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I once worked as a kitchen porter over a winter season.
We had fun with thermal temperature guns (like these) which I learned can be used for measuring the temperature of something a reasonable distance away (aside from the obvious use of laser tag), which to my mind is pretty impressive.

How do they work?

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up vote 2 down vote accepted

They basically measure the intensity of the infrared blackbody radiation in some wavelength region and calculate the temperature needed to give that intensity according to Planck's law.

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Or, more precisely (nit-picky), Wien's displacement law – Wojciech Morawiec Apr 5 '13 at 11:10
@WojciechMorawiec No, I think that is incorrect to say. To get the maximum of the Planck curve, which would be needed in order to apply Wien's displacement law, the gun would need to measure some sort of spectrum. That would require a more sophisticated device. It's much simpler to just measure the total intensity in one fixed wavelength region. – jkej Apr 5 '13 at 11:19
You are right, I guess I was thinking too much in the direction of the color index of stars. There seem to be quite a few methods to do pyrometric measurements, but diodes are not my speciality :) – Wojciech Morawiec Apr 5 '13 at 12:46
A minor correction - thermometers also have to be calibrated for the type of surface being measured, as Planck's law is inexact for surfaces that are not perfectly 'black'. I imagine that most are calibrated for water. (Materials that reflect some of the incident radiation necessarily emit less in that region than a blackbody would). – Dmytry Apr 5 '13 at 14:04

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