the difference between a stone wall and a tree after a hot day, or after the day has gotten hot. Infrared vision could give you a thermal picture of the environment.

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    $\begingroup$ One issue that comes with having big pupils is that your depth of field is much reduced, requiring better focussing optics. All the problems that humans have with failing eyesight will hit sooner, and will probably manifest as night blindness before it shows up as daytime vision issues. $\endgroup$ – Starfish Prime Jun 19 '19 at 19:21
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    $\begingroup$ A fully dark-adapted human eye has an aperture of about *f*/2, which is very wide. Go into a dark room and let your eyes adapt, then look in a mirror. You'll be surprised how large your pupils are. I don't think that you can gain more that one f-stop in this way. $\endgroup$ – AlexP Jun 19 '19 at 20:55
  • $\begingroup$ And then someone shines a bright light at you, and you are blinded. Flashbangs would be much more effective against your heroes. $\endgroup$ – vsz Jun 20 '19 at 6:18
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    $\begingroup$ @AlexP they're still no more than half the radius of my eye socket. $\endgroup$ – OrangeDog Jun 20 '19 at 9:45
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    $\begingroup$ A note for anybody reading this answer: you actually can't have infrared vision in human-sized eyes, not if you also want the standard visual spectrum. As wavelength increases (from visual to infrared), larger sensors are required. Cones and rods won't cut it for infrared vision: you need separate organs like what snakes have, unless you want to sacrifice conventional vision. If you want detailed infrared vision at any distance, said organs will probably be as a combined whole larger than eyes; my choice of location on a humanoid would be the cheeks, since those offer plenty of surface area. $\endgroup$ – Palarran Jun 21 '19 at 0:04
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The problem with tapetum lucidum is that the reflected photon will interact with the photoreceptor in a slightly different position due to the reflection, actually blurring the image.

Therefore, instead of increasing the amount of captured light by reflecting it back after the retina, why not make a double stacked layer retina? In this way the result would be the same (a photon would cross twice the sensitive layer, doubling the chances of being sensed) but without having the blurring due to reflection.

Moreover, in daylight condition the nerves of the second layer could be switched off to keep a good vision.

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    $\begingroup$ While not a bad solution, I believe this would still cause blurring because your lense can only focus to a single focal depth, and because your first reina would still scatter some of the light. $\endgroup$ – Nosajimiki Jun 19 '19 at 20:12
  • $\begingroup$ @Nosajimiki, but one can still play with the depth of focus. $\endgroup$ – L.Dutch - Reinstate Monica♦ Jun 20 '19 at 5:10
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    $\begingroup$ Increasing the depth of field requires a smaller aperture which will itself reduce night vision. Don't get me wrong, I think doing a double retina where one can turn off is a viable answer to the OP. It would give night vision comparable to a tapetum lucidum while preventing over-sensitivity during the day, but the night vision will still be blurred. $\endgroup$ – Nosajimiki Jun 20 '19 at 14:31
  • $\begingroup$ @Nosajimiki One layer will be in focus. The other won't, but the brain is amazing. $\endgroup$ – wizzwizz4 Jun 21 '19 at 15:29
  • $\begingroup$ @Nosajimiki Wouldn't a double-retina with one being able to "turn off" be equivalent to a tapetum lucidum with its own "eyelid" back there? $\endgroup$ – Delioth Jun 21 '19 at 19:16
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Three things spring to mind, none are easy fixes though.

The first thing is that the layout of human eyes (and indeed the eyes of all vertebrates) is a bit odd... there's lots of infrastructure in the form of neurons in front of the photoreceptors. This isn't quite as stupid as it sounds, as those neurons don't just get in the way of the light, but serve some useful focussing and filtering purposes. I have a sneaking suspicion that vertebrate eyes have evolved to make the most of their silly-backwards layout, rather than the backwards layout being somehow desirable in and of itself. Certainly, it is a bit hard to evolve out of the backwards layout once you've got it. A right-way-round retina could also evolve to be just as good at focussing, without any unnecessary absorbtion of light.

By way of a bonus, by putting the optic nerve at the back rather than needing it to punch through the eye and spread out across the inside means that you can get rid of the silly blind spot. That'll mean you can use more of the light coming through the pupil too... not much, but every little counts.

So: photoreceptors in front, nerves behind, more light gets to the photoreceptors and there's no stupid blind spot.

Next: long wavelength (or near-infrared, if you like) light sensitivity.

