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For other uses, seeInfrared (disambiguation).
Afalse-colorimage of two people taken in long-wavelength infrared (body-temperature thermal) radiation
This pseudocolor infraredspace telescopeimage has blue, green, and red corresponding to wavelengths of 3.4, 4.6, and 12μm,respectively.

Infrared(IR;sometimes calledinfrared light) iselectromagnetic radiation(EMR) withwavelengthslonger than that ofvisible lightbut shorter thanmicrowaves.The infraredspectral bandbegins with waves that are just longer than those ofredlight (the longest waves in thevisible spectrum), so IR is invisible to the human eye. IR is generally understood to include wavelengths from around 750nm(400THz) to 1mm(300GHz).[1][2]IR is commonly divided between longer-wavelength thermal IR, emitted from terrestrial sources, and shorter-wavelength IR or near-IR, part of thesolar spectrum.[3]Longer IR wavelengths (30–100 μm) are sometimes included as part of theterahertz radiationband.[4]Almost allblack-body radiationfrom objects nearroom temperatureis in the IR band. As a form of electromagnetic radiation, IR carriesenergyandmomentum,exertsradiation pressure,and has properties corresponding toboththose of awaveand of aparticle,thephoton.

It was long known that fires emit invisibleheat;in 1681 the pioneering experimenterEdme Mariotteshowed that glass, though transparent to sunlight, obstructed radiant heat.[5][6]In 1800 the astronomer SirWilliam Herscheldiscovered that infrared radiation is a type of invisible radiation in the spectrum lower in energy than red light, by means of its effect on athermometer.[7]Slightly more than half of the energy from theSunwas eventually found, through Herschel's studies, to arrive onEarthin the form of infrared. The balance between absorbed and emitted infrared radiation has an important effect on Earth'sclimate.

Infrared radiation is emitted or absorbed bymoleculeswhen changing rotational-vibrational movements. It excitesvibrationalmodes in a molecule through a change in thedipole moment,making it a useful frequency range for study of these energy states for molecules of the proper symmetry.Infrared spectroscopyexamines absorption and transmission ofphotonsin the infrared range.[8]

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 astronomyuses sensor-equippedtelescopesto penetrate dusty regions of space such asmolecular clouds,to detect objects such asplanets,and to view highlyred-shiftedobjects from the early days of theuniverse.[9]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.[10]Military and civilian applications includetarget acquisition,surveillance,night vision,homing,and tracking. Humans at normal body temperature radiate chiefly at wavelengths around 10 μm. Non-military uses includethermal efficiencyanalysis, environmental monitoring, industrial facility inspections, detection ofgrow-ops,remote temperature sensing, short-rangewireless communication,spectroscopy,andweather forecasting.

Definition and relationship to the electromagnetic spectrum[edit]

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 700 nm to 1 mm. This range of wavelengths corresponds to afrequencyrange of approximately 430 THz down to 300 GHz. Beyond infrared is the microwave portion of theelectromagnetic 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).

Light comparison[11]
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[edit]

Sunlight,at an effective temperature of 5,780K(5,510 °C, 9,940 °F), is composed of near-thermal-spectrum radiation that is slightly more than half infrared. Atzenith,sunlight provides anirradianceof just over 1kWper square meter at sea level. Of this energy, 527 W is infrared radiation, 445 W is visible light, and 32 W isultravioletradiation.[12]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.[13]

Regions[edit]

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 withWien'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[edit]

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 be quite bright. And even IR at wavelengths up to 1,050 nm from pulsed lasers can be seen by humans under certain conditions.[14][15][16]

Commonly used subdivision scheme[edit]

A commonly used subdivision scheme is:[17][18]

