Editing Infrared homing (section)

== Seeker types ==
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The three main materials used in the infrared sensor are [[lead(II) sulfide]] (PbS), [[indium antimonide]] (InSb) and [[mercury cadmium telluride]] (HgCdTe). Older sensors tend to use PbS, newer sensors tend to use InSb or HgCdTe. All perform better when cooled, becoming more sensitive and able to detect cooler objects.

[[File:Nag missile closeup.JPG|thumb|[[Nag (ATGM)|Nag ATGM]] with [[imaging infrared]] seeker (closeup).]]

Early infrared seekers were most effective in detecting infrared radiation with shorter wavelengths, such as the 4.2 micrometre emissions of the carbon dioxide efflux of a [[jet engine]]. This made them useful primarily in tail-chase scenarios, where the exhaust was visible and the missile's approach was carrying it toward the aircraft as well. In combat these proved extremely ineffective as pilots attempted to make shots as soon as the seeker saw the target, launching at angles where the target's engines were quickly obscured or flew out of the missile's field of view. Such seekers, which are most sensitive to the 3 to 5 micrometre range, are now called ''single-color'' seekers. This led to new seekers sensitive to both the exhaust as well as the longer 8 to 13 micrometer [[wavelength]] range, which is less absorbed by the atmosphere and thus allows dimmer sources like the fuselage itself to be detected. Such designs are known as "all-aspect" missiles. Modern seekers combine several detectors and are called ''two-color'' systems.

All-aspect seekers also tend to require cooling to give them the high degree of sensitivity required to lock onto the lower-level signals coming from the front and sides of an aircraft. Background heat from inside the sensor, or the aerodynamically heated sensor window, can overpower the weak signal entering the sensor from the target. ([[Charge-coupled device|CCDs]] in cameras have similar problems; they have much more "noise" at higher temperatures.) Modern all-aspect missiles like the [[AIM-9M Sidewinder]] and Stinger use [[Joule-Thomson effect|compressed gas]] like [[argon]] to cool their sensors in order to lock onto the target at longer ranges and all aspects. (Some such as the AIM-9J and early-model [[Molniya R-60|R-60]] used a [[Thermoelectric effect|peltier]] [[thermoelectric cooler]]).