View source for Physics:Infrared

{{short description|Form of electromagnetic radiation}}
[[File:Ir girl.png|thumb|A [[False color|false-color]] image of two people taken in long-wavelength infrared (body-temperature thermal) radiation.]]
[[File:Wide-field Infrared Survey Explorer first-light image.jpg|thumb|right|This pseudocolor infrared [[Astronomy:Space telescope|space telescope]] image has blue, green, and red corresponding to wavelengths of 3.4, 4.6, and 12&nbsp;[[Micrometre|μm]], respectively.]]

'''Infrared''' ('''IR'''; sometimes called '''infrared light''') is [[Physics:Electromagnetic radiation|electromagnetic radiation]] (EMR) in the [[Physics:Spectral band|spectral band]] between [[Physics:Microwave|microwaves]] and [[Physics:Light|visible light]] (the nominal [[Red|red]] edge of the [[Physics:Visible spectrum|visible spectrum]]). It is invisible to the human eye. IR is generally understood to encompass wavelengths from around 750 nm to 1000&nbsp;[[Micrometre|μm]] ([[Physics:Frequency|frequencies]] of 400&nbsp;[[Terahertz (unit)|THz]] to 300 [[Gigahertz|GHz]]).<ref>{{Cite journal |last1=Vatansever |first1=Fatma |last2=Hamblin |first2=Michael R. |date=2012-01-01 |title=Far infrared radiation (FIR): Its biological effects and medical applications |journal=Photonics & Lasers in Medicine |volume=1 |issue=4 |pages=255–266 |doi=10.1515/plm-2012-0034 |issn=2193-0643 |pmc=3699878 |pmid=23833705}}</ref><ref>{{Cite book |url=http://www.intechopen.com/books/infrared-radiation |title=Infrared Radiation |date=2012-02-10 |publisher=InTech |isbn=978-953-51-0060-7 |editor-last=Morozhenko |editor-first=Vasyl |language=en |doi=10.5772/2031}}</ref> 

IR is commonly divided between longer-wavelength thermal IR, emitted from terrestrial sources, and shorter-wavelength IR or near-IR, part of the [[Astronomy:Sunlight|solar spectrum]].<ref name="IPCC AR4-SYR">{{cite web |url=https://ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr_appendix.pdf |title=IPCC AR4 SYR Appendix Glossary |access-date=2008-12-14 |url-status=dead |archive-url=https://web.archive.org/web/20181117121314/http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr_appendix.pdf |archive-date=2018-11-17}}</ref> Longer IR wavelengths (30–100&nbsp;μm) are sometimes included as part of the [[Physics:Terahertz radiation|terahertz radiation]] band.<ref>{{cite book |last=Rogalski|first=Antoni |title=Infrared and terahertz detectors |date=2019 |publisher=CRC Press |location=Boca Raton, FL |isbn=9781315271330 |page=929 |edition=3rd}}</ref> 

Almost all [[Astronomy:Black-body radiation|black-body radiation]] from objects near [[Physics:Room temperature|room temperature]] is at the IR band. As a form of electromagnetic radiation, IR carries [[Physics:Energy|energy]] and [[Physics:Momentum|momentum]], exerts [[Astronomy:Radiation pressure|radiation pressure]], and has properties corresponding to [[Physics:Wave–particle duality|both]] those of a [[Wave|wave]] and of a [[Physics:Subatomic particle|particle]], the [[Physics:Photon|photon]].

It was long known that fires emit invisible [[Physics:Heat|heat]]; in 1681 the pioneering experimenter [[Biography:Edme Mariotte|Edme Mariotte]] showed that glass, though transparent to sunlight, obstructed radiant heat.<ref>{{cite web|last=Calel|first=Raphael|date=19 February 2014|title=The Founding Fathers v. The Climate Change Skeptics|url=https://publicdomainreview.org/2014/02/19/the-founding-fathers-v-the-climate-change-skeptics/|access-date=16 September 2019|website=The Public Domain Review}}</ref><ref>{{cite web|last=Fleming|first=James R.|date=17 March 2008|title=Climate Change and Anthropogenic Greenhouse Warming: A Selection of Key Articles, 1824–1995, with Interpretive Essays|url=http://nsdl.library.cornell.edu/websites/wiki/index.php/PALE_ClassicArticles/GlobalWarming.html|access-date=1 February 2022|website=National Science Digital Library Project Archive PALE:ClassicArticles}} [http://nsdl.library.cornell.edu/websites/wiki/index.php/PALE_ClassicArticles/GlobalWarming/Article1.html Article 1: General remarks on the temperature of the earth and outer space].</ref> 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 [[Physics:Thermometer|thermometer]].<ref>Michael Rowan-Robinson (2013). ''Night Vision: Exploring the Infrared Universe''. Cambridge University Press. p. 23. {{ISBN|1107024765}}.</ref> Slightly more than half of the energy from the [[Astronomy:Sun|Sun]] was eventually found, through Herschel's studies, to arrive on [[Astronomy:Earth|Earth]] in the form of infrared. The balance between absorbed and emitted infrared radiation has an important effect on Earth's [[Earth:Climate|climate]].

Infrared radiation is emitted or absorbed by [[Physics:Molecule|molecule]]s when changing rotational-vibrational movements. It excites [[Vibration|vibration]]al 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. [[Physics:Infrared spectroscopy|Infrared spectroscopy]] examines absorption and transmission of [[Physics:Photon|photon]]s in the infrared range.<ref>{{cite web |last=Reusch |first=William |year=1999 |url=http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/InfraRed/infrared.htm |title=Infrared Spectroscopy |publisher=Michigan State University |access-date=2006-10-27 |url-status=dead |archive-url=https://web.archive.org/web/20071027110406/http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/InfraRed/infrared.htm |archive-date=2007-10-27 }}</ref>

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. [[Astronomy:Infrared astronomy|Infrared astronomy]] uses sensor-equipped [[Astronomy:Telescope|telescope]]s to penetrate dusty regions of space such as [[Astronomy:Molecular cloud|molecular cloud]]s, to detect objects such as [[Astronomy:Planet|planet]]s, and to view highly red-shifted objects from the early days of the [[Astronomy:Universe|universe]].<ref name="ir_astronomy">{{cite web |url=http://www.ipac.caltech.edu/Outreach/Edu/importance.html |title=IR Astronomy: Overview |publisher=NASA Infrared Astronomy and Processing Center |access-date=2006-10-30 |url-status=dead |archive-url=https://web.archive.org/web/20061208151300/http://www.ipac.caltech.edu/Outreach/Edu/importance.html |archive-date=2006-12-08 }}</ref> 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.<ref>{{Cite web|last=Chilton|first=Alexander|date=2013-10-07|title=The Working Principle and Key Applications of Infrared Sensors|url=https://www.azosensors.com/article.aspx?ArticleID=339|access-date=2020-07-11|website=AZoSensors|language=en}}</ref>

Military and civilian applications include target acquisition, surveillance, [[Physics:Night vision|night vision]], homing, and tracking. Humans at normal body temperature radiate chiefly at wavelengths around 10&nbsp;μm. Non-military uses include [[Physics:Thermal efficiency|thermal efficiency]] analysis, environmental monitoring, industrial facility inspections, detection of grow-ops, remote temperature sensing, short-range wireless communication, [[Physics:Spectroscopy|spectroscopy]], and [[Earth:Weather forecasting|weather forecasting]].

==Definition and relationship to the electromagnetic spectrum==
There is no universally accepted definition of the range of infrared radiation. Typically, it is taken to extend from the nominal red edge of the visible spectrum at 700 nm to 1 mm. This range of wavelengths corresponds to a [[Physics:Frequency spectrum|frequency]] range of approximately 430&nbsp;THz down to 300&nbsp;GHz. Beyond infrared is the microwave portion of the [[Physics:Electromagnetic spectrum|electromagnetic spectrum]]. Increasingly, terahertz radiation is counted as part of the microwave band, not infrared, moving the band edge of infrared to 0.1&nbsp;mm (3 THz).

