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A false-color image of two people taken in long-wavelength infrared (body-temperature thermal) radiation

Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) 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 (the longest waves in the visible spectrum), so IR is invisible to the human eye. IR is generally (according to ISO, CIE) understood to include wavelengths from around 780 nm (380 THz) to 1 mm (300 GHz).[1][2] 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.[3] Longer IR wavelengths (30–100 μm) are sometimes included as part of the terahertz radiation band.[4] 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.[5]

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.[6][7] 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.[8] 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.[9]

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.[10] 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.[11] 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

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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 (3 THz).

Position in the electromagnetic spectrum[12]
Name Wavelength Frequency (Hz) Photon energy (eV)
Gamma ray less than 10 pm more than 30 EHz more than 124 keV
X-ray 10 pm – 10 nm 30 PHz – 30 EHz 124 keV – 124 eV
Ultraviolet 10 nm – 400 nm 750 THz – 30 PHz 124 eV – 3.3 eV
Visible 400 nm – 700 nm 430 THz – 750 THz 3.3 eV – 1.7 eV
Infrared 700 nm – 1 mm 300 GHz – 430 THz 1.7 eV – 1.24 meV
Microwave 1 mm – 1 meter 300 MHz – 300 GHz 1.24 meV – 1.24 μeV
Radio 1 meter and more 300 MHz and below 1.24 μeV and below

Nature

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Sunlight, at an effective temperature of 5,780 K (5,510 °C, 9,940 °F), 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, 527 W is infrared radiation, 445 W is visible light, and 32 W is ultraviolet radiation.[13] Nearly all the infrared radiation in sunlight is near infrared, shorter than 4 μm.

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.[14]

Regions

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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

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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.[15][16][17]

Commonly used subdivision scheme

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A commonly used subdivision scheme is:[18][19][20]

Division name Abbreviation Wavelength Frequency Photon energy Temperature[i] Characteristics
Near-infrared NIR, IR-A DIN 0.75–1.4 μm 214–400 THz 886–1,653 meV 3,864–2,070 K
(3,591–1,797 °C) 		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 (silica) 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 2,070–966 K
(1,797–693 °C) 		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 (see transmission windows).
Mid-wavelength infrared MWIR, IR-C DIN; MidIR.[22] Also called intermediate infrared (IIR) 3–8 μm 37–100 THz 155–413 meV 966–362 K
(693–89 °C) 		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 362–193 K
(89 – −80 °C) 		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 193–3 K
(−80.15 – −270.15 °C) 		(see also far-infrared laser and far-infrared)
A comparison of a thermal image (top) and an ordinary photograph (bottom). The plastic bag is mostly transparent to long-wavelength infrared, but the man's glasses are opaque.

NIR and SWIR together is sometimes called "reflected infrared", whereas MWIR and LWIR is sometimes referred to as "thermal infrared".

CIE division scheme

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The International Commission on Illumination (CIE) recommended the division of infrared radiation into the following three bands:[23][24]

Abbreviation Wavelength Frequency
IR-A 780–1400 nm 215–384 THz
IR-B 1400–3000 nm 100–215 THz
IR-C 3–1000 μm 0.3–100 THz

ISO 20473 scheme

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ISO 20473 specifies the following scheme:[25]

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

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Astronomers typically divide the infrared spectrum as follows:[26]

Designation Abbreviation Wavelength
Near-infrared NIR 0.7–2.5 μm
Mid-infrared MIR 3–25 μm
Far-infrared FIR above 25 μm

These divisions are not precise and can vary depending on the publication. The three regions are used for observation of different temperature ranges,[27] 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.

Sensor response division scheme

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Plot of atmospheric transmittance in part of the infrared region

A third scheme divides up the band based on the response of various detectors:[28]

  • Near-infrared: from 0.7 to 1.0 μm (from the approximate end of the response of the human eye to that of silicon).
  • Short-wave infrared: 1.0 to 3 μm (from the cut-off of silicon to that of the MWIR atmospheric window). InGaAs covers to about 1.8 μm; the less sensitive lead salts cover this region. Cryogenically cooled MCT detectors can cover the region of 1.0–2.5 μm.
  • Mid-wave infrared: 3 to 5 μm (defined by the atmospheric window and covered by indium antimonide, InSb and mercury cadmium telluride, HgCdTe, and partially by lead selenide, PbSe).
  • Long-wave infrared: 8 to 12, or 7 to 14 μm (this is the atmospheric window covered by HgCdTe and microbolometers).
  • Very-long wave infrared (VLWIR) (12 to about 30 μm, covered by doped silicon).

Near-infrared is the region closest in wavelength to the radiation detectable by the human eye. Mid- and far-infrared are progressively further from the visible spectrum. Other definitions follow different physical mechanisms (emission peaks, vs. bands, water absorption) and the newest follow technical reasons (the common silicon detectors are sensitive to about 1,050 nm, while InGaAs's sensitivity starts around 950 nm and ends between 1,700 and 2,600 nm, depending on the specific configuration). No international standards for these specifications are currently available.

