Forward looking infrared (FLIR) is an imaging technology that senses infrared radiation.
Since FLIRs use detection of thermal energy to create the "picture" assembled for the video output, they can be used to help pilots and drivers steer their vehicles at night, and in fog, or detect warm objects against a cold background when it is completely dark (such as a cloudy, moonless night). The wavelength of infrared that FLIRs detects differs significantly from that of Night vision, which operates in the visible light and near infrared ranges (0.4 to 1.0 micrometres).
There are two basic ranges of infrared; long-wave infrared and medium-wave infrared. The long-wave infrared (LWIR) cameras, sometimes called "far infrared", operate at 8 to 12 micrometre and can see heat such as hot engine parts or human body heat a few miles away, but longer distance viewing is made more difficult because the infra-red light is absorbed, scattered and refracted by the air and water vapor. Some long-wave FLIRs require their detector to be cryogenically cooled, although moderately sensitive FLIRs are produced that do not require cryogenic cooling.