# Am I right about IR vision?

I watched Kyle's Because Science video on The Predator and 1 thing he talked about was IR vision. Here was his argument:

If you can see nothing but infrared light, you can only see the ambient temperature and thus you couldn't see anything.

Here is the link to the video:

Because Science The Predator

I would argue strongly against that simply because not all things react to heat in the same way or to the same extent.

1 simple way to disprove his argument would be to have a set of materials that each react differently to heat such as these:

Metal: conductor

Glass: Traps heat

Body of human or animal: Emits heat

Water: High heat capacity

Plastic: Insulator

Make sure they are at exactly the same temperature and then see them in infrared. You would clearly see differences in what appears to be temperature.

The metal would appear to at least be several hundred degrees if not thousands of degrees. The glass would also appear very hot but not as hot as metal. The body would appear to be at the temperature it actually is at. The water would appear to be significantly colder(it might look as cold as ice even if it isn't that cold). The plastic would appear to be only slightly colder than the body. You wouldn't be seeing the temperature but rather the reaction to heat.

Another easy way to test would be to look at the body of a human or a creature at its starting position, and then continue looking at that same position as the creature or human moves while at the same time not sensing movement with any other senses. You would see a faint heat signature that dissipates within milliseconds and the colder it is, the faster the heat signature dissipates. In other words, even if you couldn't tell the exact shape of the creature or human, you could still tell that it moved by seeing a faint heat signature.

Thus I do believe that Kyle is wrong.

You could sense the movement of an object or how it reacts to heat, not just the ambient temperature. Yes, you could tell just from looking outside whether it is 40 degrees or 80 degrees. But you could also tell how an object reacts to a change in temperature or if an object has moved. Nothing would get past you since there is no object that doesn't emit heat. Even black holes emit a small amount of heat due to their immense gravitational differential. Yes, even cold blooded creatures like lizards that would blend into their environment as far as temperature, you could still see.

In other words, unless your eyes were injured, got cataracts, or are effected by genetics, you would never go blind until you die. You wouldn't even be night blind. Even in a jungle at night during a new moon, you could still see everything around you. You would even be able to see your own abdominal and chest organs. You could see if you are pregnant or gravid long before you actually look bigger. You could see if something was inflamed and what it is before you get any symptoms.

But am I right that you could see anything with infrared vision including black holes and cold blooded creatures and that night blindness would simply be impossible without something directly effecting your eyes?

• If the objects are at the same temperature then necessarily they will look the same in infrared. If you gave each object the same amount of heat energy they would have different temperature due to different heat capacities. – Triatticus Nov 22 '18 at 17:01
• @Triatticus That assumes that all objects are blackbodies. In reality, objects have nontrivial emissivities that make their spectrum deviate from a blackbody spectrum. – probably_someone Nov 22 '18 at 17:13
• I'm mostly saying it in response to the definitely incorrect statement about metals there, the sensors I've seen do not display these "several hundreds of degrees" but the actual surface temperature within some tolerance. But certainly the emissivity needs to be taken into account – Triatticus Nov 22 '18 at 17:17

Here was his argument:

If you can see nothing but infrared light, you can only see the ambient temperature and thus you couldn't see anything.

I think Kyle's argument here is that it is more difficult to detect a warm body by its IR radiation, if the background is also warm. Which is a reasonable argument.

With that understood, even if the temperature of an object is the same as the temperature of a background, the object could be picked up by an IR camera in the dark, unless its surface is identical to the surface of the background.

Each surface is characterized by its emissivity and, if the emissivity of two surfaces is different, they will have different intensity of IR radiation, even if they have the same temperature.

In general, surfaces that absorb more light (including IR), also radiate more light. According to this, rough dark surfaces should radiate much more than shiny light surfaces. So, an ideal black body is also a perfect radiator (emissivity is defined as $$1$$), while a mirror is a poor radiator (emissivity around $$0.02$$).

Here is a link to a Wikipedia article on emissivity, which includes a table listing emissivity of common materials.

I would argue strongly against that simply because not all things react to heat in the same way or to the same extent.

A few years back I got to look through a serious IR sensor on a armored recce vehicle. It was a very hot day. I had absolutely no problem seeing people even though the air was probably pretty close to body temperature.

So these things can apparently pick up pretty minute differences in temperature. I'd expect the ones used by an interstellar species to be even better. So I'd say Arnie was toast, mud or no.

Make sure they are at exactly the same temperature and then see them in infrared. You would clearly see differences in what appears to be temperature.

Nope - as it turns out, this is wrong. Thermal emission is extremely information-lossy, and it is fully controlled by the temperature, with the total radiated power fixed by the Stefan-Boltzmann law, and the spectral distribution given by the Planck spectrum. Neither of those two depends on the material in any way beyond the temperature.

I appreciate that this is rather counter-intuitive, but that's just how things are.

So, can you get around this? You can, if what you're looking for is reflected IR, in the same way that visible light works, but then you need to be working with illumination that's brighter than the thermal emission at the wavelengths of interest, and that becomes increasingly infeasible the longer the wavelength becomes. Or you can look for differences in the temperature within your field of view, as not all objects in a given scene will be at the same temperature (which is how existing IR cameras work). But that's it, I'm afraid.