The onset of infrared is defined (according to different standards) at various values typically between 700 nm and 800 nm, but the boundary between visible and infrared light is not precisely defined. The human eye is markedly less sensitive to light above 700 nm wavelength, so longer wavelengths make insignificant contributions to scenes illuminated by common light sources. Particularly intense near-IR light (e.g., from lasers, LEDs or bright daylight with the visible light filtered out) can be detected up to approximately 780 nm, and will be perceived as red light. Intense light sources providing wavelengths as long as 1,050 nm can be seen as a dull red glow, causing some difficulty in near-IR illumination of scenes in the dark (usually this practical problem is solved by indirect illumination). Leaves are particularly bright in the near IR, and if all visible light leaks from around an IR-filter are blocked, and the eye is given a moment to adjust to the extremely dim image coming through a visually opaque IR-passing photographic filter, it is possible to see the Wood effect, which consists of IR-glowing foliage.[29]

Telecommunication bands

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In optical communications, the part of the infrared spectrum that is used is divided into seven bands based on availability of light sources, transmitting/absorbing materials (fibers), and detectors:[30]

Band Descriptor Wavelength range
O band Original 1,260–1,360 nm
E band Extended 1,360–1,460 nm
S band Short wavelength 1,460–1,530 nm
C band Conventional 1,530–1,565 nm
L band Long wavelength 1,565–1,625 nm
U band Ultralong wavelength 1,625–1,675 nm

The C-band is the dominant band for long-distance telecommunications networks. The S and L bands are based on less well established technology, and are not as widely deployed.

Heat

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Main article: Thermal radiation
Materials with higher emissivity appear closer to their true temperature than materials that reflect more of their different-temperature surroundings. In this thermal image, the more reflective ceramic cylinder, reflecting the cooler surroundings, appears to be colder than its cubic container (made of more emissive silicon carbide), while in fact, they have the same temperature.

Infrared radiation is popularly known as "heat radiation",[31] but light and electromagnetic waves of any frequency will heat surfaces that absorb them. Infrared light from the Sun accounts for 49%[32] of the heating of Earth, with the rest being caused by visible light that is absorbed then re-radiated at longer wavelengths. Visible light or ultraviolet-emitting lasers can char paper and incandescently hot objects emit visible radiation. Objects at room temperature will emit radiation concentrated mostly in the 8 to 25 μm band, but this is not distinct from the emission of visible light by incandescent objects and ultraviolet by even hotter objects (see black body and Wien's displacement law).[33]

Heat is energy in transit that flows due to a temperature difference. Unlike heat transmitted by thermal conduction or thermal convection, thermal radiation can propagate through a vacuum. Thermal radiation is characterized by a particular spectrum of many wavelengths that are associated with emission from an object, due to the vibration of its molecules at a given temperature. Thermal radiation can be emitted from objects at any wavelength, and at very high temperatures such radiation is associated with spectra far above the infrared, extending into visible, ultraviolet, and even X-ray regions (e.g. the solar corona). Thus, the popular association of infrared radiation with thermal radiation is only a coincidence based on typical (comparatively low) temperatures often found near the surface of planet Earth.

The concept of emissivity is important in understanding the infrared emissions of objects. This is a property of a surface that describes how its thermal emissions deviate from the ideal of a black body. To further explain, two objects at the same physical temperature may not show the same infrared image if they have differing emissivity. For example, for any pre-set emissivity value, objects with higher emissivity will appear hotter, and those with a lower emissivity will appear cooler (assuming, as is often the case, that the surrounding environment is cooler than the objects being viewed). When an object has less than perfect emissivity, it obtains properties of reflectivity and/or transparency, and so the temperature of the surrounding environment is partially reflected by and/or transmitted through the object. If the object were in a hotter environment, then a lower emissivity object at the same temperature would likely appear to be hotter than a more emissive one. For that reason, incorrect selection of emissivity and not accounting for environmental temperatures will give inaccurate results when using infrared cameras and pyrometers.

Applications

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Night vision

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Main article: Night vision
Active-infrared night vision: the camera illuminates the scene at infrared wavelengths invisible to the human eye. Despite a dark back-lit scene, active-infrared night vision delivers identifying details, as seen on the display monitor.

Infrared is used in night vision equipment when there is insufficient visible light to see.[34] Night vision devices operate through a process involving the conversion of ambient light photons into electrons that are then amplified by a chemical and electrical process and then converted back into visible light.[34] Infrared light sources can be used to augment the available ambient light for conversion by night vision devices, increasing in-the-dark visibility without actually using a visible light source.[34][1]

The use of infrared light and night vision devices should not be confused with thermal imaging, which creates images based on differences in surface temperature by detecting infrared radiation (heat) that emanates from objects and their surrounding environment.[35][8]

Thermography

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Thermography helped to determine the temperature profile of the Space Shuttle thermal protection system during re-entry.
Main article: Thermography

Infrared radiation can be used to remotely determine the temperature of objects (if the emissivity is known). This is termed thermography, or in the case of very hot objects in the NIR or visible it is termed pyrometry. Thermography (thermal imaging) is mainly used in military and industrial applications but the technology is reaching the public market in the form of infrared cameras on cars due to greatly reduced production costs.

Thermographic cameras detect radiation in the infrared range of the electromagnetic spectrum (roughly 9,000–14,000 nm or 9–14 μm) and produce images of that radiation. Since infrared radiation is emitted by all objects based on their temperatures, according to the black-body radiation law, thermography makes it possible to "see" one's environment with or without visible illumination. The amount of radiation emitted by an object increases with temperature, therefore thermography allows one to see variations in temperature (hence the name).

