How do those infrared thermometers work? How does flashing a red light on someones head give you their body temperature?
In: Technology
The red light you see is only a part of what’s going on. The red light is used to tell you two things: the thermometer is on and it shows you what you’re pointing at so you know you’re reading the right temperature.
The actual temperature reading is done through different light that our eyes can’t see. Infrared light is invisible, but we can feel it as heat. The thermometer is measuring the infrared light being given off by a person/food being cooked/whatever, and translates that to a temperature. Higher energy infrared is associated with higher temperatures.
The red dot is just a laser targeting tool, allowing you to easily see what you are measuring the temperature of.
They work by having a sensor that detects infra-red light. Everything gives off light, it is just that unless they are really hot, that light is too low in energy for us to see – it is infra (that is, below) red light. You can feel this, however, because it still shines on your skin, and is able to warm it up.
The sensor detects the amount of a certain ‘colour’ of infra-red light that is being given off, and the hotter something is, the more light it gives off. From that, the computer inside the device can calculate and display a temperature.
So… basically everything you know is either wrong, incomplete, or basically oversimplified to the point of being basically a lie.
Temperature as most people know it isn’t a thing, not like a ball is a thing. You can’t even measure it directly like one can count balls, bricks, or trees. What people think of as temperature is a comparative measure of energy. If an object is “hotter” then it’s giving off a lot of energy and if it’s “colder” it’s absorbing energy.
All a themometer is, whether we’re talking a alcohol or mercury thermometer, is a calibrated tube where the expansion of the fluid as a function of internal energy (ie. temperature) is well known and marked off. IR thermometers aren’t much different, in a fashion, but to get there we need to cover a few things.
The energy you think of as movement is the same energy you perceive as temperature. The exact same.
Suppose every atom in an object has the same amount of energy, and they’re all headed in the same direction, it’s called movement. If the energy in each atom is directed in a different direction, but it’s insufficient to tear it apart, that’s temperature. Basically, the more the atoms in an object jiggle, the “hotter” it is.
Now, how does the IR thermometer work? Well…
It’s easier to visualize the jiggling action as if the atoms were bound together with springs. The more energy the atom has, the farther it will travel before it gets yanked back, and it will do so faster. Its position will move back and forth and that motion will be, more or less, repetitive… and it will cross the “same” place with a certain frequency.
Now, the entire time it’s moving back and forth, the atom is emitting a constant stream of photons and that stream will ripple outwards. Other atoms will catch some of those photons, absorb them, gain energy… and eventually shed other photons to lose it.
So… visible light is just photons. So are microwaves, etc. The only thing that makes light visible to us is the fact the rods/cones in your eyes are exceptionally good at catching photons of a specific frequency range and the structures then translate that into a local electrical impulse that gets relayed back into your brain for processing.
IR thermometers aren’t really any different, really. It has a detector that’s tuned for a specific frequency of photon emission that just so happens to be outside the frequency band we can see. So when you point the IR thermometer at an object, all it’s doing is basically checking how “bright” the object appears to the detector. That gets relayed into other circuits, just like how your eye relays to the brain.
The internal circuitry translates the detector’s “how bright” into the displayed “how warm” by essentially doing a complicated chart lookup.
The details are a fair bit more complicated than that and vary by model/manufacturer, but that’s mostly in a “Please God, can we avoid doing math today” level nutshell. Much more than that and explaining it without a chalkboard or dry erase markers gets annoying.
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