An opaque object that is subject to radiant heat may show an increase in temperature.

Like a reporter near a Hawaiian lava flow.
That reporter then referred to the radiant heat as "up to some amount of °C". (What pleasantly caught my attention first was that he specified the unit used.)
He felt it's hotter the nearer he got, so it naively makes sense to assume an increase in temperature.
Note I refer to the sensory input of the person, not his experience of temperature, which may be pain for example. But part of that sensation was the effect of the IR radiation on his skin, or even the heat receptors of his skin.

I do not see how this "hot" can be expressed as a temperature.
One could wait until the temperature of the object comes to an equilibrium with available cooling, and then measure some well defined temperature, like the maximum of the surface temperature. But that is very different from the situation at hand, and depends a lot on properties of the object.

  • Is it indeed possible to specify a meaningful amount of °C?
  • If not, what would be best used instead?
  • Or is the situation of the reporter just too ill-defined, even though he validly has the sensation of increased temperature?
  • $\begingroup$ Possible duplicate of How does a radiometric infrared camera estimate an objects temperature? $\endgroup$ Commented May 12, 2018 at 5:05
  • $\begingroup$ @sammygerbil That seems related, but not duplicate. $\endgroup$ Commented May 12, 2018 at 5:08
  • $\begingroup$ Why do you think so? $\endgroup$ Commented May 12, 2018 at 5:13
  • $\begingroup$ While that question certainly handles central effect, I think this has some side aspects that make it less clear to me. For example, the person does not experience only the radiation. $\endgroup$ Commented May 12, 2018 at 5:22
  • $\begingroup$ What the person experiences is subjective and physiological. Questions on this site are required to ask about physics. $\endgroup$ Commented May 12, 2018 at 5:26

2 Answers 2


Well, there is local heating of the air--and I am going to leave that as "it is what is is". Regarding the wall of lava, what counts is the surface temperature and the solid angle is subtends. Let's say the surface is 1000C. If you are in a bubble ($4\pi$ solid angle), you are at 1000C whether the bubble diameter is 10 feet or 10 miles--that is the nature of radiant heat. If it is a wall of lava, then the closer you get, the greater the solid angle, so the hotter it feels. If there is absorption in the air, then you get hotter air and less exponential decay--so the apparent temp goes up faster than the solid angle.

Note that this effect occurs in forrest fires too: if the wall of flames is high enough, and wide enough--radiant heat can ignite structures from a surprising distance.

The material question is also a factor--but it just delays the inevitable. Volcanologist don't wear black, they wear insulating reflective suits, but that just buys them time. If they don't leave the scene, they will eventual be the same temperature as a Goth kid wearing all black.


Sure, but there are lots of "caveats" going with the subject.

The first place to look is the subject of pyrometer. Here is the wiki article, but be sure to google the subject and pick out articles at a level useful to you. And search through this forum for "pyrometer." There are several posts that seem to be related but not directly answers to this question.


The basic notion is, a hot object produces thermal radiation. If you compare this emitted radiation to a standard, you can estimate the temperature by noting the shape of the spectrum emitted.

There are lots of things to think about before applying such methods. For example, if the target is not hot enough you may get non-useful numbers. You need to be in a range that the target is producing adquate thermal radiation. There are effects due to the specific nature of the target. Different materials may produce different results at the same temperature. You may require some sort of calibration process to get acceptable results. And even then, you may only get results with a substantial uncertainty.

But there are lots of situations where such a method is applicable. One example is, heating objects to "red hot" type temperatures. So, smelting metal, heat treating certain types of material, control of a furnace for temperatures in that range, and various situations with similar temperature ranges.


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