how do laser pointed thermometer guns find a reading of the temperature?
In: Technology
What the lasers do: The lasers pointers are there so you know where you are measuring from and what distance, if you have two laser beams which cross 30cm (1foot) from the device it is really easy to get the user to always take a measurment from 30cm away from the object, becuse at this range the two merge into one. They use lasers because they emit only a single frequency of light so it wont interfere with the reading.
How the reading works: objects emit radiation in proportion to their temperature (to the power 4), hot objects (like the sun) emit more light at higher energies (more blue). cold objects like you emit most of their light in the, low energy, infra red (redder than red) wavelengths. All the thermometer is doing is measuring light from one particular wavelength in the infra red part of the spectrum. It then uses this number and a model called ‘black body radiation’ to estimate the temperature of the surface that emited the light. This model just says how much light will be emitted at each wavelength for a given temperature.
There are lots of problems with this which lead to limitations of how these thermometers can be used, firstly this is just light so if you point it at somthing that is reflecting the sun it’s going to say it’s really hot even if it isn’t, if you point it at somthign which is opaque to visible light but clear to IR light it wont see it at all/ it will say it’s very cold. Also the distance from the object matters a lot, which is why they put the lasers on. But lastly, most things are not perfect ‘black bodies’, this means that they don’t emit light in a neat predictable curve so you will get different measurments for different materials of the same temperature, so these devices are typically calibrated for a particular task eg, skin temperature, or they are just quite inaccurate. Sometimes these differences are small and don’t matter sometimes they matter a lot (imagine measuring the temperatre of an infra red LED!, it’s not hot but it is emitting loads of infra red light. The thermometer is blind to all the other light, so it will just think that the LED is emiting loads of light at every wavelength, because thats what a black body would do!
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