{| class=wikitable style="float:center; margin:2px; text-align:center;"
|+ [[Physics:Electromagnetic spectrum|Light comparison]]<ref>{{cite book|ref=Haynes|editor=Haynes, William M.|year=2011|title= CRC Handbook of Chemistry and Physics |edition=92nd|publisher= CRC Press|isbn=978-1-4398-5511-9|page=10.233}}</ref>
|-
! Name || [[Wavelength]] || [[Hertz|Frequency (Hz)]] || [[Physics:Electronvolt|Photon energy (eV)]]
|-
| [[Physics:Gamma ray|Gamma ray]] || less than 10&nbsp;pm || more than 30&nbsp;EHz || more than 124&nbsp;keV
|-
| [[Physics:X-ray|X-ray]] || 10&nbsp;pm – 10&nbsp;nm || 30&nbsp;PHz – 30&nbsp;EHz || 124&nbsp;keV – 124&nbsp;eV
|-
| [[Physics:Ultraviolet|Ultraviolet]] || 10&nbsp;nm – 400&nbsp;nm || 750&nbsp;THz – 30&nbsp;PHz || 124&nbsp;eV – 3.3&nbsp;eV
|-
| [[Physics:Visible light|Visible]] || 400&nbsp;nm – 700&nbsp;nm || 430&nbsp;THz – 750&nbsp;THz || 3.3&nbsp;eV – 1.7&nbsp;eV
|- style="background:#FFE8E8;"
| '''Infrared''' || 700&nbsp;nm – 1&nbsp;mm || 300&nbsp;GHz – 430&nbsp;THz || 1.7&nbsp;eV – 1.24&nbsp;meV
|-
| [[Physics:Microwave|Microwave]] || 1&nbsp;mm – 1&nbsp;meter || 300&nbsp;MHz – 300&nbsp;GHz || 1.24&nbsp;meV – 1.24&nbsp;μeV
|-
<!-- radio waves _include_ microwaves, so following ranges overlap those above -->
| Radio || 1&nbsp;meter and more || 300&nbsp;MHz and below || 1.24&nbsp;μeV and below
|}

==Nature==
[[Astronomy:Sunlight|Sunlight]], at an effective temperature of 5,780&nbsp;[[Kelvin|K]] (5,510&nbsp;°C, 9,940&nbsp;°F), is composed of near-thermal-spectrum radiation that is slightly more than half infrared. At [[Zenith|zenith]], sunlight provides an [[Physics:Irradiance|irradiance]] of just over 1&nbsp;[[Kilowatt|kW]] per square meter at sea level. Of this energy, 527 W is infrared radiation, 445 W is visible light, and 32 W is [[Physics:Ultraviolet|ultraviolet]] radiation.<ref>{{cite web |url=http://rredc.nrel.gov/solar/spectra/am1.5/ |title=Reference Solar Spectral Irradiance: Air Mass 1.5 |access-date=2009-11-12}}</ref> 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.<ref>{{Cite web|url=https://www.e-education.psu.edu/astro801/content/l3_p5.html|title = Blackbody Radiation &#124; Astronomy 801: Planets, Stars, Galaxies, and the Universe}}</ref>

==Regions==<!--Mid-infrared redirects here-->

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 [[Physics:Wien's displacement law|Wien's displacement law]]. The infrared band is often subdivided into smaller sections, although how the IR spectrum is thereby divided varies between different areas in which IR is employed.

===Visible limit===
Infrared radiation is generally considered to begin with wavelengths longer than visible by the human eye. There is no hard wavelength limit to what is visible, as the eye's sensitivity decreases rapidly but smoothly, for wavelengths exceeding about 700&nbsp;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&nbsp;nm from pulsed lasers can be seen by humans under certain conditions.<ref name="Sliney1976">{{cite journal| last1=Sliney | first1=David H. | last2=Wangemann | first2=Robert T. | last3=Franks | first3=James K. | last4 =Wolbarsht | first4=Myron L. | year=1976 | title=Visual sensitivity of the eye to infrared laser radiation | journal=[[Organization:Journal of the Optical Society of America|Journal of the Optical Society of America]] | volume=66 | issue=4 | pages=339–341 | doi=10.1364/JOSA.66.000339 | pmid=1262982 | quote=The foveal sensitivity to several near-infrared laser wavelengths was measured. It was found that the eye could respond to radiation at wavelengths at least as far as 1064 nm. A continuous 1064 nm laser source appeared red, but a 1060 nm pulsed laser source appeared green, which suggests the presence of second harmonic generation in the retina. | bibcode=1976JOSA...66..339S }}</ref><ref name="LynchLivingston2001">{{cite book|last1=Lynch|first1=David K.|last2=Livingston|first2=William Charles|title=Color and Light in Nature|url=https://books.google.com/books?id=4Abp5FdhskAC&pg=PA231|access-date=12 October 2013|edition=2nd|year=2001|publisher=Cambridge University Press|location=Cambridge, UK|isbn=978-0-521-77504-5|page=231|quote=Limits of the eye's overall range of sensitivity extends from about 310 to 1,050 nanometers}}</ref><ref name="Saidman1933">{{cite journal| last1=Saidman | first1=Jean | date=15 May 1933 | title=Sur la visibilité de l'ultraviolet jusqu'à la longueur d'onde 3130 |trans-title=The visibility of the ultraviolet to the wave length of 3130 | journal=Comptes rendus de l'Académie des sciences | volume=196 | pages=1537–9 | language=fr | url =http://visualiseur.bnf.fr/ark:/12148/bpt6k3148d}}</ref>

=== Commonly used subdivision scheme ===

A commonly used subdivision scheme is:<ref name="Byrnes">{{Cite book|last=Byrnes |first=James |title=Unexploded Ordnance Detection and Mitigation |publisher=Springer |year=2009 |pages=21–22 |isbn=978-1-4020-9252-7|bibcode=2009uodm.book.....B }}</ref><ref name="RP-photonics">{{cite web |title=Infrared Light |url=https://www.rp-photonics.com/infrared_light.html |website=RP Photonics Encyclopedia |publisher=RP Photonics |access-date=20 July 2021}}</ref>

{| class="wikitable"
|-
! Division&nbsp;name
! Abbreviation
! Wavelength
! Frequency
! Photon&nbsp;energy
! Temperature{{efn-lr|name=†|Temperatures of black bodies for which spectral peaks fall at the given wavelengths, according to the wavelength form of [[Physics:Wien's displacement law|Wien's displacement law]]<ref>{{cite web|title=Peaks of Blackbody Radiation Intensity|url=http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/wien3.html|access-date=27 July 2016}}</ref>}}
! Characteristics
|-
! Near-infrared
|NIR, IR-A ''DIN''
| 0.75–1.4&nbsp;μm
| 214–400&nbsp;[[Terahertz (unit)|THz]]
| 886–1,653&nbsp;[[Physics:MeV|meV]]
| {{convert|3864|–|2070|K|C|lk=on|disp=br()}}
| Goes up to the wavelength of the first [[Physics:Electromagnetic absorption by water|water absorption]] band, and commonly used in fiber optic telecommunication because of low attenuation losses in the SiO<sub>2</sub> glass ([[Chemistry:Silica|silica]]) medium. [[Physics:Image intensifier|Image intensifier]]s are sensitive to this area of the spectrum; examples include [[Physics:Night vision|night vision]] devices such as night vision goggles. [[Physics:Near-infrared spectroscopy|Near-infrared spectroscopy]] is another common application.
|-
! Short-wavelength infrared
| SWIR, IR-B ''DIN''
| 1.4–3&nbsp;μm
| 100–214&nbsp;THz
| 413–886&nbsp;meV
| {{convert|2070|–|966|K|C|lk=on|disp=br()}}
| Water absorption increases significantly at 1,450&nbsp;nm. The 1,530 to 1,560&nbsp;nm range is the dominant spectral region for long-distance telecommunications (see [[Physics:Fiber-optic communication#Transmission windows|transmission windows]]).
|-
! {{anchor|MidIR|MWIR|IIR|IR-C}} Mid-wavelength infrared
|MWIR, IR-C ''DIN''; MidIR.<ref name=rdmag20120908>{{Cite news |date=August 14, 2012 |title=Photoacoustic technique 'hears' the sound of dangerous chemical agents |periodical=R&D Magazine |at=rdmag.com |url=http://www.rdmag.com/News/2012/08/Chemistry-Test-Measurement-Photonics-Photoacoustic-technique-hears-the-sound-of-dangerous-chemical-agents/?et_cid=2797047&et_rid=54719290&linkid=http%3a%2f%2fwww.rdmag.com%2fNews%2f2012%2f08%2fChemistry-Test-Measurement-Photonics-Photoacoustic-technique-hears-the-sound-of-dangerous-chemical-agents |access-date=September 8, 2012 }}</ref> Also called intermediate infrared (IIR)
| 3–8&nbsp;μm
| 37–100&nbsp;THz
| 155–413&nbsp;meV
| {{convert|966|–|362|K|C|lk=on|disp=br()}}
| In guided missile technology the 3–5&nbsp;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 [[Physics:Infrared signature|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&nbsp;μm
| 20–37&nbsp;THz
| 83–155&nbsp;meV
| {{convert|362|–|193|K|C|lk=on|disp=br()}}
| 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, moon, or infrared illuminator. This region is also called the "thermal infrared".
|-
! [[Physics:Far infrared|Far-infrared]]
| FIR
| 15–1,000&nbsp;μm
| 0.3–20&nbsp;THz
| 1.2–83&nbsp;meV
| {{convert|193|–|3|K|C|lk=on|disp=br()}}
| (see also [[Physics:Far-infrared laser|far-infrared laser]] and [[Physics:Far infrared|far-infrared]])
|}
{{thermal image comparison}}
NIR and SWIR together is sometimes called "reflected infrared", whereas MWIR and LWIR is sometimes referred to as "thermal infrared".

