Infrared

Infrared is electromagnetic radiation with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those of red light, so IR is invisible to the human eye. IR is generally understood to include wavelengths from around to. IR is commonly divided between longer-wavelength thermal IR, emitted from terrestrial sources, and shorter-wavelength IR, or near IR, part of the solar spectrum. Longer IR wavelengths are sometimes included as part of the terahertz radiation band. Almost all black-body radiation from objects near room temperature is in the IR band. As a form of EMR, IR carries energy and momentum, exerts radiation pressure, and has properties corresponding to both those of a wave and of a particle, the photon.
It was long known that fires emit invisible heat; in 1681 the pioneering experimenter Edme Mariotte showed that glass, though transparent to sunlight, obstructed radiant heat. In 1800 the astronomer Sir William Herschel discovered that infrared radiation is a type of invisible radiation in the spectrum lower in energy than red light, by means of its effect on a thermometer. Slightly more than half of the energy from the Sun was eventually found, through Herschel's studies, to arrive on Earth in the form of infrared. The balance between absorbed and emitted infrared radiation has an important effect on Earth's climate.
Infrared radiation is emitted or absorbed by molecules when changing rotational-vibrational movements. It excites vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states for molecules of the proper symmetry. Infrared spectroscopy examines absorption and transmission of photons in the infrared range.
Infrared radiation is used in industrial, scientific, military, commercial, and medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected. Infrared astronomy uses sensor-equipped telescopes to penetrate dusty regions of space such as molecular clouds, to detect objects such as planets, and to view highly red-shifted objects from the early days of the universe. Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, to assist firefighting, and to detect the overheating of electrical components. Military and civilian applications include target acquisition, surveillance, night vision, homing, and tracking. Humans at normal body temperature radiate chiefly at wavelengths around 10 μm. Non-military uses include thermal efficiency analysis, environmental monitoring, industrial facility inspections, detection of grow-ops, remote temperature sensing, short-range wireless communication, spectroscopy, and weather forecasting.

Definition and relationship to the electromagnetic spectrum

There is no universally accepted definition of the range of infrared radiation. Typically, it is taken to extend from the nominal red edge of the visible spectrum at 780 nm to 1 mm. This range of wavelengths corresponds to a frequency range of approximately 430 THz down to 300 GHz. Beyond infrared is the microwave portion of the electromagnetic spectrum. Increasingly, terahertz radiation is counted as part of the microwave band, not infrared, moving the band edge of infrared to 0.1 mm.

Name	Wavelength	Frequency	Photon energy
Gamma ray	less than 10 pm	more than 30 EHz	more than 124 keV
X-ray	10 pm10 nm	30 EHz30 PHz	124 keV124 eV
Ultraviolet	10 nm400 nm	30 PHz750 THz	124 eV3.3 eV
Visible	400 nm700 nm	750 THz430 THz	3.3 eV1.7 eV
Infrared	700 nm1 mm	430 THz300 GHz	1.7 eV1.24 meV
Microwave	1 mm1 meter	300 GHz300 MHz	1.24 meV1.24 μeV
Radio	1 meter and more	300 MHz and below	1.24 μeV and below

Nature

, at an effective temperature of 5,780 K, is composed of near-thermal-spectrum radiation that is slightly more than half infrared. At zenith, sunlight provides an irradiance of just over 1 kW per square meter at sea level. Of this energy, 527W is infrared radiation, 445W is visible light, and 32W is ultraviolet radiation. Nearly all the infrared radiation in sunlight is near infrared, shorter than 4μm.
On the surface of Earth, at far lower temperatures than the surface of the Sun, some thermal radiation consists of infrared in the mid-infrared region, much longer than in sunlight. Black-body, or thermal, radiation is continuous: it radiates at all wavelengths. Of these natural thermal radiation processes, only lightning and natural fires are hot enough to produce much visible energy, and fires produce far more infrared than visible-light energy.

