How / why do infrared thermometers work? How can reading the intensity of infrared radiation coming from an object let you calculate its temperature?   Could any wavelength be used to measure temperature - why infrared, and not, for instance, green, or low frequency radio waves?  And would an infrared thermometer give an accurate temperature of, say, an infrared LED?
 A: *

*Every object with nonzero temperature (so, every object) radiates EM radiation

*The spectrum of a black body (that is, a body that absorbs all light at all wavelengths) is given by Planck's law.

*Generally, nothing is completely black (except for empty space, which is why cosmic microwave background has a very nice black body radiation spectrum). In general, emissivity varies with wavelength: everything that reflects some light (has color in any part of the spectrum) doesn't follow Planck's law anymore, it multiplies it by some factor for each wavelength.


Conclusion: you can measure the temperature using the Planck's law by measuring emitted light at some wavelength (or many wavelengths for an accurate reading), only if the object is black at that wavelength. It's handy to do this in that portion of the infrared spectrum, where most materials have no special behaviour. In green wavelengths, it would only work for black, red and blue objects (but not for green/yellow/cyan/anything that reflects green). In radio wavelengths, the signals would be way too weak to detect with such a small sensor, and also, due to large wavelength, there's no way to target the measurement to a small object (you'd basically get an average of half the sky due to diffraction), and if an object itself is smaller than the wavelength, it's emission footprint is small (it's basically transparent for that wavelength, so the assumptions required for the Planck's law to work don't hold.