===CIE division scheme===

The International Commission on Illumination (CIE) recommended the division of infrared radiation into the following three bands:<ref>{{cite web|last=Henderson |first=Roy |url=http://info.tuwien.ac.at/iflt/safety/section1/1_1_1.htm |title=Wavelength considerations |publisher=Instituts für Umform- und Hochleistungs |access-date=2007-10-18 |archive-url = https://web.archive.org/web/20071028072110/http://info.tuwien.ac.at/iflt/safety/section1/1_1_1.htm <!-- Bot retrieved archive --> |archive-date = 2007-10-28}}</ref><ref>{{cite web |last1=CIE (International Commission on Illumination) |title=infrared radiation IR radiation IRR |url=https://cie.co.at/eilvterm/17-21-004 |website=17-21-004 |access-date=18 October 2022}}</ref>
{| class="wikitable"
|-
!Abbreviation
!Wavelength
!Frequency
|-
| IR-A|| 780&nbsp;nm – 1,400&nbsp;nm<br />(0.78&nbsp;μm – 1.4&nbsp;μm) || 215&nbsp;THz – 430&nbsp;THz
|-
| IR-B|| 1,400&nbsp;nm – 3,000&nbsp;nm<br />(1.4&nbsp;μm – 3&nbsp;μm)|| 100&nbsp;THz – 215&nbsp;THz
|-
| IR-C|| 3,000&nbsp;nm – 1&nbsp;mm<br />(3&nbsp;μm – 1,000&nbsp;μm)|| 300&nbsp;GHz – 100&nbsp;THz
|}

===ISO 20473 scheme===

[[Organization:International Organization for Standardization|ISO]] 20473 specifies the following scheme:<ref>{{cite ISO standard|csnumber=39482|title=ISO 20473:2007 – Optics and photonics – Spectral bands}}</ref>

{| class="wikitable"
|-
! style="width:100pt; text-align:left;"| Designation
! style="width:100pt; text-align:center;"| Abbreviation
! style="width:150pt; text-align:center;"| Wavelength
|-
|align="left"| Near-Infrared
| style="text-align:center;"| NIR
| style="text-align:center;"| 0.78–3&nbsp;μm
|-
|align="left"| Mid-Infrared
| style="text-align:center;"| MIR
| style="text-align:center;"| 3–50&nbsp;μm
|-
|align="left"| Far-Infrared
| style="text-align:center;"| FIR
| style="text-align:center;"| 50–1,000&nbsp;μm
|}

===Astronomy division scheme===

Astronomers typically divide the infrared spectrum as follows:<ref>{{cite web |url=http://www.ipac.caltech.edu/Outreach/Edu/Regions/irregions.html |title=Near, Mid and Far-Infrared |publisher=NASA IPAC |access-date=2007-04-04 |url-status=dead |archive-url=https://archive.today/20120529/http://www.ipac.caltech.edu/Outreach/Edu/Regions/irregions.html |archive-date=2012-05-29 }}</ref>

{| class="wikitable"
|-
! style="width:100pt; text-align:left;"| Designation
! style="width:100pt; text-align:center;"| Abbreviation
! style="width:150pt; text-align:center;"| Wavelength
|-
|align="left"| Near-Infrared
| style="text-align:center;"| NIR
| style="text-align:center;"| 0.7 to 2.5&nbsp;μm
|-
|align="left"| Mid-Infrared
| style="text-align:center;"| MIR
| style="text-align:center;"| 3 to 25&nbsp;μm
|-
|align="left"| Far-Infrared
| style="text-align:center;"| FIR
| style="text-align:center;"| above 25&nbsp;μm.
|}

These divisions are not precise and can vary depending on the publication. The three regions are used for observation of different temperature ranges{{Citation needed|date=December 2020}}, 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 [[Earth:Infrared window|atmospheric windows]] and appear, for instance, in the titles of many papers.<!--A Wikipedia search for JHK finds several examples-->

===Sensor response division scheme===
[[File:Atmosfaerisk spredning-en.svg|thumb|Plot of atmospheric transmittance in part of the infrared region]]

A third scheme divides up the band based on the response of various detectors:<ref name="Miller">Miller, ''Principles of Infrared Technology'' (Van Nostrand Reinhold, 1992), and Miller and Friedman, ''Photonic Rules of Thumb'', 2004. {{ISBN|978-0-442-01210-6}}{{Page needed|date=September 2010}}</ref>
* Near-infrared: from 0.7 to 1.0&nbsp;μm (from the approximate end of the response of the human eye to that of silicon).
* Short-wave infrared: 1.0 to 3&nbsp;μm (from the cut-off of silicon to that of the MWIR atmospheric window). InGaAs covers to about 1.8&nbsp;μm; the less sensitive lead salts cover this region. Cryogenically cooled [[Physics:Mercury cadmium telluride|MCT]] detectors can cover the region of 1.0–2.5{{nbsp}}μm.
* Mid-wave infrared: 3 to 5&nbsp;μm (defined by the atmospheric window and covered by [[Chemistry:Indium antimonide|indium antimonide]], InSb and [[Physics:Mercury cadmium telluride|mercury cadmium telluride]], HgCdTe, and partially by [[Chemistry:Lead selenide|lead selenide]], PbSe).
* Long-wave infrared: 8 to 12, or 7 to 14&nbsp;μm (this is the atmospheric window covered by HgCdTe and [[Physics:Microbolometer|microbolometer]]s).
* Very-long wave infrared (VLWIR) (12 to about 30&nbsp;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 [[Chemistry:Silicon|silicon]] detectors are sensitive to about 1,050&nbsp;nm, while [[Chemistry:Indium gallium arsenide|InGaAs]]'s sensitivity starts around 950&nbsp;nm and ends between 1,700 and 2,600&nbsp;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&nbsp;nm and 800&nbsp;nm, but the boundary between visible and infrared light is not precisely defined. The human eye is markedly less sensitive to light above 700&nbsp;nm wavelength, so longer wavelengths make insignificant contributions to scenes illuminated by common light sources. Particularly intense near-IR light (e.g., from [[Physics:Laser|laser]]s, LEDs or bright daylight with the visible light filtered out) can be detected up to approximately 780&nbsp;nm, and will be perceived as red light. Intense light sources providing wavelengths as long as 1,050&nbsp;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 that consists of IR-glowing foliage.<ref>{{Cite journal|title=The Sensitivity of the Human Eye to Infra-Red Radiation |journal= Journal of the Optical Society of America |volume=37 |issue=7 |pages=546–553 |year=1947 |doi=10.1364/JOSA.37.000546|pmid= 20256359 |last1=Griffin|first1=Donald R.|last2=Hubbard|first2=Ruth|last3=Wald|first3=George|bibcode= 1947JOSA...37..546G }}</ref>

===Telecommunication bands===

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:<ref>{{cite journal|last=Ramaswami |first=Rajiv |date=May 2002 |doi=10.1109/MCOM.2002.1006983 |title=Optical Fiber Communication: From Transmission to Networking |journal=IEEE Communications Magazine |volume=40 |issue=5 |pages=138–147 |s2cid=29838317 }}</ref>

{| class="wikitable"
|-
!Band
!Descriptor
!Wavelength range
|-
|O band
|Original
|1,260–1,360&nbsp;nm
|-
|E band
|Extended
|1,360–1,460&nbsp;nm
|-
|S band
|Short wavelength
|1,460–1,530&nbsp;nm
|-
|[[Physics:C band (infrared)|C band]]
|Conventional
|1,530–1,565&nbsp;nm
|-
|L band
|Long wavelength
|1,565–1,625&nbsp;nm
|-
|U band
|Ultralong wavelength
|1,625–1,675&nbsp;nm
|}

The C-band is the dominant band for long-distance [[Engineering:Telecommunication|telecommunication]] networks. The S and L bands are based on less well established technology, and are not as widely deployed.