Hyperspectral imaging

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Main article: Hyperspectral imaging
Hyperspectral thermal infrared emission measurement, an outdoor scan in winter conditions, ambient temperature −15 °C, image produced with a Specim LWIR hyperspectral imager. Relative radiance spectra from various targets in the image are shown with arrows. The infrared spectra of the different objects such as the watch clasp have clearly distinctive characteristics. The contrast level indicates the temperature of the object.[36]
Infrared light from the LED of a remote control as recorded by a digital camera

A hyperspectral image is a "picture" containing continuous spectrum through a wide spectral range at each pixel. Hyperspectral imaging is gaining importance in the field of applied spectroscopy particularly with NIR, SWIR, MWIR, and LWIR spectral regions. Typical applications include biological, mineralogical, defence, and industrial measurements.

Thermal infrared hyperspectral imaging can be similarly performed using a thermographic camera, with the fundamental difference that each pixel contains a full LWIR spectrum. Consequently, chemical identification of the object can be performed without a need for an external light source such as the Sun or the Moon. Such cameras are typically applied for geological measurements, outdoor surveillance and UAV applications.[37]

Other imaging

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Reflected light photograph in various infrared spectra to illustrate the appearance as the wavelength of light changes

In infrared photography, infrared filters are used to capture the near-infrared spectrum. Digital cameras often use infrared blockers. Cheaper digital cameras and camera phones have less effective filters and can view intense near-infrared, appearing as a bright purple-white color. This is especially pronounced when taking pictures of subjects near IR-bright areas (such as near a lamp), where the resulting infrared interference can wash out the image. There is also a technique called 'T-ray' imaging, which is imaging using far-infrared or terahertz radiation. Lack of bright sources can make terahertz photography more challenging than most other infrared imaging techniques. Recently T-ray imaging has been of considerable interest due to a number of new developments such as terahertz time-domain spectroscopy.


Tracking

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The IR seeker head on a FIM-92 MANPADS
Main article: Infrared homing

Infrared tracking, also known as infrared homing, refers to a passive missile guidance system, which uses the emission from a target of electromagnetic radiation in the infrared part of the spectrum to track it. Missiles that use infrared seeking are often referred to as "heat-seekers" since infrared (IR) is just below the visible spectrum of light in frequency and is radiated strongly by hot bodies. Many objects such as people, vehicle engines, and aircraft generate and retain heat, and as such, are especially visible in the infrared wavelengths of light compared to objects in the background.[38]

Heating

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Main article: Infrared heating
Infrared hair dryer for hair salons, c. 2010s

Infrared radiation can be used as a deliberate heating source. For example, it is used in infrared saunas to heat the occupants. It may also be used in other heating applications, such as to remove ice from the wings of aircraft (de-icing).[39]

Infrared heating is also becoming more popular in industrial manufacturing processes, e.g. curing of coatings, forming of plastics, annealing, plastic welding, and print drying. In these applications, infrared heaters replace convection ovens and contact heating.

Cooling

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Main article: Passive daytime radiative cooling

A variety of technologies or proposed technologies take advantage of infrared emissions to cool buildings or other systems. The LWIR (8–15 μm) region is especially useful since some radiation at these wavelengths can escape into space through the atmosphere's infrared window. This is how passive daytime radiative cooling (PDRC) surfaces are able to achieve sub-ambient cooling temperatures under direct solar intensity, enhancing terrestrial heat flow to outer space with zero energy consumption or pollution.[40][41] PDRC surfaces maximize shortwave solar reflectance to lessen heat gain while maintaining strong longwave infrared (LWIR) thermal radiation heat transfer.[42][43] When imagined on a worldwide scale, this cooling method has been proposed as a way to slow and even reverse global warming, with some estimates proposing a global surface area coverage of 1-2% to balance global heat fluxes.[44][45]

Communications

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Further information: Consumer IR

IR data transmission is also employed in short-range communication among computer peripherals and personal digital assistants. These devices usually conform to standards published by IrDA, the Infrared Data Association. Remote controls and IrDA devices use infrared light-emitting diodes (LEDs) to emit infrared radiation that may be concentrated by a lens into a beam that the user aims at the detector. The beam is modulated, i.e. switched on and off, according to a code which the receiver interprets. Usually very near-IR is used (below 800 nm) for practical reasons. This wavelength is efficiently detected by inexpensive silicon photodiodes, which the receiver uses to convert the detected radiation to an electric current. That electrical signal is passed through a high-pass filter which retains the rapid pulsations due to the IR transmitter but filters out slowly changing infrared radiation from ambient light. Infrared communications are useful for indoor use in areas of high population density. IR does not penetrate walls and so does not interfere with other devices in adjoining rooms. Infrared is the most common way for remote controls to command appliances. Infrared remote control protocols like RC-5, SIRC, are used to communicate with infrared.

Free-space optical communication using infrared lasers can be a relatively inexpensive way to install a communications link in an urban area operating at up to 4 gigabit/s, compared to the cost of burying fiber optic cable, except for the radiation damage. "Since the eye cannot detect IR, blinking or closing the eyes to help prevent or reduce damage may not happen."[46]

Infrared lasers are used to provide the light for optical fiber communications systems. Wavelengths around 1,330 nm (least dispersion) or 1,550 nm (best transmission) are the best choices for standard silica fibers.

IR data transmission of audio versions of printed signs is being researched as an aid for visually impaired people through the Remote infrared audible signage project. Transmitting IR data from one device to another is sometimes referred to as beaming.