Division name Abbreviation Wavelength Frequency Photon energy Temperature[i] Characteristics
Near-infrared NIR, IR-ADIN 0.75–1.4μm 214–400THz 886–1,653meV 3,864–2,070K
(3,591–1,797°C) 		Goes up to the wavelength of the firstwater absorptionband, and commonly used infiber optictelecommunication because of low attenuation losses in the SiO2glass (silica) medium.Image intensifiersare sensitive to this area of the spectrum; examples includenight visiondevices such as night vision goggles.Near-infrared spectroscopyis another common application.
Short-wavelength infrared SWIR, IR-BDIN 1.4–3 μm 100–214 THz 413–886 meV 2,070–966K
(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 (seetransmission windows).
Mid-wavelength infrared MWIR, IR-CDIN;MidIR.[20]Also called intermediate infrared (IIR) 3–8 μm 37–100 THz 155–413 meV 966–362K
(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 theinfrared signatureof the target aircraft, typically the jet engine exhaust plume. This region is also known as thermal infrared.
Long-wavelength infrared LWIR, IR-CDIN 8–15 μm 20–37 THz 83–155 meV 362–193K
(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 193–3K
(−80.15 – −270.15°C) 		(see alsofar-infrared laserandfar-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[edit]

TheInternational Commission on Illumination(CIE) recommended the division of infrared radiation into the following three bands:[21][22]

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[edit]

ISO20473 specifies the following scheme:[23]

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[edit]

Astronomers typically divide the infrared spectrum as follows:[24]

Designation Abbreviation Wavelength
Near-infrared NIR 0.7–2.5 μm
Mid-infrared MIR 3–25 μm
Far-infrared FIR above25 μm

These divisions are not precise and can vary depending on the publication. The three regions are used for observation of different temperature ranges,[25]and hence different environments in space.

The most common photometric system used in astronomy allocates capitalletters to different spectral regionsaccording 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 toatmospheric windowsand appear, for instance, in the titles of manypapers.

Sensor response division scheme[edit]

Plot of atmospheric transmittance in part of the infrared region

A third scheme divides up the band based on the response of various detectors:[26]

  • 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).InGaAscovers to about 1.8 μm; the less sensitive lead salts cover this region. Cryogenically cooledMCTdetectors can cover the region of 1.0–2.5μm.
  • Mid-wave infrared: 3 to 5 μm (defined by the atmospheric window and covered byindium antimonide,InSb andmercury cadmium telluride,HgCdTe, and partially bylead selenide,PbSe).
  • Long-wave infrared: 8 to 12, or 7 to 14 μm (this is the atmospheric window covered by HgCdTe andmicrobolometers).
  • 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 commonsilicondetectors are sensitive to about 1,050 nm, whileInGaAs'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., fromlasers,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 theWood effectthat consists of IR-glowing foliage.[27]

Telecommunication bands[edit]

Inoptical 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:[28]

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-distancetelecommunicationnetworks. The S and L bands are based on less well established technology, and are not as widely deployed.

Heat[edit]

Main article:Thermal radiation
Materials with higheremissivityappear 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",[29]but light and electromagnetic waves of any frequency will heat surfaces that absorb them. Infrared light from the Sun accounts for 49%[30]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 roomtemperaturewillemitradiationconcentrated 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 (seeblack bodyandWien's displacement law).[31]

Heatis energy in transit that flows due to a temperature difference. Unlike heat transmitted bythermal conductionorthermal convection,thermal radiation can propagate through avacuum.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. thesolar 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 ofemissivityis 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 ablack 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[edit]

Night vision[edit]

Main article:Night vision
Active-infrared night vision: the camera illuminates the scene at infrared wavelengths invisible to thehuman 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.[32]Night vision devicesoperate 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.[32]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.[32]

The use of infrared light and night vision devices should not be confused withthermal imaging,which creates images based on differences in surface temperature by detecting infrared radiation (heat) that emanates from objects and their surrounding environment.[33]

Thermography[edit]

Thermography helped to determine the temperature profile of theSpace Shuttle thermal protection systemduring 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 termedpyrometry.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 camerasdetect 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[edit]

Main article:Hyperspectral imaging
Hyperspectral thermal infraredemissionmeasurement, an outdoor scan in winter conditions, ambient temperature −15 °C, image produced with aSpecimLWIR hyperspectral imager. Relative radiance spectra from various targets in the image are shown with arrows. Theinfrared spectraof the different objects such as the watch clasp have clearly distinctive characteristics. The contrast level indicates the temperature of the object.[34]
Infrared light from theLEDof aremote controlas recorded by a digital camera

A hyperspectral image is a "picture" containing continuousspectrumthrough 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 athermographic 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 andUAVapplications.[35]

Other imaging[edit]

Ininfrared photography,infrared filtersare used to capture the near-infrared spectrum.Digital camerasoften use infraredblockers.Cheaper digital cameras andcamera phoneshave 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 usingfar-infraredorterahertz 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 asterahertz time-domain spectroscopy.