== Heat ==
{{Main|Physics:Thermal radiation}}

[[File:Effect of emissivity on apparent temperature.jpg|thumb|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",<ref>{{Cite book|title=Infrared Radiation. Van Nostrand's Scientific Encyclopedia|publisher=John Wiley & Sons, Inc.|doi=10.1002/0471743984.vse4181.pub2|chapter=Infrared Radiation|year=2007|isbn=978-0471743989}}</ref> but light and electromagnetic waves of any frequency will heat surfaces that absorb them. Infrared light from the Sun accounts for 49%<ref>{{cite web |title=Introduction to Solar Energy |website=Passive Solar Heating & Cooling Manual |publisher=Rodale Press, Inc. |year=1980 |url=http://www.azsolarcenter.com/design/documents/passive.DOC |format=DOC |access-date=2007-08-12 |archive-url=https://web.archive.org/web/20090318200719/http://www.azsolarcenter.com/design/documents/passive.DOC |archive-date=2009-03-18 |url-status=dead }}</ref> 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 [[Physics:Temperature|temperature]] will [[Physics:Spontaneous emission|emit]] [[Physics:Thermal radiation|radiation]] concentrated mostly in the 8 to 25&nbsp;μm band, but this is not distinct from the emission of visible light by incandescent objects and ultraviolet by even hotter objects (see [[Astronomy:Black body|black body]] and [[Physics:Wien's displacement law|Wien's displacement law]]).<ref>{{cite web |last=McCreary |first=Jeremy |date=October 30, 2004 |url=http://dpfwiw.com/ir.htm |title=Infrared (IR) basics for digital photographers-capturing the unseen (Sidebar: Black Body Radiation) |publisher=Digital Photography For What It's Worth |access-date=2006-11-07 |archive-date=2008-12-18 |archive-url=https://web.archive.org/web/20081218010429/http://dpfwiw.com/ir.htm |url-status=dead }}</ref>

[[Physics:Heat|Heat]] is energy in transit that flows due to a temperature difference. Unlike heat transmitted by [[Physics:Thermal conduction|thermal conduction]] or thermal convection, thermal radiation can propagate through a [[Physics:Vacuum|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 [[Physics:Solar corona|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 [[Physics:Emissivity|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 [[Astronomy:Black body|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==

===Night vision===
{{Main|Physics:Night vision}}
[[File:Active-Infrared-Night-Vision.jpg|thumb|Active-infrared night vision: the camera illuminates the scene at infrared wavelengths invisible to the [[Biology:Human eye|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.<ref name="how night vision works">{{cite web|title=How Night Vision Works |publisher=American Technologies Network Corporation |url=http://www.atncorp.com/HowNightVisionWorks |access-date=2007-08-12}}</ref> 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.<ref name="how night vision works"/> 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.<ref name="how night vision works"/>

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 ([[Physics:Heat|heat]]) that emanates from objects and their surrounding environment.<ref>{{cite web |last=Bryant |first=Lynn |title=How does thermal imaging work? A closer look at what is behind this remarkable technology |date=2007-06-11 |url=http://www.video-surveillance-guide.com/how-does-thermal-imaging-work.htm |access-date=2007-08-12 |archive-url=https://web.archive.org/web/20070728055934/http://www.video-surveillance-guide.com/how-does-thermal-imaging-work.htm |archive-date=2007-07-28 |url-status=dead }}</ref>

===Thermography===
[[File:STS-3 infrared on reentry.jpg|thumb|left|upright=0.7|Thermography helped to determine the temperature profile of the [[Engineering:Space Shuttle thermal protection system|Space Shuttle thermal protection system]] during re-entry.]]{{Main|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&nbsp;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===
{{Main|Physics:Hyperspectral imaging}}

[[File:Specim aisaowl outdoor.png|thumb|left| Hyperspectral thermal infrared [[Physics:Emission spectrum|emission]] measurement, an outdoor scan in winter conditions, ambient temperature −15&nbsp;°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.<ref name=Holma>Holma, H., (May 2011), [http://www.photonik.de/index.php?id=11&np=5&artid=848&L=1 Thermische Hyperspektralbildgebung im langwelligen Infrarot] {{webarchive|url=https://web.archive.org/web/20110726171326/http://www.photonik.de/index.php?id=11&np=5&artid=848&L=1 |date=2011-07-26 }}, Photonik</ref>]]
[[File:Blue infrared light.jpg|thumb|Infrared light from the [[Engineering:LED|LED]] of a remote control as recorded by a digital camera]]

A hyperspectral image is a "picture" containing continuous [[Physics:Infrared spectroscopy|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 [[Physics:Thermographic camera|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.<ref name="Frost&Sullivan Specim Owl">Frost&Sullivan, Technical Insights, Aerospace&Defence (Feb 2011): [http://www.frost.com/prod/servlet/segment-toc.pag?segid=D870-00-48-00-00&ctxixpLink=FcmCtx3&ctxixpLabel=FcmCtx4 World First Thermal Hyperspectral Camera for Unmanned Aerial Vehicles].</ref>

===Other imaging===
In [[Infrared photography|infrared photography]], infrared filters are used to capture the near-infrared spectrum. [[Engineering:Digital camera|Digital camera]]s 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 '[[Physics:Terahertz radiation|T-ray]]' imaging, which is imaging using far-infrared or [[Physics:Terahertz radiation|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 [[Physics:Terahertz time-domain spectroscopy|terahertz time-domain spectroscopy]].
[[File:Infrared portrait comparison.jpg|thumb|Reflected light photograph in various infrared spectra to illustrate the appearance as the wavelength of light changes.]]

===Tracking===
{{Main|Physics: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.<ref>{{cite journal|author1=Mahulikar, S.P. |author2=Sonawane, H.R. |author3=Rao, G.A.|year=2007|title=Infrared signature studies of aerospace vehicles|journal=Progress in Aerospace Sciences|volume=43|issue=7–8|pages= 218–245|url=http://dspace.library.iitb.ac.in/xmlui/bitstream/handle/10054/613/5740.pdf|doi=10.1016/j.paerosci.2007.06.002|bibcode = 2007PrAeS..43..218M |citeseerx=10.1.1.456.9135 }}</ref>

