How Does Thermal Imaging Work?
Thermal imaging equipment sees heat rather than visible light. As humans we see light in the wavelengths typically around 390 – 700 nanometres. Infrared light radiates beyond the top of the visible spectrum 700 nanometres to 1 micrometre. As a comparison, ultraviolet (or black) light operates just below the visible spectrum at 10 – 400 nanometres.
Thermal imaging often involves additional image-enhancement to make the output image clearer and easier to read (often with additional information such as the temperature at each pixel point included with the image). While some sources may use the terms image-enhancement and thermal imaging almost interchangeably, they are different technologies.
Thermal imaging uses infrared light to create a temperature-map of the surrounding area, while image-enhancement is a computer-technique used to improve the appearance of a digital image. Both have a place and both can be used in night and day surveillance operations.
Everything has a temperature, and that temperature can be detected in the infrared end of the spectrum. Thermal imaging focuses the radiated infrared energy using a lens. Infrared detectors then scan the focussed infrared light and create a temperature pattern or map known as a thermogram. The thermogram is then digitised and sent to a signal-processing unit where, using image-enhancement techniques, the thermogram is reconstructed into an image we can see and interact with.
Infrared scanners detect temperatures from a range of -20C to 2,000C and can detect a change in temperature of as little as 0.2C. Most infrared scanners are uncooled, which means they operate at room temperature. This is useful as the scanners are also silent which makes them useful for night-detection work and wildlife filming.
It is possible, however, to have an infrared scanner that is cooled cryogenically. Cooled to 0C, these devices have a much higher resolution as they can see temperature differences of 0.1C from distances of up to 300 metres. They can even detect what a person is holding in near and total dark situations.
While clever, thermal imaging cameras cannot see through walls. The cameras will pick up the heat generated by the building, but not anything within it. They cannot even see through glass since they see the thermal image of the glass itself and not what is on the other side. Similarly a thermal imaging camera cannot see through clothing. It can be used to locate cooler objects hidden under clothing, like a gun, but will not see your skin, for example.
Clearly the most important use of thermal imaging is in the field of law enforcement. At night a thermal imaging camera mounted on a helicopter will pick out individuals hiding in bushes or under other coverings. In fact it can be used to see behind objects like boxes or rubbish bins if the person hiding is emitting enough heat energy (after they have been running, for example).
If police are tracking a stolen car, thermal imaging can even see the heat residue left by skidding tyres on all surfaces and will detect the heat of an engine under its bonnet to show it was recently running. Thermal imaging is also useful in finding hidden compartments behind walls or floor surfaces that have been disturbed. Moving the soil around to cover tracks, for example, alters the thermal profile of the soil and a camera can detect this. Compartments in walls will radiate at a different temperature to the wall and create cold or hot spots that are detected by thermal imaging cameras.
Moving away from law enforcement, thermal imaging is also important in environmental situations. Chemical wastes and oil radiate at a different temperature to the soil and water around them and can be used to track them back to their source. In homes, thermal imaging can be used to identify areas where heat is escaping, helping you to keep warm in winter time without spending a fortune on bills. Search and rescue also use thermal imaging to locate people who are lost under snow or building rubble.
Thermal imaging has a wide variety of uses, coming up with technologies to disguise the thermal output of an object is now the new area of research.