Take a look at this answer on the biology stack exchange

Rods vs cones light sensitivity

Those low-light sensitive rods cells are great when it is gloomy but they're actually less sensitive to some longer, redder wavelengths of visible light than colour-sensitive cone cells are.

Could you make a better rod cell? Maybe. Some enterprising biologists have engineered rod cells to express cone cell red pigments, but the results weren't great... the problem is that at low light levels, those long wavelength signals are noisy. If you just had a bunch of these modified rod cells you'd be able to see near infrared light much better, but the quality of your low-light vision in those circumstances would be quite poor. Blurry, most probably, rather than the sort of static snow effects that digital cameras get in low light.

Maybe though, with a bit of clever work, you could come up with a better rod cell, or a new type of cell and pigment that shows better low-level, long-wavelength sensitivity without suffering from too much thermal noise.

You can't go too long though, because you'll start having focussing issues... making a lens that will work well for short blue wavelengths as well as it works for longer near-IR ones will be tricky. Go far enough into the mid and long-IR range and you won't be able to focus at all because you've got eyes made of the wrong materials. For those wavelengths, take a leaf out of the pit-vipers book, and grow some pit organs.

The third option is really more like a two-and-a-halfth option. Follow the lead of security camera manufacturers: Security camera with near-IR illuminator

Get you some bioluminescense that emits a decent amount of near-IR light, and add it to the skin cells in convenient places (like your face, but not too close to your eyes). Easier said than done, but if you can do it, the noise sensitivity issues of those modified rod cells suddenly go away. It'll require more energy to run (bioluminescense ain't free) and it might be visible to people with conventional eyes (and you'll have a brightly glowing face form the point of view of other members of your species) but maybe it'll look awesome into the bargain...

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  • $\begingroup$ Having "inverted" retina allows vertebrates to fit more photoreceptors into an eye, the effect scaling inversely with the size of the eye - in other words, the smaller the animal, the more space the arrangement saves(the smallest eyes achieving a whopping 50% reduction in ocular volume compared to an invertebrate with the same focal length)! [br] sciencedirect.com/science/article/pii/S0042698909003162 :D [br] Also makes for speedier processing since smaller eyes=less distance signals have to travel. Eyes are so amazing $\endgroup$ – Tero Jun 20 '19 at 4:10
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    $\begingroup$ @Tero its an interesting idea that's hard to read any more about (because obviously every copy of the paper is hidden behind a paywall, because science should be secret or expensive) though I remain somewhat dubious. By simple geometry, the benefits scale poorly with the radius of the eye, and you still have the issue of the large blind spot where the optic nerve pierces the retina. The additional receptor density is clearly insufficient by itself, because your eye still has a fovea centralis to minimise obstructing clutter. $\endgroup$ – Starfish Prime Jun 20 '19 at 6:25
  • $\begingroup$ The whole thing is freely available to anyone willing to click the link, though? As I said, the effects are strongest in small animals, but even then humans have 11.3% smaller eyes than if everted. Though all eyes are subject to diminishing returns with regards to size, inverted retinas or not. The blind spot is small and not only is it in the peripherals to begin with but in species with binocular vision the other eye covers the spot. (๏w๏) And as for the fovea... You might be interested in learning that there are invertebrates with analogues to foveas. And vertebrates who lack them. $\endgroup$ – Tero Jun 20 '19 at 13:07
  • $\begingroup$ @Tero huh, from a different browser I get to see the paper instead of just the abstract. No idea what's happening there. But anyway, your 11.3% difference is the volume of the eye, not the area of the retina. Given the quoted thickness of the retina (0.5mm, though nih.gov suggests more like 0.3 max and mostly thinner) the entire surface area of the eye would be reduced by more like 4%. It is harder to quantify how much light is absorbed by travelling through the "back" of the retina first, but it will be non-negligible. $\endgroup$ – Starfish Prime Jun 20 '19 at 18:15
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    $\begingroup$ Cephalopods have everted rather than inverted retinas, as you describe (and there is no evidence of loss of acuity due to a slightly smaller number of cones). They also get to see polarization (having receptors aligned along both of two perpendicular axes). Studies of these eyes abound (because octopi can regrow lost or damaged eyes, so n>1 repeats are possible). $\endgroup$ – Eric Towers Jun 21 '19 at 2:08
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I like your thought, but I think you'd be better served trying another system entirely. Instead of working within the human vision spectrum, why not consider expanding it to infrared? Have the rod cells be able to pick up light far beyond that meager 380-740nm wavelength, and double it or even triple it to allow up to 1000nm, or even 10,000nm. The increase light sensitivity will give humans night vision. Are there downsides? Yes. Two.

