Infra-red devices employ the wavelength just beyond the red end of the spectrum, and provide their users with the ability to acquire targets in darkness or low-light conditions. Infra-red is used for a variety of military purposes, particularly for sighting systems and reconnaissance purposes. The first infra-red sights employed active detector systems, emitting beams invisible to the human eye which bathed the target in infra-red light. Reflections from the target were focused on a converter unit containing a photocathode, which provided the viewer with a reconstructed image. The first infra-red sights were bulky, and image quality was lacking. Nonetheless, such sights provided advantages to the user, in that whereas the enemy had been cloaked by poor light or darkness, he was now visible, even if not with the utmost clarity. Technological advance enabled the size of such sights to be reduced, although it did not overcome the fundamental difficulty that infra-red sights of this nature are emitters, and can thus be detected by an enemy with suitable equipment.
This drawback gave rise to passive systems, which have evolved into the thermal imager. As the name suggests, the thermal imager creates images based on heat (the most common infra-red source on the battlefield), enabling clear definition of objects against the cooler background. Infra-red photography and imaging systems, used both in aircraft and by ground systems, enable the provision of ‘round-the-clock’ reconnaissance information, while modern targeting systems, such as the British Thermal Observation and Gunnery System (TOGS) for tanks and the Thermal Imaging and Laser Designation (TIALD) pod for aircraft, employ similar technology for finding targets in poor light conditions or darkness. In the latter case, the combination of a laser designator and thermal imager allows the aircrew to acquire their target in low-light conditions, and to then designate it for attack by laser-guided bombs.
Further employment of the infra-red spectrum may be found in aerial combat with the infra-red homing missile. These missiles, such as the AIM-9 Sidewinder, use a seeker to detect heat from enemy aircraft. The first such missiles had to be fired from the rear at the enemy aircraft's engine, but refinements have increased the sensitivity of the seeker, to permit all-aspect engagement even of the friction over a wing. Although technological advances have increased the capabilities of sighting systems, air-to-air missiles, and reconnaissance capabilities immensely, the fundamental technology remains the same as it was in the 1940s when infra-red first became a military tool.
— David Jordan
