Infrared is an energy similar to visible light, but with a longer wavelength.
Infrared energy is invisible to the human eye, however, while visible light energy
is emitted by objects only at a very high temperature,
infrared energy is emitted by all objects at ordinary temperatures.
Since thermal imagers sense infrared energy which varies with the temperature of objects in a scene,
the image generated provides a thermal signature of the scene. This image can be displayed on a standard video monitor.
Infrared energy from objects in a scene are focused by optics onto an infrared detector.
The infrared information is then passed to sensor electronics for image processing.
The signal processing circuitry translates the infrared detector data into an image that can be viewed on a standard video monitor.