IR is sometimes used for assistive audio as an alternative to an audio induction loop.

Spectroscopy

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Infrared vibrational spectroscopy (see also near-infrared spectroscopy) is a technique that can be used to identify molecules by analysis of their constituent bonds. Each chemical bond in a molecule vibrates at a frequency characteristic of that bond. A group of atoms in a molecule (e.g., CH2) may have multiple modes of oscillation caused by the stretching and bending motions of the group as a whole. If an oscillation leads to a change in dipole in the molecule then it will absorb a photon that has the same frequency. The vibrational frequencies of most molecules correspond to the frequencies of infrared light. Typically, the technique is used to study organic compounds using light radiation from the mid-infrared, 4,000–400 cm−1. A spectrum of all the frequencies of absorption in a sample is recorded. This can be used to gain information about the sample composition in terms of chemical groups present and also its purity (for example, a wet sample will show a broad O-H absorption around 3200 cm−1). The unit for expressing radiation in this application, cm−1, is the spectroscopic wavenumber. It is the frequency divided by the speed of light in vacuum.

Thin film metrology

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In the semiconductor industry, infrared light can be used to characterize materials such as thin films and periodic trench structures. By measuring the reflectance of light from the surface of a semiconductor wafer, the index of refraction (n) and the extinction Coefficient (k) can be determined via the Forouhi–Bloomer dispersion equations. The reflectance from the infrared light can also be used to determine the critical dimension, depth, and sidewall angle of high aspect ratio trench structures.

Meteorology

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IR satellite picture of cumulonimbus clouds over the Great Plains of the United States

Weather satellites equipped with scanning radiometers produce thermal or infrared images, which can then enable a trained analyst to determine cloud heights and types, to calculate land and surface water temperatures, and to locate ocean surface features. The scanning is typically in the range 10.3–12.5 μm (IR4 and IR5 channels).

Clouds with high and cold tops, such as cyclones or cumulonimbus clouds, are often displayed as red or black, lower warmer clouds such as stratus or stratocumulus are displayed as blue or grey, with intermediate clouds shaded accordingly. Hot land surfaces are shown as dark-grey or black. One disadvantage of infrared imagery is that low clouds such as stratus or fog can have a temperature similar to the surrounding land or sea surface and do not show up. However, using the difference in brightness of the IR4 channel (10.3–11.5 μm) and the near-infrared channel (1.58–1.64 μm), low clouds can be distinguished, producing a fog satellite picture. The main advantage of infrared is that images can be produced at night, allowing a continuous sequence of weather to be studied.

These infrared pictures can depict ocean eddies or vortices and map currents such as the Gulf Stream, which are valuable to the shipping industry. Fishermen and farmers are interested in knowing land and water temperatures to protect their crops against frost or increase their catch from the sea. Even El Niño phenomena can be spotted. Using color-digitized techniques, the gray-shaded thermal images can be converted to color for easier identification of desired information.

The main water vapour channel at 6.40 to 7.08 μm can be imaged by some weather satellites and shows the amount of moisture in the atmosphere.

Climatology

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The greenhouse effect with molecules of methane, water, and carbon dioxide re-radiating solar heat

In the field of climatology, atmospheric infrared radiation is monitored to detect trends in the energy exchange between the Earth and the atmosphere. These trends provide information on long-term changes in Earth's climate. It is one of the primary parameters studied in research into global warming, together with solar radiation.

A pyrgeometer is utilized in this field of research to perform continuous outdoor measurements. This is a broadband infrared radiometer with sensitivity for infrared radiation between approximately 4.5 μm and 50 μm.

Astronomy

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Main articles: Infrared astronomy and far-infrared astronomy
Beta Pictoris with its planet Beta Pictoris b, the light-blue dot off-center, as seen in infrared. It combines two images, the inner disc is at 3.6 μm.

Astronomers observe objects in the infrared portion of the electromagnetic spectrum using optical components, including mirrors, lenses and solid state digital detectors. For this reason it is classified as part of optical astronomy. To form an image, the components of an infrared telescope need to be carefully shielded from heat sources, and the detectors are chilled using liquid helium.

The sensitivity of Earth-based infrared telescopes is significantly limited by water vapor in the atmosphere, which absorbs a portion of the infrared radiation arriving from space outside of selected atmospheric windows. This limitation can be partially alleviated by placing the telescope observatory at a high altitude, or by carrying the telescope aloft with a balloon or an aircraft. Space telescopes do not suffer from this handicap, and so outer space is considered the ideal location for infrared astronomy.

The infrared portion of the spectrum has several useful benefits for astronomers. Cold, dark molecular clouds of gas and dust in our galaxy will glow with radiated heat as they are irradiated by imbedded stars. Infrared can also be used to detect protostars before they begin to emit visible light. Stars emit a smaller portion of their energy in the infrared spectrum, so nearby cool objects such as planets can be more readily detected. (In the visible light spectrum, the glare from the star will drown out the reflected light from a planet.)

