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The thing about infrared thermometers that bugs me is how can you get the same temperature reading regardless of the distance to the object. Shouldn't there be a difference when measuring from two different standing points since energy flux density decreases with ${1\over distance^2}$ and infrared thermometers work by focusing IR light on a thermopile, which then results in decreased (when measuring from further away) absorbed energy and therefore lower temperature and finally lower voltage across thermopile. Is there something I am getting wrong about this, or do IR thermometers make use of some other physics law like Wien's displacement law, by somehow measuring $\lambda_{peak}$ to determine the temperature?

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I believe the basic answer is that, within limits, as you move away from an extended source, the IR sensor can collect flux from a greater amount of the surface. Your $\frac{1}{distance^2}$ formula only holds for a point source.

In the limit of an infinite flat surface of uniform temperature, the thermometer would receive the same power regardless of distance from the surface, as can be seen by considering the area in its cone of view (Field of View).

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  • $\begingroup$ This is a right answer. When we walk toward a wall it doesn't look brighter and a photographic exposure meter won't show a dramatic change either. An IR thermometer is roughly analogous to that, except for longer wavelength light. It assumes an emissivity of 0.9 or 0.95 and integrates a wide range of wavelengths, (something like 5 to 15 um) and uses a look-up table to calculate what temperature would produce that amount of collected light. $\endgroup$
    – uhoh
    Nov 11, 2020 at 15:34
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    $\begingroup$ The lens is probably designed so that the device has a "field of view" of close to 3deg (the best you can obtain) meaning that even at a distance of 1m the spot is only 10cm in diameter so you don't need a very large flat surface in order to measure the same temperature regardless of distance.. Also worth mentioning is that the red dot is just for pointing and it has nothing to do with measuring the temperature. $\endgroup$
    – Vinzent
    Nov 11, 2020 at 15:35
  • $\begingroup$ Thank you for the answer, I see where I got things wrong, at least I think so. Therefore I have a little follow up question. Lets say I'm measuring a temperature of a radiator with an IR thermometer with distance to spot ratio of 10:1, from 5 meters away, so that the thermometer still measures only radiator temperature. Since the radiator cannot be approximated with an infinite flat source in this case, I should get a lower reading compared to one close to the surface, right? $\endgroup$ Nov 11, 2020 at 21:00

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