A forward looking infrared (FLIR) system is a camera that takes pictures using the infrared portion of the electromagnetic spectrum. In Europe, these are typically called Thermal imagers. Often these contain sub-systems known as Thermal imaging common modules or TICM. FLIRs are often described as "infrared cameras". 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). Note that a FLIR's wavelength range differs from a night vision camera, which detects wavelengths up to around 1-1.5 micrometres (slightly higher than the human eye can detect).

There are two basic ranges of infrared. 8-12 micrometre cameras (or "far infra-red" or LWIR) can see engine exhaust, or human body heat a few miles away, but longer distance viewing becomes blurred because the infra-red light is absorbed, scattered and refracted by the air.

3-5 micrometre infrared ("MWIR") cameras can see almost as well, and are far less absorbed by air, but generally require a much more expensive sensor array, and lower-temperature cooling.

Many FLIR systems use digital image processing to improve the image quality. FLIR sensor arrays often have inconsistent responses from pixel to pixel. To fix this, the response of each pixel is measured at the factory, and a transform, mostly linear, maps the measured brightness.

FLIRs are often used in naval vessels, fixed-wing aircraft, helicopters, and armored fighting vehicles. In warfare, they have three large advantages. First, the imager itself is difficult for the enemy to detect. Second, they see heat, which is hard to camouflage. Thirdly, FLIR systems can see through smoke, fog, haze, and other atmospheric obscurants better than a visible light camera can.

Cite: Wikipedia