Infrared light is also useful for observing the cores of active galaxies, which are often cloaked in gas and dust. Distant galaxies with a high redshift will have the peak portion of their spectrum shifted toward longer wavelengths, so they are more readily observed in the infrared.[10]

Cleaning

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Infrared cleaning is a technique used by some motion picture film scanners, film scanners and flatbed scanners to reduce or remove the effect of dust and scratches upon the finished scan. It works by collecting an additional infrared channel from the scan at the same position and resolution as the three visible color channels (red, green, and blue). The infrared channel, in combination with the other channels, is used to detect the location of scratches and dust. Once located, those defects can be corrected by scaling or replaced by inpainting.[47]

Art conservation and analysis

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An infrared reflectogram of Mona Lisa by Leonardo da Vinci

Infrared reflectography[48] can be applied to paintings to reveal underlying layers in a non-destructive manner, in particular the artist's underdrawing or outline drawn as a guide. Art conservators use the technique to examine how the visible layers of paint differ from the underdrawing or layers in between (such alterations are called pentimenti when made by the original artist). This is very useful information in deciding whether a painting is the prime version by the original artist or a copy, and whether it has been altered by over-enthusiastic restoration work. In general, the more pentimenti, the more likely a painting is to be the prime version. It also gives useful insights into working practices.[49] Reflectography often reveals the artist's use of carbon black, which shows up well in reflectograms, as long as it has not also been used in the ground underlying the whole painting. Infrared reflectography can be realized by modified commercial digital cameras in the NIR spectral region or by dedicated instruments in the SWIR spectral region.[50] The recent extension of reflectography into the MWIR spectral region[51][52] has proved capable of detecting subtle differences in surface materials.

Finally, NIR reflectography can be performed with good results using smartphone cameras .[53]

Recent progress in the design of infrared-sensitive cameras makes it possible to discover and depict not only underpaintings and pentimenti, but entire paintings that were later overpainted by the artist.[54] Notable examples are Picasso's Woman Ironing and Blue Room, where in both cases a portrait of a man has been made visible under the painting as it is known today.

Similar uses of infrared are made by conservators and scientists on various types of objects, especially very old written documents such as the Dead Sea Scrolls, the Roman works in the Villa of the Papyri, and the Silk Road texts found in the Dunhuang Caves.[55] Carbon black used in ink can show up extremely well.

Biological systems

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Further information: Infrared sensing in snakes
Thermographic image of a snake eating a mouse

The pit viper has a pair of infrared sensory pits on its head. There is uncertainty regarding the exact thermal sensitivity of this biological infrared detection system.[56][57]

Other organisms that have thermoreceptive organs are pythons (family Pythonidae), some boas (family Boidae), the Common Vampire Bat (Desmodus rotundus), a variety of jewel beetles (Melanophila acuminata),[58] darkly pigmented butterflies (Pachliopta aristolochiae and Troides rhadamantus plateni), and possibly blood-sucking bugs (Triatoma infestans).[59] By detecting the heat that their prey emits, crotaline and boid snakes identify and capture their prey using their IR-sensitive pit organs. Comparably, IR-sensitive pits on the Common Vampire Bat (Desmodus rotundus) aid in the identification of blood-rich regions on its warm-blooded victim. The jewel beetle, Melanophila acuminata, locates forest fires via infrared pit organs, where on recently burnt trees, they deposit their eggs. Thermoreceptors on the wings and antennae of butterflies with dark pigmentation, such Pachliopta aristolochiae and Troides rhadamantus plateni, shield them from heat damage as they sunbathe in the sun. Additionally, it's hypothesised that thermoreceptors let bloodsucking bugs (Triatoma infestans) locate their warm-blooded victims by sensing their body heat.[59]

Some fungi like Venturia inaequalis require near-infrared light for ejection.[60]

Although near-infrared vision (780–1,000 nm) has long been deemed impossible due to noise in visual pigments,[61] sensation of near-infrared light was reported in the common carp and in three cichlid species.[61][62][63][64][65] Fish use NIR to capture prey[61] and for phototactic swimming orientation.[65] NIR sensation in fish may be relevant under poor lighting conditions during twilight[61] and in turbid surface waters.[65]

Photobiomodulation

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Near-infrared light, or photobiomodulation, is used for treatment of chemotherapy-induced oral ulceration as well as wound healing. There is some work relating to anti-herpes virus treatment.[66] Research projects include work on central nervous system healing effects via cytochrome c oxidase upregulation and other possible mechanisms.[67]

Health hazards

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Strong infrared radiation in certain industry high-heat settings may be hazardous to the eyes, resulting in damage or blindness to the user. Since the radiation is invisible, special IR-proof goggles must be worn in such places.[68]

Scientific history

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The discovery of infrared radiation is ascribed to William Herschel, the astronomer, in the early 19th century. Herschel published his results in 1800 before the Royal Society of London. Herschel used a prism to refract light from the sun and detected the infrared, beyond the red part of the spectrum, through an increase in the temperature recorded on a thermometer. He was surprised at the result and called them "Calorific Rays".[69][70] The term "infrared" did not appear until late 19th century.[71] The Latin prefix infra- means below, as it is light below red on the spectrum.[72] An earlier experiment in 1790 by Marc-Auguste Pictet demonstrated the reflection and focusing of radiant heat via mirrors in the absence of visible light.[73]

Other important dates include:[28]

Infrared radiation was discovered in 1800 by William Herschel.

