Like any other camera, a thermal or temperature alarm camera collects electromagnetic radiation, which is formed into an image. But while a conventional camera works in the range of visible light, that is with wavelengths between approximately 400 and 700 nanometers (0.4–0.7 μm), a thermal camera is designed to detect radiation with longer wavelengths. Thermal cameras work, most of the time, in either mid wavelength IR (MWIR) domain, which is approximately 3-5 µm, or in the long wavelength IR (LWIR) domain, which is approximately 8-14 µm.
The biggest difference between MWIR and LWIR compared to shorter wavelength radiation is that MWIR and LWIR are primarily emitted radiation and not reflected. Thermal cameras can detect emitted radiation. Thermal imaging works because all objects – organic or inorganic – emit a certain amount of infra-red radiation as a function of their temperature. Since the object itself emits the light, thermal cameras are not dependent on visible light and they can work in all lighting conditions, day or night.
Thermal cameras come with two different main types of sensors, high-end cooled sensors mainly used in military and scientific use cases, and the less expensive uncooled sensors. Axis thermal cameras are uncooled cameras with so called micro bolometer technology working in the LWIR domain.
The ability to emit absorbed energy is called emissivity (e). All materials have more or less emissivity, which ranges between 0 and 1. The human skin absorbs all incident radiation and has an e=close to 1
(~ 0,97), whereas a more reflective material has a lower e.
The thermal radiation of an object also depends on its temperature – the hotter it is, the more thermal radiation it emits. Humans cannot see this, but we can sense it, for example, when we approach a campfire or enter a sauna. The greater the temperature difference, the higher the contrast in the image, which makes the object more visible.
Thermal images are sometimes associated with bright, intense colors – which may seem a bit odd considering that the camera works outside the spectrum of visible light. The answer is that the colors are created digitally, in so-called pseudo-colors or palettes. Each color or shade of the palette represents a different temperature, usually white and red for higher temperatures, over green, blue and violet for colder ones. The reason is primarily practical since the human eye is better at distinguishing different shades of color than different shades of grey.
Thermal cameras do not only perform well in dark scenes – they are a great tool for detecting people and objects in 24/7 surveillance anywhere, even if the person is wearing camouflaged clothing. When it comes to detection, thermal cameras are superior to conventional cameras under a wide range of difficult weather conditions, such as snow and fog.
Keep in mind that thermal sensors are subject to export control regulations, which makes it good practice always to consult and comply with the regulations of the appropriate local export control authorities.