The first is that everything won't really have colors. Infrared binoculars look green, but that's because the infrared the binoculars picked up have been processed and are hitting your cones. The infrared that your rods pick up won't touch the cones, so it'll have the same color as it would have had if you didn't have nightvision, i.e. none at all.

The second is light. As in, daylight. If your eyes are that receptive to infrared, than you'll basically be blind from the sheer wave of light during the day. Luckily, we've got a solution. Tapetum Lucidum. (Have you heard of this before? I feel like you might have.) All jokes aside, the reflective layer will work if we reverse it. Have a third eyelid ( nictating membrane) which reflects (or just blocks) the infrared spectrum that comes down during the day will stop your night-vision humans from going blind.

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    $\begingroup$ The human night vision (scotopic) system doesn't work in daylight at all -- it's turned off. The problem with infrared is that humans are warm-blooded animals, so that the eye itself is full of infrared light... Stray light in a photo camera is not a good thing. $\endgroup$ – AlexP Jun 19 '19 at 20:58
  • $\begingroup$ @AlexP: It's a common mistake to assume that cold-blooded animals are in fact cold. Cold-blooded animals have less internal ability to regulate their body temperature. In warm environments (deserts/tropics), this capability is simply not needed as the animal can simply function at the ambient temperature. $\endgroup$ – MSalters Jun 20 '19 at 12:09
  • $\begingroup$ @MSalters: I know what poikilothermy is. The point is that humans and other homeotherms operate at higher temperature than the environment almost all the time. That is why they are visible in infrared. $\endgroup$ – AlexP Jun 20 '19 at 14:09
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    $\begingroup$ You can't use conventional eyes to see thermal IR. There's the little problem of the eye seeing itself. There's a reason why the pit vipers use what's effectively a pinhole lens for their pit organ. $\endgroup$ – Mark Jun 20 '19 at 20:07
  • $\begingroup$ @AlexP don't confuse far-IR (thermal) with near-IR ("night vision"). Warm-blooded creatures don't glow in near-IR. $\endgroup$ – Starfish Prime Jun 21 '19 at 9:48
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This is an addition to Willk's answer;

Giant pupils would be a very good start, with the addition that the iris would have to be able to get the pupil small enough to allow function in the daylight.

There could also be a secondary iris on the back side of the eye which could cover/reveal a tapetum lucidum layer. The reason why you might not want it visible all the time is that it would cause blurry vision as light that misses photo receptors is reflected to gets a second chance to be absorbed.

Lastly, you could throw in infrared vision, which is found in some species of fish and frogs. Also, while technically not part of the eye itself, you could have infrared detection pits like are found in several kinds of snakes. This would allow for some limited amount of vision even in areas with no visible light at all.

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  • $\begingroup$ +1 For suggesting the location of the tap-luc could have developed way back there, that's a natural extension of pre-existing evolution, getting it to grow a muscle and discontinuity with surrounding tissue - well that's sort of genius. $\endgroup$ – Tantalus' touch. Jun 19 '19 at 19:37
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Obscurable Tapetum Lucidum

Place your Tapetum Lucidum in a capsule of black, highly opaque fluid. In the light your eye squeezes the black fluid in front of the Tapetum Lucidum to give you a non-reflective backdrop for optimal clarity, then at night, you push the fluid behind the reflective sheet for double reception.

Larger Irises + Controllable Proptosis

Since you don't want to change the size or depth of the eye, just give it a much larger iris than the socket would suggest. Normally, this would accomplish nothing without also increasing the size of the eye socket. However, proptosis is when they eyeball is extended beyond the eye-socket. Normally this is the result of a medical condition, but in some people like, Kim Goodman, it's just a thing you can do. By pushing the eye out of the socket, you expose the larger light collecting area despite having apparently normal human sized eyes, but during the day you pull them back in giving them full human like protection from injury and sun-light.

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Literally cool night vision.

(I may have some details wrong, and numbers are completely made up, working from memory.)

Some fish can adjust the temperature of their eyes. When they are cold, the photoreceptor pigments stay active for a longer time than they do when they are warm. This have the effect of a longer exposure time ('shutter speed') at colder temperature.

When they are diving down in the inky depths, they let their eyes cool down, and they can see an image accumulating all the photons that have hit the retina in the last second. This makes their eyes very sensitive, but fast moving things are blurred.