See also

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Notes

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Infrared

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Infrared , often abbreviated as IR, is a form of with wavelengths longer than those of visible red light but shorter than microwaves, typically spanning from approximately 700 nanometers (nm) to 1 millimeter (mm). This places it immediately beyond the in the , where it is invisible to the but commonly experienced as or . Discovered in 1800 by astronomer through experiments dispersing sunlight with a prism and measuring variations, infrared is emitted by all objects with temperatures above , following principles of where emission intensity increases with . Infrared is subdivided into regions such as near-infrared (0.7–1.4 μm), short-wavelength infrared (1.4–3 μm), mid-infrared (3–8 μm), long-wavelength infrared (8–15 μm), and far-infrared (15–1,000 μm); detailed schemes are discussed in the Spectral Regions section. These regions have distinct properties and applications due to varying energies from about 1.2 meV to 1.7 eV. Its longer wavelengths allow it to penetrate and dense gases with less than visible , making it valuable for observing obscured cosmic phenomena, while its characteristics enable detection of signatures from warm bodies. Natural sources include the Sun's emission peaking in the near-infrared, Earth's atmospheric and surface in the thermal infrared, and cool celestial objects like planets and star-forming regions. Artificial sources encompass lamps emitting broadly from 500 nm to 3,000 nm, as well as electronic devices like remote controls operating in the near-infrared around 940 nm. Key applications of infrared radiation span multiple fields, including thermal imaging for and via sensors detecting 8–15 μm wavelengths, astronomy through telescopes like the that reveal planet-forming disks hidden by dust, and Earth observation satellites such as MODIS for monitoring wildfires and vegetation health. In , low-level infrared utilizes near-infrared wavelengths (760–1,000 nm) for photobiomodulation to promote tissue repair and reduce , while far-infrared (>15 μm) is explored for its potential biological effects like improved circulation. Additionally, identifies molecular structures by measuring absorption in the mid- to far-infrared, and it powers consumer technologies from motion sensors to fiber-optic communications in the near-infrared band. Despite its benefits, prolonged exposure to intense infrared can cause thermal burns, highlighting the need for safety protocols in industrial and medical uses.

Fundamentals

Definition and Electromagnetic Spectrum

Infrared radiation, often abbreviated as IR, is a form of characterized by wavelengths ranging from approximately 700 nanometers (nm) to 1 millimeter (mm). This corresponds to a frequency range of about 430 terahertz (THz) to 300 gigahertz (GHz). These parameters place infrared just beyond the red end of the , where human eyes perceive light up to around 700 nm. Within the broader , infrared occupies the band between visible light—with its shorter from about 400 nm to 700 nm—and microwaves, which extend from 1 mm to 1 meter in . This positioning means infrared has longer and lower frequencies than visible light but higher frequencies than microwaves, influencing its interactions with , such as penetration and absorption properties. Infrared radiation is emitted by all objects above due to motion, though this aspect is explored further elsewhere. As with all , infrared demonstrates wave-particle duality, manifesting as propagating waves or discrete packets of known as . The EE of an individual infrared is determined by the formula
E=hν,E = h \nu,
where hh is Planck's constant (6.626×10−346.626 \times 10^{-34} J·s) and ν\nu is the 's frequency. This relationship underscores the quantized nature of infrared , with energies typically ranging from about 1.24 millielectronvolts (meV) to 1.7 electronvolts (eV) across the infrared band. Infrared wavelengths are typically measured in nanometers (nm) for near-infrared regions or micrometers (μm) for longer wavelengths, with 1 μm equaling 1000 nm. In , the reciprocal of , known as and expressed in inverse centimeters (cm⁻¹), is commonly used; for example, the infrared range spans roughly 14,000 cm⁻¹ to 10 cm⁻¹. This unit facilitates analysis of molecular vibrations, as higher wavenumbers correspond to higher energies.

Physical Properties

Infrared radiation, as a form of electromagnetic waves, propagates through at the , c=3×108c = 3 \times 10^8 m/s. In various media, its speed is reduced according to v=c/nv = c / n, where nn is the of the medium, which for infrared wavelengths typically ranges from about 1.3 to 4 depending on the material, such as or semiconductors used in . This velocity reduction leads to phenomena like when infrared waves pass through interfaces between media, enabling applications in lenses and waveguides. Infrared radiation interacts with matter primarily through absorption, reflection, transmission, and processes. Absorption occurs when infrared photons excite molecular vibrations, converting into ; reflection and transmission depend on the material's surface properties and contrast; while redirects waves without significant loss, often via Rayleigh or Mie mechanisms for small particles. In the Earth's atmosphere, these interactions are pronounced, with strong absorption bands due to (particularly in the 2.7 μm and 6.3 μm regions) and (around 4.3 μm and 15 μm), which attenuate infrared transmission over long paths and contribute to the . Atmospheric aerosols further enhance and absorption, reducing visibility in the infrared spectrum. A key aspect of infrared-matter interaction is , which varies by and material. Near-infrared (0.7–1.4 μm) penetrates biological tissues deeper, up to 10–15 mm in models like bovine tissue, due to lower absorption by and compared to longer wavelengths. In contrast, far-infrared (above 3 μm) is absorbed more superficially, up to several centimeters (e.g., ~4 cm) in , as it strongly excites molecule vibrations, enabling heating effects in deeper tissues while primarily interacting with surface layers. Infrared radiation exhibits polarization and coherence properties inherent to electromagnetic waves. Polarization describes the orientation of the electric field vector, which can be linear, circular, or elliptical; natural sources like thermal emitters produce unpolarized infrared, but it can be polarized through reflection, , or devices like wire-grid polarizers. Coherence refers to the of waves: thermal infrared sources, such as blackbodies, are typically incoherent with short coherence lengths (on the order of micrometers), leading to diffuse emission, whereas infrared lasers generate highly coherent radiation with long coherence lengths (centimeters to meters), enabling and precise imaging.

