How does a infrared thermometer also measure humidity?

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and how accurate are these?

Edit: an infrared*

In: Technology

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Every heat source radiates electromagnetic waves with a wavelength respective to their temperature. The hotter the source, the shorter the wavelength. For the air around us it appears that the EM-radiation is in the infra-red range of the spectrum (wavelength longer than visible red light). Thus we can build a sensor that can see and measure the infra-red light and calculate the respective temperature from that measurement.

Now, when it comes to humidity things become tricky. Every (except dark) matter reflects light over the entire spectrum with very specific wavelengths where it doesn’t. Every element and every molecole has it’s very own EM reflection spectrum. Humidity is nothing but gaseous water dissolved in air. Water has a narrow notch in its reflective spectrum at a wavelength right in the infra-red spectrum. If we had a lamp that only emitts light of said wavelength, we’d expect strong reflection of that light from dry air and weak reflection from wet air e.g. steam.

The good thing is, we do know exactly how much humidity would absorb how much light of said wavelength. The “bad” thing is, to build lamps that emit light of a certain spectrum is a science under construction itself. (It’s actually a great thing which led us to build transistors smaller than 100 atoms.)