When they are near the surface, where there is light, they heat their eyes. The image they see is from all the photons in the past 1/100 s. This is not as sensitive, but that doesn't matter because there is plenty of light. What matters is that moving things are no longer blurry.

While you're at it, make the tapetum thermochromic: silvery when cold and black when hot.

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  • $\begingroup$ Fish have it ieasier because they are surrounded by a nice medium that is well suited for use as a coolant. $\endgroup$ – Starfish Prime Jun 21 '19 at 9:42
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Don't use photons at all. Do it like bats do, and make a mental image of your surroundings through sonar.

In this way, you can use your eyes by light and your ears in the dark.

Alternatively, take a page from snakes. They have cells that are sensitive to radiation at the 5 to 30 micrometers range. For comparison, the "reddest" we can see is around the 700 nanometers range! In practice, this means snakes are literally able to see in infrared.

Infrared sensing in snakes is not done via photochemical reactions, though, so it may not be as sharp as visible light sight. However, it does allow snakes to find prey and strike accurately in total darkness.

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    $\begingroup$ People can learn echo-location with practice: rnib.org.uk/rnibconnect/blind-echolocation $\endgroup$ – Robin Bennett Jun 20 '19 at 15:08
  • $\begingroup$ Trouble is resolution. The highest frequency humans can hear on average is 20kHz, which has a wavelength of 1.7cm. That means your resolution is limited to about that size, you'll see everything in in 1.7cm blobs. Bats can hear up to 200kHz, which is better, you'll see individual objects well, but won't discern any details, like hair, texture, etc. Also you're limited by range, latency and the need to emit the sound. $\endgroup$ – Curiosity Jun 20 '19 at 17:39
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Let them have bat-like sonar. While they may need some differently shaped ears there are also cases where blind people developed something similar with normal human ears. Their brain would have no issues overlaying the information of sonar and vision, giving them the ability to navigate perfectly, while a normal human could only see vague schemes. They might be sensitive to loud noise as a drawback, but this solution could be used for a nice plot twist (even if friendly they might just use the term "nightseeing" because it is all natual for them).

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In addition to the other answers like larger irises and infrared sensitivity, one other thing you can do is improve the sensitivity of the eye. Increasing the number of nerves connected to the rods will boost their sensitivity.

Another thing is improving the brain's image processing capabilities. Rewiring the brain is definitely not an easy task, but if done well could vastly improve the quality of the image the brain receives without modifying the eyes at all.

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Injected Nanoparticles

For short term night vision, you can get injected with infrared sensitive nanoparticles that can extend your vision into the infrared range

See Infrared Night Vision With Nanoparticles

It lasts about ten weeks and infrared sources appear as green light.

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Squid eyes.

The most obvious design flaw with human eyes is that someone lazily designed the receptors inside out; the nerves are all on the inside of the eye, so the light needs to go through that before it even gets to the receptor cells, and then the nerve bundle needs to be routed through the eyeball itself which leaves an inconvenient blind spot.

Squid eyes are much more sensibly designed -- the receptor cells are on the inside, the nerves are at the back, which gives better sensitivity and no blind spot.

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    $\begingroup$ Lets add some math to your answer. Humans eyes absorb 90% of all photons before they reach the photon receptors. And we need at least 9 photons hitting an individual receptor before it will register a light source, (before it "sees something). Which means that sticking the receptors farther forward we could (optimally) increase light sensitivity ten fold. And if you add in some method for the eye to drastically increase its opacity, you do not run into sunlight sensitivity. math.ucr.edu/home/baez/physics/Quantum/see_a_photon.html $\endgroup$ – Garret Gang Jun 20 '19 at 18:14
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Two sets of eyes!

They could still be humanoid eyes, just one pair is much more sensitive to light. Maybe when they are sleeping at night, they keep their day eyes open ... not super userful for seeing at night, but allows them to sleep with their eyes open literally and if something occurs that might wake them up they'll open their night eyes and close their day eyes.

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Why not have double eyelids like reptiles, and have the inner pair act as a pair of shades? This way you have superior night vision (increased no of rods, retro reflectors, etc) and then a pair of translucent but lossy eyelids, then regular eyelids?