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

Tracking[edit]

Main article:Infrared homing

Infrared tracking, also known as infrared homing, refers to apassive missile guidance system,which uses theemissionfrom 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.[36]

Heating[edit]

Main article:Infrared heating
Infraredhair dryerforhair salons,c. 2010s

Infrared radiation can be used as a deliberate heating source. For example, it is used ininfrared saunasto heat the occupants. It may also be used in other heating applications, such as to remove ice from the wings of aircraft (de-icing).[37]Infrared radiation is used in cooking, known as broiling orgrilling.One energy advantage is that the IR energy heats only opaque objects, such as food, rather than the air around them.

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[edit]

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'sinfrared window.This is howpassive daytime radiative cooling(PDRC) surfaces are able to achieve sub-ambient cooling temperatures under direct solar intensity, enhancing terrestrialheat flowto outer space with zeroenergy consumptionorpollution.[38][39]PDRC surfaces minimize shortwavesolar reflectanceto lessen heat gain while maintaining strong longwave infrared (LWIR)thermal radiationheat transfer.[40][41]When imagined on a worldwide scale, this cooling method has been proposed as a way to slow and even reverseglobal warming,with some estimates proposing a global surface area coverage of 1-2% to balance global heat fluxes.[42][43]

Communications[edit]

Further information:Consumer IR

IR data transmission is also employed in short-range communication among computer peripherals andpersonal digital assistants.These devices usually conform to standards published byIrDA,the Infrared Data Association. Remote controls and IrDA devices use infraredlight-emitting diodes(LEDs) to emit infrared radiation that may be concentrated by alensinto a beam that the user aims at the detector. The beam ismodulated,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 inexpensivesiliconphotodiodes,which the receiver uses to convert the detected radiation to anelectric current.That electrical signal is passed through ahigh-pass filterwhich 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 forremote controlsto command appliances. Infrared remote control protocols likeRC-5,SIRC,are used to communicate with infrared.

Free space optical communicationusing infraredlaserscan 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."[44]

Infrared lasers are used to provide the light foroptical fibercommunications systems. Infrared light with a wavelength around 1,330 nm (leastdispersion) or 1,550 nm (best transmission) are the best choices for standardsilicafibers.

IR data transmission of encoded audio versions of printed signs is being researched as an aid for visually impaired people through theRIAS (Remote Infrared Audible Signage)project. Transmitting IR data from one device to another is sometimes referred to asbeaming.

Spectroscopy[edit]

Infrared vibrational spectroscopy(see alsonear-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 indipolein the molecule then it will absorb aphotonthat has the same frequency. The vibrational frequencies of most molecules correspond to the frequencies of infrared light. Typically, the technique is used to studyorganic compoundsusing 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 spectroscopicwavenumber.It is the frequency divided by the speed of light in vacuum.

Thin film metrology[edit]

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 theForouhi–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[edit]

IR satellite picture of cumulonimbus clouds over theGreat Plainsof the United States.

Weather satellitesequipped 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 ascyclonesorcumulonimbus clouds,are often displayed as red or black, lower warmer clouds such asstratusorstratocumulusare 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 cloud such as stratus orfogcan have a temperature similar to the surrounding land or sea surface and does 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 cloud can be distinguished, producing afogsatellite 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. EvenEl Niñophenomena 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[edit]

Thegreenhouse effectwith 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 intoglobal warming,together withsolar radiation.

Apyrgeometeris 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[edit]

Main articles:Infrared astronomyandfar-infrared astronomy
Beta Pictoriswith 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 ofoptical 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 liquidhelium.

