How much energy is radiated, and at which wavelengths, depends on the emissivity of the surface and on its temperature. Thermal infrared sensors record differences in the received infrared radiation from various objects. Since these differences are often considerable, an infrared image can exhibit a wide range of contrasts.
Infrared energy is recorded in two ways.
The first is with a radiometer, a device that records the radiation received and compares it to a fixed or known standard. The other recording device is an infrared scanner that uses a system of mirrors that rotate, or oscillate, and focus the incoming radiation on a detector. The infrared energy striking the detector creates an electrical charge that can be amplified and recorded on tape or film. With this kind of system it is possible to get a picture of the thermal environment that we cannot experience with our normal human sensors. The ability to record variations in infrared radiation has tremendous application in extending our observation of many types of phenomena in which minor temperature variations may be extremely significant in understanding our environment.