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A nictating membrane that shields your eyes from the infrared spectrum (700 to 1000 nanometers) during times of bright light in what we call the visible spectrum (380 to 700 nm), and retracts when you want to see in infrared.

enter image description here

Fortunately, humans have a vestigial third eyelid that you can repurpose for the task.

enter image description here

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  • $\begingroup$ The issue is not one of filtering, but of thermal noise in photoreceptors making it difficult to work with low-intensity, long-wavelength light. $\endgroup$ – Starfish Prime Jun 21 '19 at 9:44
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I think some answers touch the approcha I am to suggest: Polychromacy

People generally can see in three-dimensional colourspace with bases of Red, Green and Blue. Therefore Homo Sapiens Sapiens is considered a trichromat. Some people suffer from colour-blindness. A subsidiary to this group can recognise only two colours and they may be considered dichromats. On the other hands there are people reported to see a fourth colour. You can see https://en.wikipedia.org/wiki/Pentachromacy too.

So, consider what electromagnetic waves can illuminate your world and how they interact with the surfaces. Then let your species be polychromats with retina cells sensitive to human-visible spectra plus far infrared (cold surfaces), near infrared (hot and superhot surfaces) on one side of the spectra and ultraviolets on the other.

Also note that infrared optics, "visible light" optics and UV optics needn't be compatible. Even light optics may suffer from colour abberation - different focal points for different wavelengths. This may work for you benefit - in daytime the eye is focusing the sunlight spectra to the retina while keeping the rest defocused and in the night it focuses the nightlight spectra.

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I present to you, the list of ultimate eyes in the animal kingdom::

1. Eagle eyes:

Eagle eyes

The eagle eye is among the strongest in the animal kingdom, with an eyesight estimated at 4 to 8 times stronger than that of the average human.
Rods and cones: Check, you have that already, just mentioning it for completeness

2. Mantis shrimp eyes:

Mantis shrimp eyes

Compared to the three types of photoreceptor cells that humans possess in their eyes, the eyes of a mantis shrimp have between 12 and 16 types of photoreceptors cells: We can see Red, Green and Blue, they can see red, green, blue, polarized, infrared, ultra-violet, septarine, octarine, nonarine, decarine, undecarine, dodecarine, .... (Nope: I have no clue what this last bunch of colours are neither as my physiology just cannot see them neither)

3. Owl eyes

Owl eyes

Tapetum lucidum: Check, you have that already.

4. Pigeon eyes

Pigeon eye

OK, not really their eyes, but pigeons can sense magnetic fields and it seems to be related to light.

Now combine all of the above:

and add additional double / triple / quadruple eyelids to shield these sensitive instruments from overload. and you'll end up with something like this:

Grey Alien

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  • $\begingroup$ Um, correct me if I'm wrong here, but none of those animals, bar owls, hunt or feed at night, and the OP already rejected the tapetum lucidum as causing undesirable burring of vision. $\endgroup$ – Starfish Prime Jun 21 '19 at 9:45
  • $\begingroup$ @StarfishPrime Answer edited. Been out of country, didn't have time to lead it to its conclusion. $\endgroup$ – Fabby Jun 22 '19 at 14:12
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Lots of good answers given already, but I'll add this! The first thing you need to decide is: what do you want your humanoids to have cool night-vision for?

Take the tapetum lucidum. If the point of having night-vision is so your humanoids can run about a forest at night without crashing into trees, tripping over roots, falling off cliffs, or mistaking that approaching angry bear for a placid cow, then the fact that the tapetum gives you slightly blurry images is not a problem. For instance, I'm short sighted, so trees, roots, cliffs, bears, etc are blurry when I'm not wearing my glasses. I do not, however, walk into obstacles all the time!

Therefore if all you need your humanoids to be able to do in the dark is the kind of stuff a lion, fox or wildebeest can do in the dark, then the 'lame drawback' is not a drawback at all.

However, if you want your humanoids to be able to focus on tiny details in the dark - like read a book or do embroidery or tell if that moth flying by is a lesser spotted or greater spotted woozlemoth - then the blurriness is a problem. Most of those types of things are only stuff that humans worry about. Unless you are an animal who eats woozlemoths and the greater spotted ones are poisonous.

To avoid the blurriness, you simply have no tapetum behind the fovea. The fovea is the area of the retina designed to see tiny details. So in daylight, you can read small print in books etc, but at night you cannot.

Alternatively, use elephant hawk moth trichromatic vision. Those moths use a system which lets them see in colour in the dark because of the physics of the way its eyes work: a very high photon flux to avoid the downside of trying to receive a colour signal at low light intensities.

The moths can do it because of the structure of the lenses in their multifaceted eyes. Perhaps someone who knows more about physics than I do can tell you if it is possible to duplicate the effect in a vertebrate eye?

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