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 selectedatmospheric 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, darkmolecular cloudsof gas and dust in our galaxy will glow with radiated heat as they are irradiated by imbedded stars. Infrared can also be used to detectprotostarsbefore they begin to emit visible light. Stars emit a smaller portion of their energy in the infrared spectrum, so nearby cool objects such asplanetscan 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 ofactive galaxies,which are often cloaked in gas and dust. Distant galaxies with a highredshiftwill have the peak portion of their spectrum shifted toward longer wavelengths, so they are more readily observed in the infrared.[9]

Cleaning[edit]

Infrared cleaningis a technique used by somemotion picture film scanners,film scannersandflatbed scannersto reduce or remove the effect of dust and scratches upon the finishedscan.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 byinpainting.[45]

Art conservation and analysis[edit]

An infrared reflectogram ofMona LisabyLeonardo da Vinci

Infrared reflectography[46]can be applied to paintings to reveal underlying layers in a non-destructive manner, in particular the artist'sunderdrawingor 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 calledpentimentiwhen made by the original artist). This is very useful information in deciding whether a painting is theprime versionby 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.[47]Reflectography often reveals the artist's use ofcarbon black,which shows up well in reflectograms, as long as it has not also been used in the ground underlying the whole painting.

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.[48]Notable examples arePicasso'sWoman IroningandBlue 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 theDead Sea Scrolls,the Roman works in theVilla of the Papyri,and the Silk Road texts found in theDunhuang Caves.[49]Carbon black used in ink can show up extremely well.

Biological systems[edit]

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

Thepit viperhas a pair of infrared sensory pits on its head. There is uncertainty regarding the exact thermal sensitivity of this biological infrared detection system.[50][51]

Other organisms that have thermoreceptive organs are Python s (familyPythonidae), some boas (familyBoidae), theCommon Vampire Bat(Desmodus rotundus), a variety ofjewel beetles(Melanophila acuminata),[52]darkly pigmented butterflies (Pachliopta aristolochiaeandTroides rhadamantus plateni), and possibly blood-sucking bugs (Triatoma infestans).[53]By detecting the heat that their prey emits,crotalineandboid snakesidentify and capture their prey using theirIR-sensitive pit organs.Comparably, IR-sensitive pits on theCommon Vampire Bat(Desmodus rotundus) aid in the identification of blood-rich regions on its warm-blooded victim. The jewel beetle,Melanophila acuminata,locatesforest firesvia infrared pit organs, where on recently burnt trees, they deposit their eggs.Thermoreceptorson the wings and antennae of butterflies with dark pigmentation, suchPachliopta aristolochiaeandTroides 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 theirwarm-bloodedvictims by sensing their body heat.[53]

Some fungi likeVenturia inaequalisrequire near-infrared light for ejection.[54]

Although near-infrared vision (780–1,000 nm) has long been deemed impossible due to noise in visual pigments,[55]sensation of near-infrared light was reported in the common carp and in three cichlid species.[55][56][57][58][59]Fish use NIR to capture prey[55]and for phototactic swimming orientation.[59]NIR sensation in fish may be relevant under poor lighting conditions during twilight[55]and in turbid surface waters.[59]

Photobiomodulation[edit]

Near-infrared light, orphotobiomodulation,is used for treatment of chemotherapy-induced oral ulceration as well as wound healing. There is some work relating to anti-herpes virus treatment.[60]Research projects include work on central nervous system healing effects via cytochrome c oxidase upregulation and other possible mechanisms.[61]

Health hazards[edit]

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.[62]

Scientific history[edit]

The discovery of infrared radiation is ascribed toWilliam Herschel,theastronomer,in the early 19th century. Herschel published his results in 1800 before theRoyal Society of London.Herschel used aprismtorefractlight from thesunand detected the infrared, beyond theredpart of the spectrum, through an increase in the temperature recorded on athermometer.He was surprised at the result and called them "Calorific Rays".[63][64]The term "infrared" did not appear until late 19th century.[65]Anearlier experiment in 1790byMarc-Auguste Pictetdemonstrated the reflection and focusing of radiant heat via mirrors in the absence of visible light.[66]

Other important dates include:[26]

Infrared radiation was discovered in 1800 by William Herschel.

See also[edit]

Notes[edit]

  1. ^Temperatures of black bodies for which spectral peaks fall at the given wavelengths, according to the wavelength form ofWien's displacement law[19]

References[edit]

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External links[edit]

Retrieved from "https://en.wikipedia.org/w/index.php?title=Infrared&oldid=1230897740"