Forward looking infrared
Forward looking infrared (FLIR) cameras, typically used on military and civilian aircraft, use an imaging technology that senses infrared radiation.
The sensors installed in forward-looking infrared cameras—as well as those of other thermal imaging cameras—use detection of infrared radiation, typically emitted from a heat source, to create a "picture" assembled for video output. They can be used to help pilots and drivers steer their vehicles at night and in fog, or to detect warm objects against a cooler background. The wavelength of infrared that thermal imaging cameras detect differs significantly from that of night vision, which operates in the visible light and near-infrared ranges (0.4 to 1.0 μm).
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 μm and can see heat sources, such as hot engine parts or human body heat, a few miles away, but longer-distance viewing is made more difficult because the infrared light is absorbed, scattered, and refracted by the air and water vapor. Some long-wave cameras require their detector to be cryogenically cooled, typically for several minutes before use, although moderately sensitive infrared cameras are produced that do not require cryogenic cooling. Many thermal imagers including some forward looking infrared cameras are uncooled such as some LWIR Enhanced Vision Systems (EVS).
Cameras which operate in the 3 to 5 μm range are called medium-wave infrared (MWIR) and can see almost as well, as those frequencies suffer less from water-vapor absorption, but generally require a more expensive sensor array and cryogenic cooling.
Many camera systems use digital image processing to improve the image quality. infrared imaging sensor arrays often have wildly inconsistent sensitivities from pixel to pixel, due to limitations in the manufacturing process. To remedy this, the response of each pixel is measured at the factory, and a transform, most often linear, maps the measured input signal to an output level.
Some companies offer advanced "fusion" technologies that blend a visible-spectrum image with an infrared-spectrum image to produce better results than a single-spectrum image alone. 
Thermal imaging cameras, such as the Raytheon AN/AAQ-26, are often used in a variety of applications, including naval vessels, fixed-wing aircraft, helicopters, and armored fighting vehicles. In warfare, they have three distinct advantages over other imaging technologies. First, the imager itself is nearly impossible for the enemy to detect, as it detects energy emitted from the target rather than sending out energy that is reflected from the target. Second, it sees heat, which is difficult to camouflage. Third, these camera systems can see through smoke, fog, haze, and other atmospheric obscurants better than a visible light camera can.
Origin of the term 
The term "forward looking" is used to distinguish fixed forward looking thermal imaging systems from sideways tracking infrared systems, also known as "push broom" imagers, and other thermal imaging systems such as gimbal-mounted imaging systems, handheld imaging systems and the like. Pushbroom systems typically have been used on aircraft and satellites. They normally involve a one-dimensional (1D) array of pixels which uses the motion of the aircraft or satellite to move the view of the 1D array across the ground to build up a 2D image over time. Such systems cannot be used for real time imaging, and must look perpendicular to the direction of travel.
History of the forward looking infrared systems 
In 1956 Texas Instruments began research on infrared technology that led to several line scanner contracts and with the addition of a second scan mirror the invention of the first forward looking infrared camera in 1963 with production beginning in 1966. In 1972 TI invented the Common Module concept, greatly reducing cost and allowing reuse of common components.
- Surveillance of living things (e.g. people or other animals)
- Drainage basin temperature monitoring and monitoring wild game habitats
- Detection of energy loss or insulation defects in buildings in order to reduce HVAC energy consumption
- Target acquisition and tracking by military aircraft
- Piloting of aircraft in low visibility (IFR) conditions
- Warning drivers about sudden road obstructions caused by animals (e.g. deer)
- Locating living beings (through smoke) and pinpointing sources of ignition during firefighting operations
- Search and rescue operations for missing persons especially in wooded areas or water.
- Detecting leaks of natural gas and other gasses.
- Monitoring active volcanoes.
- Detecting heat in faulty electrical joints.
- Searching for drug-labs and indoor cannabis producers at night.
The cost of thermal imaging equipment in general has fallen dramatically. Older camera designs used rotating mirrors to scan the image to a small sensor. More modern cameras no longer use this method; the simplification helps reduce cost. Uncooled technology available in many EVS products have reduced the costs to fractions of the price of older cooled technology, with similar performance. EVS is rapidly becoming mainstream on many fixed wing and rotary wing operators from Cirrus and Cessna aircraft to large business jets.
In 2001, the United States Supreme Court decided that performing surveillance of private property (ostensibly to detect high emission grow lights used in clandestine cannabis farming) using thermal imaging cameras without a search warrant by law enforcement violates the Fourth Amendment's protection from unreasonable searches and seizures. Kyllo v. United States, 533 U.S. 27, 121 S.Ct. 2038, 150 L.Ed.2d 94 (2001).
In R v. Tessling Canada's Supreme court determined that the use of thermal imagers in surveillance by police was permitted without requiring a search warrant. The Court determined that the general nature of the data gathered by thermal imagers did not reveal personal information of the occupants and therefore was not in violation of Tessling's Section 8 rights afforded under the Charter of Rights and Freedoms (1982).
See also 
- "Night Vision & Electronic Sensors Directorate – Fort Belvoir, VA". Nvl.army.mil. Retrieved 2008-12-11.
- "Three-Band Video Fusion Demo : Sarnoff Corporation". Sarnoff.com. Retrieved 2011-11-24.
- "Multiscale thermal refugia and stream habitat associations".
- "Donate to the Legal Information Institute". Law.cornell.edu. Retrieved 2008-12-11.