Eli5: how do thermal cameras capture heat without any physical contact
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Heat is infrared radiation, another form of light. You sense heat from a fire (or any hot object) without physical contact, and thermal cameras do the same thing. It’s literally light waves in a frequency that your eyes can’t see, but thermal cameras have sensors that are sensitive to those wavelengths of light.
Thermal cameras work the same way as a regular camera does. Every object in the universe emits infra-red radiation. The frequency of the radiation it emits is based on it’s temperature, the camera’s sensor is tuned to capture IR instead of light.
Light and heat are both detectable energy.
Lots of energy bounces off of things or is radiated by things. Your eyes can detect that energy within a specific range and we call that range “light”. Thermal camera can detect that energy in a different range.
If you go out on a sunny day, close your eyes and face the Sun, you will feel warmth on your face than won’t be there if you turn in any other direction. That’s because the Sun emits a lot of infrared light which you can feel as warmth. Infrared cameras also capture this infrared light, but they are much more sensitive.
The same way your eyes capture visible light without any visible contact. All objects emit infrared radiation based on how much heat they have, and infrared radiation is just light that we can’t see with our eyes because it’s below visible light in the electromagnetic spectrum. Our eyes can’t see it, but we can make sensors that can.
Thermal radiation. Every object thats abow 0K emits light. Higher the temperature the higher frequency light it emits. If the object doesn’t emit light using other methods like a star or a laser based on its thermal radiation you know its temperature. For instance things at room temperature emit in the infrared range. But lets grab some iron and heat it up. As its reaches 1000°C or so it starts to visibly glow red. Red is the lowest visible frequency and as it gets hotter it produces a yellowish glow.
Just as a not to you, OP, I’ve seen more than one comment say something like “heat is infrared light.” When you read that, you should envision it the same way we say “time is money.” You can’t actually transfer 20 minutes off your lifespan to the butcher in exchange for a steak. Heat IS the average motion of the individual atoms of a substance. The onlynthi ga that don’t jiggle around just a little bit are things at absolute 0 with no thermal energy. But since nothing can reach absolute 0 then all things have thermal energy. And along with thermal energy comes light. Not necessarily visible light, but all matter above absolute 0 emits photons. The hotter something is, the higher energy the photons it emits are. Higher energy photons have a shorter wavelength. If you have something really really cold, it can emit radio waves. If it’s really really reeeeeally hot, it emits gamma rays.
Almost everything we deal with in our day do day life is just in the range of temperature that emits infrared light. You know that if it heats up above a few hundred degrees it will start emitting visible light. The sun is hot enough that it emits UV light. But since most things we deal with are infrared light, then an infrared camera can tell which things are hotter than others. In fact, a normal camera can too. If you point it at two blocks of metal and one is glowing red, that’s the hotter one. But since most things emit light that is outside the range a regular camera can detect, the information is limited. Actually, the information you can get from an infrared camera is also limited by the wavelengths of light it is designed to detect. Things that are too hot or too cold won’t show up on them either.
Edit: oh, and an infrared camera can’t tell the difference between something emitting IR light because it’s in the correct temperature zone or if it’s designed to emit light like the IR LED on the end of your TV remote. The same way your camera can’t tell the difference between something emitting red light like an LED or if it’s red hot. You usually can tell the difference with context, but the camera only sees red.
Thermal infrared cameras are interesting. A lot of people have replied and said they work just like regular cameras. That is correct only for a small number of infrared cameras. A regular visible light camera directly converts photons into elections which it then counts. It is really hard to make a material which does this for thermal radiation. Instead thermal camera makers have two other tricks up there sleeves. The Pyroelectric effect, and microbolomeeters.
Pyroelectricity is the ability of certain materials to generate a temporary voltage when they are heated or cooled. This combined with a shutter so you can rapidly cycle the sensor between heating and cooling can result in a thermal image.
Microbolomeeters are essentially arrays of tiny resistors held in a vacuum chamber. As the thermal light strikes them they very slightly change resistance which can be used to make an image. These cameras are the most prevalent in the market because they are cheaper to make.
I can answer follow-ups in more detail if you are interested.
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