Spectral Regions

Common Subdivision Schemes

The infrared spectrum is commonly subdivided into three primary regions—near-infrared (NIR), mid-infrared (), and far-infrared ()—based on wavelength ranges that align with distinct physical interactions and technological capabilities. The near-infrared region spans approximately 0.7 to 1.4 μm, overlapping with the and enabling applications like fiber optics and due to its proximity to visible light. The mid-infrared region extends from 1.4 to 15 μm, often further divided into short-wavelength infrared (1.4–3 μm) and longer mid-wave (3–15 μm) sub-bands to reflect varying absorption characteristics. The far-infrared region covers 15 to 1000 μm, where thermal emission dominates and wavelengths approach millimeter scales. These divisions arise from key physical and practical considerations, including the excitation of molecular , which are most prominently observed in the mid-infrared where fundamental stretching and bending modes occur, providing a basis for spectroscopic identification of chemical bonds. Atmospheric transmission windows also influence the boundaries, with relatively clear paths in the near-infrared (around 0.7–1.1 μm) and mid-infrared (3–5 μm and 8–14 μm) allowing for effective ground-based observations and , while stronger absorption by and CO₂ limits transmission elsewhere. Detector sensitivities further justify the scheme, as silicon-based photodiodes excel in the near-infrared, mercury (MCT) detectors perform optimally in the mid-infrared, and microbolometers or cryogenic sensors are required for the far-infrared due to lower photon energies. In spectroscopic contexts, these wavelength ranges correspond to specific intervals, with the mid-infrared commonly expressed as 4000–400 cm⁻¹ to encompass the and regions for molecular analysis. Boundaries exhibit overlaps and variations across disciplines; for instance, often uses 0.78–3 μm for near- to mid-infrared transitions to align with sources, while may extend near-infrared to 2.5 μm to include vibrations. Such inconsistencies stem from historical conventions and application-specific needs, like eye safety standards (e.g., IR-A for near-infrared) versus thermal imaging bands.

Specialized Division Schemes

The (CIE) defines infrared radiation as optical radiation with wavelengths longer than the , starting at 780 nm where human visual response diminishes, though sensation may extend slightly beyond for intense sources; it further subdivides IR into IR-A (780 nm to 1.4 μm), IR-B (1.4 to 3 μm), and IR-C (3 μm to 1 mm) to align with physiological effects on the eye. This scheme prioritizes boundaries tied to human perception and rather than uniform wavelength intervals. The ISO 20473:2007 standard, developed for and applications excluding lighting and telecommunications, divides infrared into short-wave (near-infrared, 0.78–3 μm), mid-wave (3–50 μm), and long-wave (50–1000 μm) regions to standardize descriptions across technical fields. Unlike broader schemes, this emphasizes practical delimitation for instrument calibration and material testing, with mid-wave encompassing vibrational bands and long-wave covering thermal emissions. In astronomy, infrared is segmented into near-IR (0.8–5 μm), mid-IR (5–30 μm), and far-IR (30–1000 μm) to match observational challenges like atmospheric absorption and capabilities; the far-IR regime is particularly vital for detecting thermal emissions from grains, which absorb ultraviolet-visible and re-emit at these longer wavelengths, revealing interstellar structures such as molecular clouds and protostellar envelopes. Telecommunications employs fine-grained near-IR bands optimized for low-loss propagation in silica fiber optics, including the O-band (1260–1360 nm) for short-haul metropolitan networks and the C-band (1530–1565 nm) for long-haul amplification via erbium-doped fibers, enabling high-capacity data transmission with minimal signal attenuation. Infrared divisions based on sensor response align with detector material sensitivities, such as (InSb) photodiodes effective in the mid-IR (1–5.5 μm) for high-speed, cooled applications like thermal imaging, while (HgCdTe) extends to long-wave IR (up to 15 μm) and bolometers handle far-IR beyond 20 μm via thermal absorption. These material-driven boundaries facilitate selection for specific detection tasks, prioritizing quantum efficiency over arbitrary wavelength cuts.

Thermal Radiation

Infrared as Heat Radiation

Infrared radiation serves as the primary mechanism for thermal emission from objects at , where the peak wavelength of emission falls within the long-wavelength infrared range around 10 μm, as determined by . This law states that the wavelength at which a blackbody emits the maximum intensity of radiation, λ_max, is inversely proportional to its absolute temperature T, given by the formula λ_max T = 2.897 × 10^{-3} m·K. For typical s of approximately 300 K, this yields λ_max ≈ 9.7 μm, placing the strongest emission in the infrared spectrum and explaining why everyday objects like walls or furniture radiate primarily as infrared waves. Kirchhoff's law of thermal radiation further elucidates this process by establishing that, at and for a given , the ε of a surface—which measures its in emitting —equals its absorptivity α, the fraction of incident it absorbs. Thus, surfaces that are good absorbers of infrared , such as dark or matte materials, are also efficient emitters of thermal infrared at the same wavelengths, ensuring a balance in radiative heat exchange. This underlies why blackened surfaces, which absorb nearly all incident (α ≈ 1), emit strongly when heated, while reflective surfaces like polished metals emit poorly. A practical example is the , which at a normal of about 310 emits peak infrared radiation in the long-wavelength infrared band at approximately 9.3 μm, contributing to loss in environments. In daily life, infrared facilitates through , such as the warmth felt from on the skin—where solar infrared accounts for much of the heating effect—or the radiative cooling of a hot cup of coffee to the surrounding air, independent of conduction or . It is important to distinguish thermal infrared sources, which arise from the random thermal motion of particles in matter and follow blackbody-like spectra dependent on temperature, from non-thermal sources that produce infrared without significant heat generation. Non-thermal examples include infrared lasers or light-emitting diodes (LEDs) used in remote controls, which emit coherent or directed beams at specific wavelengths like 940 nm through electronic excitation rather than thermal processes.

