Infrared (IR) radiation is electromagnetic radiation whose wavelength is longer than that of visible light, but shorter than that of terahertz radiation and microwaves. The name means "below red" (from the Latin infra, "below"), red being the color of visible light with the longest wavelength. Infrared radiation has wavelengths between about 750 nm and 1 mm, spanning three orders of magnitude. Humans at normal body temperature can radiate at a wavelength of 10 micrometres.

Overview

Infrared imaging is used extensively for both military and civilian purposes. Military applications include target acquisition, surveillance, night vision, homing and tracking. Non-military uses include thermal efficiency analysis, remote temperature sensing, short-ranged wireless communication, spectroscopy, and weather forecasting. Infrared astronomy uses sensor-equipped telescopes to penetrate dusty regions of space, such as molecular clouds; detect cool objects such as planets, and to view highly red-shifted objects from the early days of the universe.

At the atomic level, infrared energy elicits vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states. Infrared spectroscopy examines absorption and transmission of photons in the infrared energy range, based on their frequency and intensity.

Origins of the term

The name means below red (from the Latin infra, "below"), red being the color of the longest wavelengths of visible light. IR light has a longer wavelength than that of red light. A longer wavelength means it has a lower frequency than red, hence below.

Different regions in the infrared

Objects generally emit infrared radiation across a spectrum of wavelengths, but only a specific region of the spectrum is of interest because sensors are usually designed only to collect radiation within a specific bandwidth. As a result, the infrared band is often subdivided into smaller sections.

The International Commission on Illumination (CIE) recommended the division of optical radiation into the following three bands:

• IR-A: 700 nm–1400 nm
• IR-B: 1400 nm–3000 nm
• IR-C: 3000 nm–1 mm

A commonly used sub-division scheme is:Fact: date=April 2007

• Near-infrared (NIR, IR-A DIN): 0.75-1.4 µm in wavelength, defined by the water absorption, and commonly used in fiber optic telecommunication because of low attenuation losses in the SiO₂ glass (silica) medium. Image intensifiers are sensitive to this area of the spectrum. Examples include night vision devices such as night vision goggles.