===Heating===
[[File:Hooded dryer for infrared hair drying at hair salon - shown from three perspectives.jpg|thumb|Infrared [[Engineering:Hair dryer|hair dryer]] for hair salons, c. 2010s]]
Infrared radiation can be used as a deliberate heating source. For example, it is used in [[Philosophy:Infrared sauna|infrared sauna]]s 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).<ref>White, Richard P. (2000) "Infrared deicing system for aircraft" {{US Patent|6092765}}</ref> Infrared radiation is used in cooking, known as broiling or grilling. 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===
{{Main|Earth: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&nbsp;μm) region is especially useful since some radiation at these wavelengths can escape into space through the atmosphere's [[Earth:Infrared window|infrared window]]. This is how [[Earth:Passive daytime radiative cooling|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 [[Physics:Efficient energy use|energy consumption]] or pollution.<ref name=":5">{{Cite journal |last1=Chen |first1=Meijie |last2=Pang |first2=Dan |last3=Chen |first3=Xingyu |last4=Yan |first4=Hongjie |last5=Yang |first5=Yuan |title=Passive daytime radiative cooling: Fundamentals, material designs, and applications |journal=EcoMat |year=2022 |volume=4 |doi=10.1002/eom2.12153 |s2cid=240331557 |quote=Passive daytime radiative cooling (PDRC) dissipates terrestrial heat to the extremely cold outer space without using any energy input or producing pollution. It has the potential to simultaneously alleviate the two major problems of energy crisis and global warming. |doi-access=free }}</ref><ref name=":023">{{Cite journal |last=Munday |first=Jeremy |date=2019 |title=Tackling Climate Change through Radiative Cooling |journal=Joule |volume=3 |issue=9 |pages=2057–2060 |doi=10.1016/j.joule.2019.07.010 |s2cid=201590290 |quote=By covering the Earth with a small fraction of thermally emitting materials, the heat flow away from the Earth can be increased, and the net radiative flux can be reduced to zero (or even made negative), thus stabilizing (or cooling) the Earth. |doi-access=free }}</ref> PDRC surfaces minimize shortwave solar reflectance to lessen heat gain while maintaining strong longwave infrared (LWIR) [[Physics:Thermal radiation|thermal radiation]] [[Physics:Heat transfer|heat transfer]].<ref name=":1">{{Cite journal |last1=Wang |first1=Tong |last2=Wu |first2=Yi |last3=Shi |first3=Lan |last4=Hu |first4=Xinhua |last5=Chen |first5=Min |last6=Wu |first6=Limin |date=2021 |title=A structural polymer for highly efficient all-day passive radiative cooling |journal=Nature Communications |volume=12 |issue=365 |page=365 |doi=10.1038/s41467-020-20646-7 |pmid=33446648 |pmc=7809060 |quote=Accordingly, designing and fabricating efficient PDRC with sufficiently high solar reflectance (𝜌¯solar) (λ ~ 0.3–2.5 μm) to minimize solar heat gain and simultaneously strong LWIR thermal emittance (ε¯LWIR) to maximize radiative heat loss is highly desirable. When the incoming radiative heat from the Sun is balanced by the outgoing radiative heat emission, the temperature of the Earth can reach its steady state. }}</ref><ref name=":4422">{{Cite journal |last1=Zevenhovena |first1=Ron |last2=Fält |first2=Martin |date=June 2018 |title=Radiative cooling through the atmospheric window: A third, less intrusive geoengineering approach |url=https://www.sciencedirect.com/science/article/abs/pii/S0360544218304936 |journal=Energy |volume=152 |quote= |via=Elsevier Science Direct}}</ref> When imagined on a worldwide scale, this cooling method has been proposed as a way to slow and even reverse [[Earth:Global warming|global warming]], with some estimates proposing a global surface area coverage of 1-2% to balance global heat fluxes.<ref name=":032">{{Cite journal |last=Munday |first=Jeremy |date=2019 |title=Tackling Climate Change through Radiative Cooling |journal=Joule |volume=3 |issue=9 |pages=2057–2060 |doi=10.1016/j.joule.2019.07.010 |s2cid=201590290 |quote=If only 1%–2% of the Earth’s surface were instead made to radiate at this rate rather than its current average value, the total heat fluxes into and away from the entire Earth would be balanced and warming would cease. |doi-access=free }}</ref><ref name=":442">{{Cite journal |last1=Zevenhovena |first1=Ron |last2=Fält |first2=Martin |date=June 2018 |title=Radiative cooling through the atmospheric window: A third, less intrusive geoengineering approach |url=https://www.sciencedirect.com/science/article/abs/pii/S0360544218304936 |journal=Energy |volume=152 |quote=With 100 W/m2 as a demonstrated passive cooling effect, a surface coverage of 0.3% would then be needed, or 1% of Earth's land mass surface. If half of it would be installed in urban, built areas which cover roughly 3% of the Earth's land mass, a 17% coverage would be needed there, with the remainder being installed in rural areas. |via=Elsevier Science Direct}}</ref>

===Communications===
{{Further|Engineering:Consumer IR}}

IR data transmission is also employed in short-range communication among computer peripherals and [[Social:Personal digital assistant|personal digital assistant]]s. These devices usually conform to standards published by [[Organization:Infrared Data Association|IrDA]], the Infrared Data Association. Remote controls and IrDA devices use infrared [[Engineering:Light-emitting diode|light-emitting diode]]s (LEDs) to emit infrared radiation that may be concentrated by a [[Physics:Lens (optics)|lens]] into a beam that the user aims at the detector. The beam is [[Engineering:On–off keying|modulated]], i.e. switched on and off, according to a code which the receiver interprets. Usually very near-IR is used (below 800&nbsp;nm) for practical reasons. This wavelength is efficiently detected by inexpensive [[Chemistry:Silicon|silicon]] [[Physics:Photodiode|photodiode]]s, which the receiver uses to convert the detected radiation to an [[Physics:Electric current|electric current]]. That electrical signal is passed through a [[High-pass filter|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 [[Engineering:Remote control|remote control]]s to command appliances.
Infrared remote control protocols like [[Engineering:RC-5|RC-5]], SIRC, are used to communicate with infrared.

Free space optical communication using infrared [[Physics:Laser|laser]]s 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."<ref>[http://www.ishn.com/articles/94815-dangers-of-overexposure-to-ultraviolet-infrared-and-high-energy-visible-light Dangers of Overexposure to ultraviolet, infrared and high-energy visible light | 2013-01-03]. ISHN. Retrieved on 2017-04-26.</ref>

Infrared lasers are used to provide the light for [[Engineering:Optical fiber|optical fiber]] communications systems. Infrared light with a wavelength around 1,330&nbsp;nm (least [[Physics:Dispersion (optics)|dispersion]]) or 1,550&nbsp;nm (best transmission) are the best choices for standard [[Chemistry:Silica|silica]] fibers.

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

===Spectroscopy===
[[Physics:Infrared spectroscopy|Infrared vibrational spectroscopy]] (see also [[Physics:Near-infrared spectroscopy|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., CH<sub>2</sub>) 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 [[Physics:Dipole|dipole]] in the molecule then it will absorb a [[Physics:Photon|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 [[Chemistry:Organic compound|organic compound]]s using light radiation from the mid-infrared, 4,000–400&nbsp;cm<sup>−1</sup>. 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&nbsp;cm<sup>−1</sup>). The unit for expressing radiation in this application, cm<sup>−1</sup>, is the spectroscopic [[Physics:Wavenumber|wavenumber]]. It is the frequency divided by the speed of light in vacuum.

===Thin film metrology===
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 [[Physics:Forouhi–Bloomer model|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===
[[File:NOAA Shares First Infrared Imagery from GOES-17 (43904870711).gif|thumb|left|IR satellite picture of cumulonimbus clouds over the Great Plains of the United States.]]
[[Engineering:Weather satellite|Weather satellite]]s 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&nbsp;μm (IR4 and IR5 channels).

Clouds with high and cold tops, such as [[Physics:Cyclone|cyclone]]s or [[Earth:Cumulonimbus cloud|cumulonimbus cloud]]s, 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 cloud such as stratus or fog can 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&nbsp;μm) and the near-infrared channel (1.58–1.64&nbsp;μm), low cloud 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 [[Earth:El Niño|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&nbsp;μm can be imaged by some weather satellites and shows the amount of moisture in the atmosphere.
{{clear}}

===Climatology===
[[File:Greenhouse-effect-t2.svg|thumb|right|upright=1.55|The [[Earth:Greenhouse effect|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 [[Earth:Global warming|global warming]], together with [[Physics:Solar radiation|solar radiation]].

A [[Physics:Pyrgeometer|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&nbsp;μm and 50&nbsp;μm.

