In physics, infrared (IR) light refers to a non-visible portion of the electromagnetic spectrum ranging from wavelengths of 750 nm to as long as 0.1 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:
|0.7 - 1.4 microns
|lead sulfide, photomultiplier tube, silicon photodiode
|1.4 - 3.0 microns
|Indium gallium arsenide, lead selenide
|3.0 - 5.0 microns
|zinc selenide, mercury cadmium telluride
|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.
Thermal detection and imaging, thermal radiation
Infrared radiation is sometimes referred to as thermal radiation or even equated with "heat", but such associations are somewhat misleading. All solid and liquid objects emit a broad spectrum of electromagnetic radiation, with the peak of the spectrum dependent mainly on the object's temperature but also on the surface properties of the object.
For objects at temperatures commonly occurring on Earth, this thermal emission peak is indeed in the infrared at a wavelength of about 10 microns. Hence the use of infrared detectors in thermal imaging cameras. Moreover, radiative heat transfer therefore takes places primarily in the infrared for everyday objects on Earth.
At other temperatures, however, the emission peak can lie outside of the infrared band, so it is a misconception that only infrared radiation is responsible for radiative heat transfer. Hotter objects have their emission peak at shorter wavelengths so for example the Sun, with a surface temperature of about 5800 kelvins (5500 degrees C, 9900 degrees F), has an emission peak at visible light wavelengths. Stars that are hotter still can peak in the ultraviolet. The cosmic microwave background, at 3 kelvins, has an emission peak in the microwave range, at about 1 mm.