Infrared light

In physics, infrared (IR) light refers to a non-visible portion of the electromagnetic spectrum ranging from wavelengths of 750 nm to 1 mm. The name infrared comes from Latin infra- meaning below, i.e., infrared has a lower frequency than red in the spectrum.

Various disciplines further subdivide the IR, but there is no consensus on the divisions. They vary from discipline-to-discipline and even widely within a given discipline. The following table shows a typical set of divisions:

Name	Acronym	Range	Representative detectors
Near Infrared	NIR	0.7 - 1.4 microns	lead sulfide, photomultiplier tube, silicon photodiode
Short-Wave Infrared	SWIR	1.4 - 3.0 microns	Indium gallium arsenide, lead selenide
Mid-Wave Infrared	MWIR	3.0 - 5.0 microns	zinc selenide, mercury cadmium telluride
Long-Wave Infrared	LWIR	5.0 - 20.0 microns	doped silicon, mercury cadmium telluride

Most detectors neede to be cooled below ambient temperature.

[edit] Viewing devices

Some, but not all, night vision devices use infrared light. Low-light television may be visible only, or extend into the NIR.

Forward-looking infrared viewing systems work in the LWIR, and, recently, MWIR. Night vision devices often are sensitive into the NIR.

[edit] Infrared guidance

Originally, infrared missile guidance depended on the extremely hot signature of a jet or rocket exhaust. Increasingly advanced systems, however, detect the heat on parts of the target heated by atmospheric friction, or simply being warm against a cold sky background.

Anti-ballistic missile terminal guidance often is infrared, as the incoming warhead is extremely hot.

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