===Astronomy===
{{Main|Astronomy:Infrared astronomy|Astronomy:Far-infrared astronomy}}
[[File:ESO - Beta Pictoris planet finally imaged (by).jpg|thumb|[[Astronomy:Beta Pictoris|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&nbsp;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 [[Chemistry:Helium|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 [[Earth:Infrared window|atmospheric window]]s. 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 [[Astronomy:Molecular cloud|molecular cloud]]s 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 [[Astronomy:Protostar|protostar]]s 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 [[Astronomy:Planet|planet]]s 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 [[Astronomy:Redshift|redshift]] will have the peak portion of their spectrum shifted toward longer wavelengths, so they are more readily observed in the infrared.<ref name="ir_astronomy" />

===Cleaning===
[[Infrared cleaning]] is a technique used by some [[Motion picture film scanner|motion picture film scanner]]s, [[Engineering:Film scanner|film scanner]]s 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|inpainting]].<ref>[https://web.archive.org/web/20110107163528/http://motion.kodak.com/US/en/motion/Products/Lab_And_Post_Production/dice.htm Digital ICE]. kodak.com</ref>

===Art conservation and analysis===
[[File:Infrared reflectograms of Mona Lisa.jpg|thumb|left|upright=0.8|An infrared reflectogram of ''Mona Lisa'' by Leonardo da Vinci]]
[[File:Infrared reflectography-en.svg|frameless|right|upright=0.9]]
Infrared reflectography<ref>{{cite web |url=http://www.sensorsinc.com/artanalysis.html |title=IR Reflectography for Non-destructive Analysis of Underdrawings in Art Objects |publisher=Sensors Unlimited, Inc. |access-date=2009-02-20 |archive-date=2008-12-08 |archive-url=https://web.archive.org/web/20081208052302/http://www.sensorsinc.com/artanalysis.html |url-status=dead }}</ref> 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.<ref>{{cite web |url=http://www.clevelandart.org/exhibcef/ConsExhib/html/grien.html |title=The Mass of Saint Gregory: Examining a Painting Using Infrared Reflectography |publisher=The Cleveland Museum of Art |access-date=2009-02-20 |archive-url=https://web.archive.org/web/20090113225836/http://www.clevelandart.org/exhibcef/ConsExhib/html/grien.html |archive-date=2009-01-13 |url-status=dead }}</ref> Reflectography often reveals the artist's use of [[Chemistry:Carbon black|carbon black]], which shows up well in reflectograms, as long as it has not also been used in the ground underlying the whole painting.
<!--this is somewhat pointless without an accompanying I.R.:  Among many other changes in the Arnolfini Portrait of 1434 (left), the man's face was originally higher by about the height of his eye; the woman's was higher, and her eyes looked more to the front. Each of his feet was underdrawn in one position, painted in another, and then overpainted in a third. These alterations are seen in infrared reflectograms.<ref>National Gallery Catalogues: The Fifteenth Century Netherlandish Paintings by Lorne Campbell, 1998, {{ISBN|1-85709-171-X}}, {{OL|392219M}}, {{OCLC|40732051}}, {{LCCN|98066510}}, (also titled ''The Fifteenth Century Netherlandish Schools''){{Page needed|date=September 2010}}.</ref>-->

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.<ref>[http://colourlex.com/project/ir-reflectography/ Infrared reflectography in analysis of paintings] at ColourLex.</ref> 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.<ref>{{cite web |url=http://idp.bl.uk/pages/technical_resources.a4d |title=International Dunhuang Project An Introduction to digital infrared photography and its application within IDP |publisher=Idp.bl.uk |access-date=2011-11-08 |archive-date=2008-12-02 |archive-url=https://web.archive.org/web/20081202000830/http://idp.bl.uk/pages/technical_resources.a4d |url-status=dead }}</ref> Carbon black used in ink can show up extremely well.

===Biological systems===
{{further|Physics:Infrared sensing in snakes}}
[[File:wiki snake eats mouse.jpg|thumb|Thermographic image of a snake eating a mouse]]
<!-- [[File:wiki bat.jpg|thumb|Thermographic image of a fruit bat.]] -->
The [[Biology:Crotalinae|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.<ref>{{Cite journal|title=Thermal Modeling of Snake Infrared Reception: Evidence for Limited Detection Range |journal=Journal of Theoretical Biology |volume=209 |issue=2 |pages=201–211 |year=2001 |doi=10.1006/jtbi.2000.2256 |pmid=11401462|last1=Jones|first1=B.S.|last2=Lynn|first2=W.F.|last3=Stone|first3=M.O. |bibcode=2001JThBi.209..201J |url=https://zenodo.org/record/1229918 }}</ref><ref>{{Cite journal|title=Biological Thermal Detection: Micromechanical and Microthermal Properties of Biological Infrared Receptors |journal=Biomacromolecules |volume=3 |issue=1 |pages=106–115 |year=2002 |doi=10.1021/bm015591f |pmid=11866562|last1=Gorbunov|first1=V.|last2=Fuchigami|first2=N.|last3=Stone|first3=M.|last4=Grace|first4=M.|last5=Tsukruk|first5=V. V.|s2cid=21737304 }}</ref>

Other organisms that have thermoreceptive organs are pythons (family [[Biology:Pythonidae|Pythonidae]]), some boas (family [[Biology:Boidae|Boidae]]), the Common Vampire Bat (''Desmodus rotundus''), a variety of jewel beetles (''[[Biology:Melanophila acuminata|Melanophila acuminata]]''),<ref name=Evans>{{Cite journal|last=Evans |first=W.G. |title=Infrared receptors in ''Melanophila acuminata'' De Geer |journal=Nature |volume=202 |page=211 |year=1966 |doi=10.1038/202211a0|pmid=14156319 |bibcode = 1964Natur.202..211E |issue=4928|s2cid=2553265 |doi-access=free }}</ref> darkly pigmented butterflies (''[[Biology:Pachliopta aristolochiae|Pachliopta aristolochiae]]'' and ''Troides rhadamantus plateni''), and possibly blood-sucking bugs (''[[Biology:Triatoma infestans|Triatoma infestans]]'').<ref>{{Cite journal |title=Biological infrared imaging and sensing |journal=Micrometre |year=2002 |volume=33 |issue=2 |pages=211–225 |doi=10.1016/S0968-4328(01)00010-5 |pmid=11567889 |last1=Campbell |first1=Angela L. |last2=Naik |first2=Rajesh R. |last3=Sowards |first3=Laura |last4=Stone |first4=Morley O.|url=https://zenodo.org/record/1260182 }}</ref>

Some fungi like ''[[Biology:Venturia inaequalis|Venturia inaequalis]]'' require near-infrared light for ejection.<ref>{{Cite journal|last=Brook|first=P. J.|date=26 April 1969|title=Stimulation of Ascospore Release in Venturia inaequalis by Far Red Light|journal=Nature|language=En|volume=222|issue=5191|pages=390–392|doi=10.1038/222390a0|issn=0028-0836|bibcode=1969Natur.222..390B|s2cid=4293713}}</ref>

Although near-infrared vision (780–1,000&nbsp;nm) has long been deemed impossible due to noise in visual pigments,<ref name="Meuthen et al.">{{Cite journal|title=Visual prey detection by near-infrared cues in a fish|journal=Naturwissenschaften |year=2012 |doi=10.1007/s00114-012-0980-7|last1=Meuthen|first1=Denis|last2=Rick|first2=Ingolf P.|last3=Thünken|first3=Timo|last4=Baldauf|first4=Sebastian A.|volume=99|issue=12|pages=1063–6|pmid=23086394|bibcode = 2012NW.....99.1063M |s2cid=4512517 }}</ref> sensation of near-infrared light was reported in the common carp and in three cichlid species.<ref name="Meuthen et al." /><ref>{{Cite journal|title= Postural control in tilapia under microgravity and the near infrared irradiated conditions |author1=Endo, M. |author2=Kobayashi R. |author3=Ariga, K. |author4=Yoshizaki, G. |author5=Takeuchi, T. |journal= Nippon Suisan Gakkaishi |volume=68 |pages=887–892| year=2002|doi= 10.2331/suisan.68.887|issue= 6 |doi-access=free }}</ref><ref>{{Cite journal|title= Sensitivity of tilapia to infrared light measured using a rotating striped drum differs between two strains |author1=Kobayashi R. |author2=Endo, M. |author3=Yoshizaki, G. |author4=Takeuchi, T. |journal= Nippon Suisan Gakkaishi |volume=68 |pages=646–651| year=2002|doi= 10.2331/suisan.68.646|issue= 5 |doi-access=free }}</ref><ref>{{Cite journal|title= The eyes of the common carp and Nile tilapia are sensitive to near-infrared |doi=10.1111/j.1444-2906.2005.00971.x |journal= Fisheries Science |volume=71 |pages=350–355| year=2005|last1= Matsumoto|first1= Taro|last2= Kawamura|first2= Gunzo|issue= 2 |s2cid=24556470 }}</ref><ref name="Shcherbakov et al.">{{Cite journal|title= Near-infrared orientation of Mozambique tilapia ''Oreochromis mossambicus'' |journal= Zoology |volume=115 |pages=233–238| year=2012 | doi=10.1016/j.zool.2012.01.005|last1= Shcherbakov|first1= Denis|last2= Knörzer|first2= Alexandra|last3= Hilbig|first3= Reinhard|last4= Haas|first4= Ulrich|last5= Blum|first5= Martin|issue= 4|pmid= 22770589}}</ref> Fish use NIR to capture prey<ref name="Meuthen et al." /> and for phototactic swimming orientation.<ref name="Shcherbakov et al." /> NIR sensation in fish may be relevant under poor lighting conditions during twilight<ref name="Meuthen et al." /> and in turbid surface waters.<ref name="Shcherbakov et al." />