Regions

In general, objects emit infrared radiation across a spectrum of wavelengths, but sometimes only a limited region of the spectrum is of interest because sensors usually collect radiation only within a specific bandwidth. Thermal infrared radiation also has a maximum emission wavelength, which is inversely proportional to the absolute temperature of object, in accordance with Wien's displacement law. The infrared band is often subdivided into smaller sections, although how the IR spectrum is thereby divided varies between different areas in which IR is employed.

Visible limit

Infrared radiation is generally considered to begin with wavelengths longer than visible by the human eye. There is no hard wavelength limit to what is visible, as the eye's sensitivity decreases rapidly but smoothly, for wavelengths exceeding about 700 nm. Therefore wavelengths just longer than that can be seen if they are sufficiently bright, though they may still be classified as infrared according to usual definitions. Light from a near-IR laser may thus appear dim red and can present a hazard since it may actually carry a large amount of energy. Even IR at wavelengths up to 1,050 nm from pulsed lasers can be seen by humans under certain conditions.

Commonly used subdivision scheme

A commonly used subdivision scheme is:

Division name	Abbreviation	Wavelength	Frequency	Photon energy	Temperature	Characteristics
Near infrared	NIR, IR-A DIN	0.75–1.4 μm	214–400 THz	886–1,653 meV		Goes up to the wavelength of the first water absorption band, and commonly used in fiber optic telecommunication because of low attenuation losses in the SiO₂ glass medium. Image intensifiers are sensitive to this area of the spectrum; examples include night vision devices such as night vision goggles. Near-infrared spectroscopy is another common application.
Short-wavelength infrared	SWIR, IR-B DIN	1.4–3 μm	100–214 THz	413–886 meV		Water absorption increases significantly at 1,450 nm. The 1,530 to 1,560 nm range is the dominant spectral region for long-distance telecommunications.
Mid-wavelength infrared	MWIR, IR-C DIN; MidIR. Also called intermediate infrared	3–8 μm	37–100 THz	155–413 meV		In guided missile technology the 3–5 μm portion of this band is the atmospheric window in which the seekers of passive IR 'heat seeking' missiles are designed to work, homing on to the infrared signature of the target aircraft, typically the jet engine exhaust plume. This region is also known as thermal infrared.
Long-wavelength infrared	LWIR, IR-C DIN	8–15 μm	20–37 THz	83–155 meV		The "thermal imaging" region, in which sensors can obtain a completely passive image of objects only slightly higher in temperature than room temperature – for example, the human body – based on thermal emissions only and requiring no illumination such as the sun or moon or an infrared illuminator. This region is also called the "thermal infrared".
Far infrared	FIR	15–1,000 μm	0.3–20 THz	1.2–83 meV

The band comprising NIR and SWIR together is sometimes called reflected infrared, whereas the band comprising MWIR and LWIR is sometimes referred to as thermal infrared.

CIE division scheme

The International Commission on Illumination recommended the division of infrared radiation into the following three bands:

Abbreviation	Wavelength	Frequency
IR-A
IR-B
IR-C

ISO 20473 scheme

20473 specifies the following scheme:

Designation	Abbreviation	Wavelength
Near infrared	NIR	0.78–3 μm
Mid infrared	MIR	3–50 μm
Far infrared	FIR	50–1,000 μm

Astronomy division scheme

Astronomers typically divide the infrared spectrum as follows:

Designation	Abbreviation	Wavelength
Near infrared	NIR
Mid infrared	MIR
Far infrared	FIR	above

These divisions are not precise and can vary depending on the publication. The three regions are used for observation of different temperature ranges, and hence different environments in space.
The most common photometric system used in astronomy allocates capital letters to different spectral regions according to filters used; I, J, H, and K cover the near-infrared wavelengths; L, M, N, and Q refer to the mid-infrared region. These letters are commonly understood in reference to atmospheric windows and appear, for instance, in the titles of many papers.