What is infrared light?
What is light?
When we talk about light, we are mainly talking about the light that we can see with our eyes. Here we as humans can distinguish colors depending on the wavelength of this light. In order to see something, we need sources of light. 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).
The visual light is only a small part of the electromagnetic spectrum (EMS). There are parts of the light spectrum that we cannot see with our eyes. For this we need sensors that are sensitive to this light.
For thermography, we use the infrared light between 0.7µm – 14µm. (The entire IR spectrum has wavelengths up to 1mm)
More information about the EMS can be found on the NASA science website.
Now let’s focus 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 sufficient transparency of the atmosphere is needed 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 relation between the infrared light and the amount of heat an object emits. All objects with more than 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
Creating a Planck curve from 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 with respect to a temperature (in Kelvin). As the temperature increases 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 that an object emits is no longer visible to our eyes, and we need a 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 include microbolometers, which are primarily sensitive to LWIR. These are sensors that are relatively inexpensive compared to sensors that require cooling.
Some properties of infrared light
Looking through glass with a thermal camera
Although it is possible for us to see through glass, it is not possible for most thermal imaging cameras. Glass becomes opaque at 5µm. (Depending on type and temperature of the glass. There are many types of glass)
Thus, it is not possible to look into people’s homes from the street with microbolometers. But there are applications to look through glass with thermographic cameras. It is necessary to understand what the spectral properties of the glass are in order to map this out.
Source: “7 things to know when selecting an IR camera for Research and devolpment – A guide to investing in Infrared- atmospheric transmission for infrared energy, FLIR booklet, p7
A perfect night vision camera
A thermal imaging camera receives light which it converts into an image. The thermal imager is therefore the perfect night vision camera, 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, …) But a thermal imaging camera can perfectly form an image without these sources.
Hence, cameras are also increasingly used for perimeter surveillance and other surveillance applications. There, they often do not have the intention of measuring temperatures. Often sensors other than temperature measurements are used for surveillance cameras.
Metals are a mirror for the IR spectrum
If you are going to use a thermal imaging camera in the field, it is important that you have sufficient 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 means they reflect a greater proportion of light than they emit their own heat via infrared light. This can give highly distorted images and misinterpretations in the field.
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