How does Thermal Imaging work?
In: Technology
All objects emit light. The colour (ie wavelength) depends on the temperature. Thing is, most objects emit a colour that human eyes can’t see- the shortest wavelengths we can see are Violet, and the longest are red (hence the rainbow.)
Thermal imaging works because they use cameras that can see colours humans can’t see, then a computer changes them to a colour we can see on the screen. This way hot objects show up red or white, cold objects show up black.
The analog value from the pixels detectors are converted to digital values, usually already in the chip. These digital values are going thought some calculation to correct for difference sensitivities in the pixels to get consistent values. So that a specific digital value always corresponds to the same temperature regardless of differences in pixels detectors. After this each value is mapped into palette that converts it to RGB that is displayed. The palette are usually selectable by the user. In addition to this there might also be some math converting the values to temperature if it is what you want. The detectors can be of different types. Very common today is bolometric detector elements. Each element are heated by the received radiation. And the change in resistance caused by this heating are measured. All this is made in real time.
( from somebody developing IR cameras at FLIR for 25 years )
Actual ELI5:
When things get hot, they let out tiny invisible waves,
These waves are captured by a special lens
The waves then hit a tiny special component, and the material gets hotter
When it heats up, its electrical resistance changes
Each tiny special component corresponds to a pixel on the display
The pixel just shows a unique colour for each resistance (/temperature)
Put many pixels together, and you have a colourful picture of resistance/temperature values in the form of colour
To summarise in a genuinely simple ELI5-way: all objects with any heat at all emit infrared waves. Thermal imaging simply uses sensors/cameras that “see” infrared instead of visible light, and converts it to light we can see.
Technically speaking, humans even emit visible light. It’s just such a ridiculously tiny amount that you can’t tell.
You build a camera that can see the color of light just slightly lower than red on the spectrum.
Then show the intensity of this light as a color scale. White for a lot of intensity, black for none.
Stuff that emits heat, also emits energy in the form of electromagnetic radiation IR (infra-red). When can’t see that form of radiation because our eyes have evolved to see in the most efficient part of the spectrum (visible) when it comes to the light (electromagnetic radiation) produced by the sun, most have evolved a long the same lines, though some can see in near IR and near UV (ultraviolet).
Thermal imaging cameras have the same basic principles as normal ones when it comes to capturing a image but instead of visible light it’s calibrated to capture IR electromagnetic radiation. Just like how each pixel captures different “colours” of visible light, thermal imaging cameras capture different levels (ie colours) of Infra red light in each pixel and use all those pixels to form a image with software that knows what each pixel infra-red level corresponds with the know level of heat that causes the level of infra-red to be emitted.
Human eyes and digital cameras have receptors that are sensitive to visible light. This is a particular array of colors in the enormous electromagnetic spectrum.
Hot things emit thermal energy, in a different part of that spectrum. Cold things emit less than hot things.
We can make digital cameras with receptors that are sensitive to only the thermal colors of light. Thermal light is invisible to human eyes, by the way, but other parts of our skin can detect it when we feel heat.
In images made by those thermal cameras, things emitting more heat look “bright” and colder things look “dark”.
Most heat is transmitted as infrared light. Infrared light isn’t visible to human eyes. It’s relatively easy with modern technology though to make a sensor that can detect it. Attach that sensor to a computer and tell the computer to display the different levels of heat as different colors and boom: thermal imaging.
Bonus science: the discovery of IR is one of my favorite science stories. If you were to guess when IR was discovered what year would you think? Whatever your guess I bet it was a lot later than the answer: 1800. William Herschel discovered it by accident with a thermometer.
He had set up a prism to refract sunlight onto a table. He put thermometers down in each of different colored bands on the table because he wanted to see if there was a difference in the colors of light. He also wanted a control temperature for the ambient room temperature so he had an additional thermometer on the table just past the red light on the end. When he checked his readings he discovered that there was indeed temperature differences between the various colors. That was interesting, but what was especially surprising was that his “control” thermometer had the highest temperature reading of all. That initially made no sense, but with some additional expiramentation he determined that there most be an additional band of non-visible light beyond the red that carried a lot of heat energy. He called it infrared for “below red”.
Imagine that. A guy discovered an invisible part of the electromagnetic spectrum by *accident* using a *thermometer.*
Electromagnetic radiation is a wave-like physical phenomenon that takes different names depending of its frequency, i.e. how fast the wave oscillates.
The radiation our eyes can detect we call it visible light. But radio waves, microwaves, x-rays, gamma rays, infrared and ultraviolet are all ‘flavours’ electromagnetic radiation of different ranges of frequency.
Every object emits spontaneously radiation, most intensely on a certain frequency that grows as the temperature grows.
That’s why heated metal starts to glow read: at a certain point our eyes can pick up its ‘thermal’ radiation.
Every room temperature object emits in a range of radiation called infrared that our eyes can’t see. It’s just ‘before’ the red color, that’s the first we can see.
If you build a detector, exactly like those of a regular camera, but tuned on the infrared instead of visible light, you can obtain a thermal picture.
Then you convert it on a scale of visible colors on screen to make our human mind understand.
Almost everything that is above -273.15°C emmits infra-red light (which had a frequency smaller than red light). IR light is out of the visible spectrum, meaning you cant see it. Hot things have more enrgy, which is why their atoms resomate more intensely and emmit more light (when things reack a certain point, they start resonating at frequencies of visible light, which is why *realy* hot things glow red). Thermal imaging takes advantage of this, and based on the amout of collected IR light, it generates an image accordingly.
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