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.
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.
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.