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