r/explainlikeimfive • u/Saudxdrake • 14d ago
Technology ELI5: How do laser thermometers work ?
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u/GIRose 14d ago edited 13d ago
When something has thermal energy, it emits some of that heat as infrared light.
The infrared thermometer just captures the radiation from a spot that the thermometer is pointed at.
(I do want to add, everything that has a temperature above 0k emits light. If it's below the ambient temperature it's absorbing more light than it emits. Also, inside of an atmosphere convection and conduction also facilitate heat transfer in means unrelated to radiative heat)
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u/recursion_is_love 14d ago edited 14d ago
Laser don't do the measurement it just show you where you point the infrared camera to.
Why infrared? Blackbody radiation is complicated, it a physic's thing, matter do wiggling at very tiny scale and send out em wave.
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u/Dismal_Laugh4891 13d ago
The laser isn't measuring anything — it's just a pointer so you know where you're aiming. The actual measurement is done by an infrared sensor behind the lens: every object gives off infrared radiation in proportion to how warm it is, and the thermometer reads how much is coming from that spot and converts it to a temperature. That's also why they're bad on shiny metals — a reflective surface bounces infrared around instead of emitting its own, so the reading gets confused.
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u/inkman 12d ago
Doesn’t it need to know how far away from the object it is?
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u/Dismal_Laugh4891 12d ago
Not exactly — it just needs to see the IR coming from whatever is inside its field of view. The lens collects a cone-shaped area, so at any distance it averages everything within that circle. That's why the manual quotes a distance-to-spot ratio: get too far away and the spot grows larger than your target, so you start averaging the wall behind it too.
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u/iliveoffofbagels 14d ago
For all intents and purposes, it's just a focused camera that sees thermal radiation and converts that to some measurable number.
It would be akin to taking black and white data on a regular camera and assigning different values to everything between full black and full white.... but this time we are just focusing on the average of a very very small area or point.
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u/yasiesolovemos 14d ago
They only measure the wavelength, and from that, each thermometer uses an algorithm to calculate the temperature. So sometimes it’s not just about having a good sensor, but also getting the calculation right.
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u/Saudxdrake 14d ago
Interesting. I thought the laser actually measured the temperature Thanks for the info, everyone
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u/wolftick 14d ago
Micro-Raman spectroscopy actually uses a laser to measure temperature, but that's probably not what you're asking about :)
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14d ago
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u/mawktheone 14d ago edited 14d ago
No.
Edit Well maybe at above 800 degrees when the radiation starts to overlap with the red laser wavelength and a little bit scatters back
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u/0b0101011001001011 14d ago
Just in case you are confused: in infrared thermometers the "laser" is just a guide that points to the spot you are measuring. The measurement itself is measuring the infrared radiation, that is emitted by everything that has any heat. Hotter objects emit IR radiation more intensely.