I know that mid wave IR, long wave IR, and far IR are all infrared light that we feel as heat. Often, even when it is cold and windy, if I face the sun in a sunny spot it feels like it is 80 degrees F because of the IR light from the sun.

I know that all heat is at least partly IR light, even our own body heat. But what wavelengths of IR light are hotter? Is it mid wave IR because of its high frequency or is it far IR because of its long wavelength?

  • $\begingroup$ Body heat is not 'partly IR radiation'. Body heat is really heat energy (Enthalpy). But all objects above $0\:\mathrm{K}$ emit electromagnetic radiation, at lowish temperature mostly IR. See Planck's Law: en.wikipedia.org/wiki/Planck%27s_law $\endgroup$
    – Gert
    Commented Nov 29, 2015 at 1:48
  • $\begingroup$ Oh yeah. Then how is it that with thermal IR technology we can see heat and pit vipers have natural IR or heat vision? $\endgroup$
    – Caters
    Commented Nov 29, 2015 at 2:14
  • 1
    $\begingroup$ There's no contradiction whatsoever: bodies (humans, pit vipers or whatever) contain Enthalpy (heat energy) and the consequence of this is that they radiate IR. The IR is a manifestation of body enthalpy but it's not the thing itself. We're not really seeing heat, we're seeing the IR that's the consequence of that heat. $\endgroup$
    – Gert
    Commented Nov 29, 2015 at 2:20
  • $\begingroup$ There's no special connection between IR and heat. Blackbody radiation at or near room temperature (including human body temperature) happens to be dominated by IR, but that's nothing fundamental; after all, at the temperature of the surface of the sun, blackbody radiation is visible and quite bright. And a lot of the heat you're getting from the sun is from visible light. Hot things produce infrared radiation, and absorbing radiation means you're absorbing energy, and that can heat you up. But there's no special connection between IR and heat. $\endgroup$
    – elifino
    Commented Nov 29, 2015 at 2:22
  • $\begingroup$ Infrared light and heat are commonly mixed up. See physics.stackexchange.com/q/80983/37364 $\endgroup$
    – mmesser314
    Commented Nov 29, 2015 at 2:40

1 Answer 1


There seems to be some confusion here - the perception of "heat" from an IR lamp relates to the amount of energy absorbed by the body. This depends on the reflectivity of the skin at that particular wavelength; also, you would have to normalize this in some way.

Now we know from Planck's Law that there is a distribution of wavelengths from a black body: the peak emission goes to shorter wavelengths as the body gets hotter. Importantly though, at any given wavelength the emission density is great eras the temperature of the black body goes up.

It is not directly possible to answer your question without somehow deciding how to normalize the intensity: if we decrease the wavelength of the IR by making the emitter hotter, we will always experience greater heating.

I recommend that you look for reflectivity curves for skin: if the skin absorbs the radiation it will feel warm. Everything else is a matter of how you normalize.


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