Infrared homing

Infrared homing is a passive weapon guidance system which uses the infrared (IR) light emission from a target to track and follow it seamlessly. Missiles which use infrared seeking are often referred to as "heatseekers" since infrared is radiated strongly by hot bodies. Many objects such as people, vehicle engines and aircraft generate and emit heat and so are especially visible in the infrared wavelengths of light compared to objects in the background.

Infrared seekers are passive devices, which, unlike radar, provide no indication that they are tracking a target. That makes them suitable for sneak attacks during visual encounters or over longer ranges when they are used with a forward looking infrared or similar cueing system. Heat-seekers are extremely effective: 90% of all United States air combat losses between 1984 and 2009 were caused by infrared-homing missiles. They are, however, subject to a number of simple countermeasures, most notably by dropping flares behind the target to provide false heat sources. That works only if the pilot is aware of the missile and deploys the countermeasures on time. The sophistication of modern seekers has rendered these countermeasures increasingly ineffective.

The first IR devices were experimented with during World War II. During the war, German engineers were working on heat-seeking missiles and proximity fuses but did not have time to complete development before the war ended. Truly practical designs did not become possible until the introduction of conical scanning and miniaturized vacuum tubes during the war. Anti-aircraft IR systems began in earnest in the late 1940s, but the electronics and the entire field of rocketry were so new that they required considerable development before the first examples entered service in the mid-1950s. The early examples had significant limitations and achieved very low success rates in combat during the 1960s. A new generation developed in the 1970s and the 1980s made great strides and significantly improved their lethality. The latest examples from the 1990s and on have the ability to attack targets out of their field of view (FOV) behind them and even to pick out vehicles on the ground.

IR seekers are also the basis for many semi-automatic command to line of sight (SACLOS) weapons. In this use, the seeker is mounted on a trainable platform on the launcher and the operator keeps it pointed in the general direction of the target manually, often using a small telescope. The seeker does not track the target, but the missile, often aided by flares to provide a clean signal. The same guidance signals are generated and sent to the missile via thin wires or radio signals, guiding the missile into the center of the operator's telescope. SACLOS systems of this sort have been used both for anti-tank missiles and surface-to-air missiles, as well as other roles.

The infrared sensor package on the tip or head of a heat-seeking missile is known as the seeker head. The NATO brevity code for an air-to-air infrared-guided missile launch is Fox Two.

The ability of certain substances to give off electrons when struck by infrared light had been discovered by the Indian polymath Jagadish Chandra Bose in 1901, who saw the effect in galena, known today as lead sulfide, PbS. There was little use and he allowed his 1904 patent to lapse. In 1917, Theodore Case, as part of his work on what became the Movietone sound system, discovered that a mix of thallium and sulfur was much more sensitive but was highly unstable electrically and proved to be of little use as a practical detector. It was used for some time by the US Navy as a secure communications system.

In 1930 the introduction of the Ag–O–Cs (silver–oxygen–cesium) photomultiplier provided the first practical solution to the detection of IR, combining it with a layer of galena as the photocathode. Amplifying the signal emitted by the galena, the photomultiplier produced a useful output that could be used for detection of hot objects at long ranges. This led to developments in a number of nations, notably Britain and Germany where it was seen as a potential solution to the problem of detecting night bombers.

In Britain, research stagnated, with even the main research team at Cavendish Labs expressing their desire to work on other projects, especially after it became clear that radar was going to be a better solution. Frederick Lindemann, Winston Churchill's favorite on the Tizard Committee, remained committed to IR and became increasingly obstructionist to the work of the Committee who were otherwise pressing for radar development. Eventually they dissolved the Committee and reformed, leaving Lindemann off the roster and filling his position with well known radio expert Edward Victor Appleton.