===Photobiomodulation===
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.<ref>{{cite journal | last1 = Hargate | first1 = G | title = A randomised double-blind study comparing the effect of 1072-nm light against placebo for the treatment of herpes labialis | journal = Clinical and Experimental Dermatology | volume = 31 | issue = 5 | pages = 638–41 | year = 2006 | pmid = 16780494 | doi = 10.1111/j.1365-2230.2006.02191.x | s2cid = 26977101 }}</ref> Research projects include work on central nervous system healing effects via cytochrome c oxidase upregulation and other possible mechanisms.<ref>{{cite journal |vauthors=Desmet KD, Paz DA, Corry JJ, Eells JT, Wong-Riley MT, Henry MM, Buchmann EV, Connelly MP, Dovi JV, Liang HL, Henshel DS, Yeager RL, Millsap DS, Lim J, Gould LJ, Das R, Jett M, Hodgson BD, Margolis D, Whelan HT | date = May 2006 | title = Clinical and experimental applications of NIR-LED photobiomodulation | journal = Photomedicine and Laser Surgery | volume = 24 | issue = 2 | pages = 121–8 | pmid = 16706690 | doi = 10.1089/pho.2006.24.121 | s2cid = 22442409 | url = https://epublications.marquette.edu/dentistry_fac/3 }}</ref>

===Health hazards===
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.<ref>{{cite book|author=Rosso, Monona l|title=The Artist's Complete Health and Safety Guide|url=https://books.google.com/books?id=E7-9unTgJrwC&pg=PA33|year=2001|publisher=Allworth Press|isbn=978-1-58115-204-3|pages=33–}}</ref>

==Scientific history==
The discovery of infrared radiation is ascribed to William Herschel, the [[Astronomy:Astronomer|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 [[Astronomy:Sun|sun]] and detected the infrared, beyond the [[Red|red]] part of the spectrum, through an increase in the temperature recorded on a [[Physics:Thermometer|thermometer]]. He was surprised at the result and called them "Calorific Rays".<ref>{{cite journal |last=Herschel |first=William |title=Experiments on the refrangibility of the invisible rays of the Sun |journal=Philosophical Transactions of the Royal Society of London |year=1800 |volume=90 |pages=284–292 |url=https://babel.hathitrust.org/cgi/pt?id=pst.000054592520;view=1up;seq=358 |jstor=107057 |doi=10.1098/rstl.1800.0015|doi-access=free }}</ref><ref>{{cite web |url=http://coolcosmos.ipac.caltech.edu/cosmic_classroom/classroom_activities/herschel_bio.html |title=Herschel Discovers Infrared Light |website=Coolcosmos.ipac.caltech.edu |access-date=2011-11-08 |url-status=dead |archive-url=https://web.archive.org/web/20120225094516/http://coolcosmos.ipac.caltech.edu/cosmic_classroom/classroom_activities/herschel_bio.html |archive-date=2012-02-25 }}</ref> The term "infrared" did not appear until late 19th century.<ref>In 1867, French physicist [[Biography:Edmond Becquerel|Edmond Becquerel]] coined the term {{lang|fr|infra-rouge}} (infra-red):
* {{cite book |last1=Becquerel |first1=Edmond |title=La Lumiere: Ses causes et ses effets |trans-title=Light: Its causes and effects |date=1867 |publisher=Didot Frères, Fils et Cie. |location=Paris, France |pages=141–145 |url=https://books.google.com/books?id=SyWP1zBJiv0C&pg=PA141 |language=fr}}
The word {{lang|fr|infra-rouge}} was translated into English as "infrared" in 1874, in a translation of an article by Vignaud Dupuy de Saint-Florent (1830–1907), an engineer in the French army, who attained the rank of lieutenant colonel and who pursued photography as a pastime.
* {{cite journal |last1=de Saint-Florent |title=Photography in natural colours |journal=The Photographic News |date=10 April 1874 |volume=18 |pages=175–176 |url=https://babel.hathitrust.org/cgi/pt?id=nyp.33433060399015;view=1up;seq=188}}  From p. 176:  "As to the infra-red rays, they may be absorbed by means of a weak solution of sulphate of copper, ..."
See also:
* {{cite journal |last1=Rosenberg |first1=Gary |title=Letter to the Editors: Infrared dating |journal=American Scientist |date=2012 |volume=100 |issue=5 |page=355 |url=https://www.americanscientist.org/article/infrared-dating}}</ref> An [[Physics:Pictet's experiment|earlier experiment in 1790]] by Marc-Auguste Pictet demonstrated the reflection and focusing of radiant heat via mirrors in the absence of visible light.<ref>{{Cite book |last=Chang |first=Hasok |title=Inventing temperature: measurement and scientific progress |date=2007 |publisher=Oxford University Press |isbn=978-0-19-533738-9 |edition=1. issued as paperback |series=Oxford studies in philosophy of science |location=Oxford |pages=166–167}}</ref>