Blackbody Emission

A blackbody is defined as an idealized physical body that absorbs all incident , regardless of frequency or angle of incidence, and re-emits energy solely based on its , with no reflection or transmission. This perfect absorber also serves as a perfect emitter, providing a theoretical benchmark for spectra. The of is described by , which quantifies the power emitted per unit area per unit per unit frequency as a function of λ\lambda and TT: B(λ,T)=2hc2λ51ehc/λkT−1,B(\lambda, T) = \frac{2hc^2}{\lambda^5} \frac{1}{e^{hc / \lambda k T} - 1}, where hh is Planck's constant, cc is the , and kk is Boltzmann's constant. This formula reveals that the peaks at a that shifts with , with significant infrared contributions dominating at lower temperatures due to the exponential term in the denominator. Wien's displacement law further characterizes this peak shift, stating that the wavelength λmax⁡\lambda_{\max} at which the spectral radiance is maximum satisfies λmax⁡T=b\lambda_{\max} T = b, where b≈2898 μm⋅Kb \approx 2898 \, \mu\mathrm{m \cdot K} is Wien's displacement constant. For terrestrial temperatures around 300 K, λmax⁡\lambda_{\max} falls in the long-wavelength infrared range near 10 μ\mum, explaining why blackbody emission from everyday objects is predominantly infrared. The total radiant power emitted by a blackbody is governed by the Stefan-Boltzmann law, P=σAT4P = \sigma A T^4, where σ=5.670×10−8 W⋅m−2⋅K−4\sigma = 5.670 \times 10^{-8} \, \mathrm{W \cdot m^{-2} \cdot K^{-4}} is the Stefan-Boltzmann constant and AA is the surface area. At temperatures below 1000 K, the majority of this power is emitted in the infrared spectrum, as shorter wavelengths become negligible.

Detection and Measurement

Infrared Detectors

Infrared detectors convert incident infrared radiation into measurable electrical signals, broadly categorized into detectors, which respond to the heat generated by absorbed photons, and quantum detectors, which rely on photon-induced electronic transitions. detectors operate by sensing changes caused by absorption and can function across all infrared spectral bands without wavelength selectivity, making them suitable for detection. Quantum detectors, in contrast, exhibit wavelength-specific responses based on material bandgaps and typically offer higher sensitivity but often require cooling to minimize thermal noise. Thermal detectors include bolometers, which measure radiation through changes in electrical resistance due to heating of a sensitive element, such as a thin film of or . The resistance variation is proportional to the rise from absorbed infrared power, enabling room-temperature operation with responsivities on the order of 10^4 to 10^5 V/W in microbolometer arrays. Pyroelectric detectors, another type of thermal device, generate a voltage in response to rapid changes via the pyroelectric effect in materials like tantalate or PZT ceramics, producing charge proportional to the rate of incident flux. These detectors are inherently AC-coupled, requiring modulation of the input signal, and achieve noise equivalent powers (NEPs) as low as 10^{-9} W/√Hz at room . Both bolometer and pyroelectric types are uncooled and versatile for near- to far-infrared detection, though they exhibit slower response times (milliseconds) compared to quantum alternatives. Quantum detectors, such as , operate by absorbing photons to excite charge carriers across a bandgap, with materials like (HgCdTe) commonly used for mid-infrared detection due to its tunable bandgap (0.1–1.5 eV) via composition control. In photovoltaic mode, the photodiode generates a voltage without external , relying on the built-in to separate electron-hole pairs, which suits low-noise, power-limited applications. Photoconductive mode applies reverse to increase carrier collection speed and gain, though it introduces higher dark current; quantum efficiency η, defined as η=number of charge carriers generatednumber of incident photons,\eta = \frac{\text{number of charge carriers generated}}{\text{number of incident photons}}, quantifies the conversion efficiency, often exceeding 70% for optimized HgCdTe devices in the 3–5 μm band. These detectors provide high speed (nanoseconds) and spectral selectivity but suffer from thermal generation noise at longer wavelengths. For mid- and far-infrared quantum detection, cryogenic cooling is essential to reduce excitation of carriers, which otherwise overwhelms the signal; typical operating temperatures are 77 K using or below 50 K with mechanical coolers for HgCdTe or InSb devices, achieving background-limited performance with dark currents reduced by orders of magnitude. Uncooled operation is feasible for short-wave infrared but degrades detectivity in longer bands due to increased noise. A key for comparing detector performance is the specific detectivity D^*, given by D∗=AΔfNEP,D^* = \frac{\sqrt{A \Delta f}}{\text{NEP}},
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