When we talk about light, we mainly talk about the light we can see with our eyes. Here we as humans can distinguish colors depending on the wavelength of this light. To see something, we need light sources. This can be the sun, but also starlight or lamp,... So you will see that for certain parts of the light spectrum no visual light is needed to see complete darkness (for our eyes).
Visual light is a small particle of the electromagnetic spectrum(EMS). There are parts of the light spectrum that we cannot see with our eyes. For that, we need sensors that are sensitive to this light.
For thermography, we use the infrared light between 0.7µm - 14µm. (The full IR spectrum has wavelengths up to 1mm)
More information about EMS can be found here on the website of NASA science.
Let us now concentrate more on the infrared spectrum of 780nm -1mm. Only a small part of this infrared spectrum is of interest for building thermal imaging cameras. This is because it requires sufficient transparency of the atmosphere to be able to image anything. For thermographic applications, we can divide the IR spectrum into 3 zones. SWIR - short-wave infrared (0.7µm-1.7µm), MWIR - Mid-wave infrared (3-5µm) and LWIR - Long-wave infrared (7.5-14µm).
There is a relationship between infrared light and the amount of heat emitted by an object. All objects above 0K (or-273.15°C) emit infrared light. The higher the temperature and energy, the more IR light an object emits.
Source: "Wikipedia, Planck's law, https://en.wikipedia.org/wiki/Planck%27s_law
Making your own Planck curve based on a temperature : https://www.spectralcalc.com/blackbody_calculator/blackbody.php
In the image above, you can see how the electromagnetic radiation of a blackbody behaves relative to a temperature (in Kelvin). As the temperature rises to 5000K, the peak of the radiation is visible in our visual spectrum. The surface temperature of the sun is around 5500K and is right in the middle of our visual spectrum. As that temperature drops to, say, 3000K, the peak of radiation moves more toward the infrared spectrum. Eventually, the amount of light emitted by an object is no longer visible to our eyes, and we need thermal imaging camera to image lower temperatures.
There are sensors that are more sensitive to SWIR (e.g., InGaAs), and certain sensors are more sensitive to MWIR (e.g., MCT and InSb detectors). But most thermal cameras on the market contain microbolometers, which are primarily sensitive to LWIR. These are sensors that are relatively inexpensive compared to sensors that require cooling.
Although it is possible for us to see through glass, it is not possible for most thermal imaging cameras. Glass becomes opaque in from 5µm. (depending on type and temperature of the glass. There are many types of glass)
For example, it is not possible to look into people's homes with microbolometers from the street. But there are applications for looking through glass with thermographic cameras. It is necessary to understand what the spectral properties of the glass are in order to map this out.
A thermal imaging camera receives light that it converts into an image. Therefore, the thermal imaging camera is the perfect night viewer because the camera does not need active visual light to form an image. A night vision camera needs light sources that it then amplifies (street light, light from stars, moonlight, light sources, etc.) But a thermal imaging camera can perfectly form an image without these sources.
Hence, cameras are also increasingly being used for perimeter surveillance and other monitoring applications. There, they often do not have the intention of measuring temperatures. Many times other sensors are also used for surveillance cameras than for temperature measurements.
If you are going to use a thermal imaging camera in the field, it is important to have adequate knowledge of radiation properties such as emission, reflection and transmission. Thermal imaging cameras image IR light. Metals are generally the mirrors of the IR spectrum. That is, they reflect a greater proportion of light than they emit their own heat via infrared light. This can produce highly distorted images and misinterpretations in the field.
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