On daytime, the surface of Earth is illuminated abundantly by light of spectrum from 250nm-1500nm, that includes near-ultraviolet spectrum (250nm-380nm), visible spectrum (380nm-720nm) and near-infrared spectrum (720nm-1500nm). Our eyes is very sensitive to so called visible light spectrum. It is known that some species of insects and birds are sensitive to ultraviolet light.

But I have not heard of any animal sensitive to near-infrared spectrum (720nm-1500nm). Is there some animal that may see near-infrared? Our world is filled with near-infrared light, for example, most remote control leds emits light of 950nm, CD readers have laser in Near-infrared light, most of plants reflects near-infrared abundantly, night security cameras uses near-infrared lamp too. Near-infrared is widely used in military too.

It seems to be very advantageous for one to be able to see near-infrared. But why we cannot see near-infrared light? Why there seems to be no vertebrates animals with eyes sensitive to near-infrared spectrum? Is there some biological or chemical reason behind that?

Actually we can perceive near-infrared as red light if it is strong enough. But our eyes have very low sensitivity to this spectrum so that it may be reasonable to just say we "cannot see" near-infrared.

p.s.: I am not referring to Far-Infrared light (heat), although sometimes the both are called Infrared light, this name may be misleading.

closed as off-topic by Kyle Kanos, BMS, Brandon Enright, Kyle Oman, John Rennie Nov 20 '14 at 6:02

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    You can actually extend your vision to 950 nm by eating a lot of fish liver, see here – Count Iblis Nov 20 '14 at 1:14
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    @CountIblis - Actually, you can't. See here. – David Hammen Nov 20 '14 at 1:34
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    This question appears to be off-topic because it is about the physiological results of evolution and not physics. – Kyle Kanos Nov 20 '14 at 1:41
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    @DavidHammen I was just about to overdose on fish liver, but your message stopped me from doing that :) . – Count Iblis Nov 20 '14 at 1:45
  • @KyleKanos - It is about physics. Absorption spectrum and thermal noise -- that's physics, not physiology. Evolution sometimes responds to physical constraints. This is one of those cases. – David Hammen Nov 20 '14 at 4:47

My previous answer was off topic because, as a comment says, it was about the far infrared, not the near, as you asked. I still leave it for those interested, at the end of this new edited answer. The problem is that water vapor has a strong absorption in the on the near infrared, so the actual light that comes from the sun has much less power than in the visible spectrum. Thus, in evolutive terms, it would not pay much to develop a near infrared photo-receptor: it is expensive and will not result in a great improvement relative to visible light (for the same reason that most of us do not have more that three photoreceptors, although some human females do have). The few nocturnal animals that have night vision in the near infrared have a different retinal structure: Many animals have a tissue layer called the tapetum lucidum in the back of the eye that reflects light back through the retina, increasing the amount of light available for it to capture, but reducing the sharpness of the focus of the image. This is found in many nocturnal animals and some deep sea animals, and is the cause of eyeshine. Humans, and monkeys, lack a tapetum lucidum. Thus, developing near infrared vision has its challenges and are only adopted by animals that really need a use for it.

On the other hand, the problem with thermal infrared light is that the entire world emits it. Everything that's warm shows up on infrared. Some animals are warmer than their surroundings (birds and mammals), but only just. It would take a fairly fine discrimination to use infrared vision effectively.

For non-warm-blooded animals, the differences are even smaller. Cold-blooded animals will still have metabolic hot spots, but they're not that much warmer than the background. Fruits and vegetables are at exactly the background temperature, and don't stand out in IR at all. So even if an animal did develop an extra set of infrared-sensing pigments, it wouldn't do it all that much good.

If you're a warm-blooded animal, the IR sensors are going to pick up your own body heat as well. You could, say, put them on stalks, but that's a lot of extra mechanism.

On earth (at least in nature), only one thing emits light, the sun; as long as you don't look at it, you're OK. Any hot thing will emit IR, and if you've got something very hot around, you risk oversaturating your IR sensor, rendering you blind until the pigments reset.

Some snakes have a very rudimentary set of IR sensors, at the tip of the nose, away from the eyes. They don't need the kind of focus that the eyes have; they just need a rough idea of where a warm mammal snack is. They can then use their other senses to narrow down precisely where it is.

  • Very good and detailed answer! Thanks! – Roman Matveev Jan 24 '16 at 21:13

One answer lies in utility and the evolutionary origin of our eyesight. Our eye is basically the same as that of a fish. Water absorbs red light. It absorbs near infrared even more strongly. Fish didn't evolve the ability to see in the near infrared because this strong absorption would make that capability rather useless. Near infrared would also be rather useless to land animals that live in humid areas, cloudy areas, or places with dense vegetation.

Another answer lies in thermal noise. Even though the thermal infrared is well-removed from the near infrared, there is a potential for heat to trigger a response in photoreceptors. This happens to some extent with the visible-range photoreceptors in our eyes. We occasionally see flashes of light that aren't there, some of which are noise generated by our own body heat. This noise would be even stronger with a near infrared photodetector, strongly reducing the utility of seeing in the near infrared. See Luo, et al. (2011), "Activation of visual pigments by light and heat," Science 332.6035:1307-1312.

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