Other important dates include:<ref name="Miller"/>
[[File:William Herschel01.jpg|thumb|upright|Infrared radiation was discovered in 1800 by William Herschel.]]
* 1830: Leopoldo Nobili made the first [[Physics:Thermopile|thermopile]] IR detector.<ref>See:
* {{cite journal |last1=Nobili |first1=Leopoldo |title=Description d'un thermo-multiplicateur ou thermoscope électrique |journal=Bibliothèque Universelle |date=1830 |volume=44 |pages=225–234 |url=https://babel.hathitrust.org/cgi/pt?id=pst.000052859885;view=1up;seq=237 |trans-title=Description of a thermo-multiplier or electric thermoscope |language=fr}}
* {{cite journal |last1=Nobili |last2=Melloni |title=Recherches sur plusieurs phénomènes calorifiques entreprises au moyen du thermo-multiplicateur |journal=Annales de Chimie et de Physique |date=1831 |volume=48 |pages=198–218 |url=https://babel.hathitrust.org/cgi/pt?id=uva.x002487856;view=1up;seq=202 |series=2nd series |trans-title=Investigations of several heat phenomena undertaken via a thermo-multiplier |language=fr}}
* {{cite book |last1=Vollmer |first1=Michael |last2=Möllmann |first2=Klaus-Peter |title=Infrared Thermal Imaging: Fundamentals, Research and Applications |date=2010 |publisher=Wiley-VCH |location=Berlin, Germany |pages=1–67 |edition=2nd |url=https://books.google.com/books?id=ClU_DwAAQBAJ&pg=SA1-PA67 |isbn=9783527693290}}</ref>
* 1840: [[Biography:John Herschel|John Herschel]] produces the first thermal image, called a thermogram.<ref>{{cite journal |last1=Herschel |first1=John F.&nbsp;W. |title=On chemical action of rays of solar spectrum on preparation of silver and other substances both metallic and nonmetallic and on some photographic processes |journal=Philosophical Transactions of the Royal Society of London |date=1840 |volume=130 |pages=1–59 |url=https://babel.hathitrust.org/cgi/pt?id=pst.000054592933;view=1up;seq=47 |bibcode=1840RSPT..130....1H |doi=10.1098/rstl.1840.0002|s2cid=98119765 }} The term "thermograph" is coined on p. 51: " ... I have discovered a process by which the calorific rays in the solar spectrum are made to leave their impress on a surface properly prepared for the purpose, so as to form what may be called a thermograph of the spectrum, ... ".</ref>
* 1860: [[Biography:Gustav Kirchhoff|Gustav Kirchhoff]] formulated the [[Physics:Kirchhoff's law of thermal radiation|blackbody theorem]] <math>E = J(T, n)</math>.<ref>See:
* {{cite journal |last1=Kirchhoff |title=Ueber den Zusammenhang von Emission und Absorption von Licht und Warme |journal=Monatsberichte der Königlich-Preussischen Akademie der Wissenschaften zu Berlin (Monthly Reports of the Royal Prussian Academy of Philosophy in Berlin) |date=1859 |pages=783–787 |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015049219333;view=1up;seq=811 |trans-title=On the relation between emission and absorption of light and heat |language=de}}
* {{cite journal |last1=Kirchhoff |first1=G. |title=Ueber das Verhältnis zwischen dem Emissionsvermögen und dem Absorptionsvermögen der Körper für Wärme und Licht |journal=Annalen der Physik und Chemie |date=1860 |volume=109 |issue=2 |pages=275–301 |url=https://babel.hathitrust.org/cgi/pt?id=umn.31951d00326548g;view=1up;seq=291 |trans-title=On the relation between bodies' emission capacity and absorption capacity for heat and light |language=de|bibcode=1860AnP...185..275K |doi=10.1002/andp.18601850205 |doi-access=free }}
* English translation: {{cite journal |last1=Kirchhoff |first1=G. |title=On the relation between the radiating and absorbing powers of different bodies for light and heat |journal=Philosophical Magazine |date=1860 |volume=20 |pages=1–21 |url=https://babel.hathitrust.org/cgi/pt?id=pst.000068485634;view=1up;seq=19 |series=4th series}}</ref>
* 1873: [[Biography:Willoughby Smith|Willoughby Smith]] discovered the photoconductivity of [[Chemistry:Selenium|selenium]].<ref>See:
* {{cite journal |last1=Smith |first1=Willoughby |title=The action of light on selenium |journal=Journal of the Society of Telegraph Engineers |date=1873 |volume=2 |issue=4 |pages=31–33 |url=https://babel.hathitrust.org/cgi/pt?id=uiug.30112007449892;view=1up;seq=67 |doi=10.1049/jste-1.1873.0023}}
* {{cite journal |last1=Smith |first1=Willoughby |title=Effect of light on selenium during the passage of an electric current |journal=Nature |date=20 February 1873 |volume=7 |issue=173 |page=303 |url=https://babel.hathitrust.org/cgi/pt?id=uiug.30112007449892;view=1up;seq=67 |doi=10.1038/007303e0|bibcode=1873Natur...7R.303. |doi-access=free }}</ref>
* 1878: [[Biography:Samuel Pierpont Langley|Samuel Pierpont Langley]] invents the first [[Physics:Bolometer|bolometer]], a device which is able to measure small temperature fluctuations, and thus the power of far infrared sources.<ref>See:
* {{cite journal |last1=Langley |first1=S.&nbsp;P. |title=The bolometer |journal=Proceedings of the American Metrological Society |date=1880 |volume=2 |pages=184–190 |url=https://babel.hathitrust.org/cgi/pt?id=nyp.33433090766035;view=1up;seq=282}}
* {{cite journal |last1=Langley |first1=S.&nbsp;P. |title=The bolometer and radiant energy |journal=Proceedings of the American Academy of Arts and Sciences |date=1881 |volume=16 |pages=342–358 |url=https://babel.hathitrust.org/cgi/pt?id=hvd.32044106428089;view=1up;seq=360 |doi=10.2307/25138616 |jstor=25138616}}</ref>
* 1879: [[Physics:Stefan–Boltzmann law|Stefan–Boltzmann law]] formulated empirically that the power radiated by a blackbody is proportional to ''T''<sup>4</sup>.<ref>{{cite journal |last1=Stefan |first1=J. |title=Über die Beziehung zwischen der Wärmestrahlung und der Temperatur |journal=Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften [Wien]: Mathematisch-naturwissenschaftlichen Classe (Proceedings of the Imperial Academy of Philosophy [in Vienna]: Mathematical-scientific Class) |date=1879 |volume=79 |pages=391–428 |url=https://babel.hathitrust.org/cgi/pt?id=hvd.32044093294874;view=1up;seq=419 |trans-title=On the relation between heat radiation and temperature |language=de}}</ref>
* 1880s and 1890s: Lord Rayleigh and [[Biography:Wilhelm Wien|Wilhelm Wien]] solved part of the blackbody equation, but both solutions diverged in parts of the electromagnetic spectrum. This problem was called the "[[Physics:Ultraviolet catastrophe|ultraviolet catastrophe]] and infrared catastrophe".<ref>See:
* {{cite journal |last1=Wien |first1=Willy |title=Ueber die Energieverteilung im Emissionsspektrum eines schwarzen Körpers |journal=Annalen der Physik und Chemie |date=1896 |volume=58 |pages=662–669 |url=https://babel.hathitrust.org/cgi/pt?id=wu.89048352850;view=1up;seq=676 |series=3rd series |trans-title=On the energy distribution in the emission spectrum of a black body |language=de}}
* English translation: {{cite journal |last1=Wien |first1=Willy |title=On the division of energy in the emission-spectrum of a black body |journal=Philosophical Magazine |date=1897 |volume=43 |issue=262 |pages=214–220 |doi=10.1080/14786449708620983 |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015024088695;view=1up;seq=226 |series=5th series}}</ref>
* 1892: Willem Henri Julius published infrared spectra of 20 organic compounds measured with a bolometer in units of angular displacement.<ref>{{Cite book |url=https://books.google.com/books?id=K1AVAAAAIAAJ&q=Bolometrisch+Ondersoek+van+Absorptiespectra&pg=PA44 |title=Bolometrisch onderzoek van absorptiespectra |last=Julius |first=Willem Henri |date=1892 |publisher=J. Müller |language=nl}}</ref>
* 1901: [[Biography:Max Planck|Max Planck]] published the [[Physics:Planck's law|blackbody equation]] and theorem. He solved the problem by quantizing the allowable energy transitions.<ref>See:
* {{cite journal |last1=Planck |first1=M. |title=Ueber eine Verbesserung der Wien'schen Spectralgleichung |journal=Verhandlungen der Deutschen Physikalischen Gesellschaft |date=1900 |volume=2 |pages=202–204 |url=https://babel.hathitrust.org/cgi/pt?id=coo.31924056107224;view=1up;seq=516 |trans-title=On an improvement of Wien's spectral equation |language=de}}
* {{cite journal |last1=Planck |first1=M. |title=Zur Theorie des Gesetzes der Energieverteilung im Normalspectrum |journal=Verhandlungen der Deutschen Physikalischen Gesellschaft |date=1900 |volume=2 |pages=237–245 |url=https://babel.hathitrust.org/cgi/pt?id=coo.31924056107224;view=1up;seq=551 |trans-title=On the theory of the law of energy distribution in the normal spectrum |language=de}}
* {{cite journal |last1=Planck |first1=Max |title=Ueber das Gesetz der Energieverteilung im Normalspectrum |journal=Annalen der Physik |date=1901 |volume=4 |issue=3 |pages=553–563 |url=https://babel.hathitrust.org/cgi/pt?id=coo.31924066378310;view=1up;seq=585 |series=4th series |trans-title=On the law of energy distribution in the normal spectrum |language=de |bibcode=1901AnP...309..553P |doi=10.1002/andp.19013090310|doi-access=free }}</ref>
* 1905: [[Biography:Albert Einstein|Albert Einstein]] developed the theory of the [[Physics:Photoelectric effect|photoelectric effect]].<ref>See:
* {{cite journal |last1=Einstein |first1=A. |title=Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt |journal=Annalen der Physik |date=1905 |volume=17 |issue=6 |pages=132–148 |url=https://archive.org/stream/annalenderphysi108unkngoog#page/n150/mode/2up |series=4th series |trans-title=On heuristic viewpoint concerning the production and transformation of light |language=de |bibcode=1905AnP...322..132E |doi=10.1002/andp.19053220607|doi-access=free }}
* English translation: {{cite journal |last1=Arons |first1=A. B. |last2=Peppard |first2=M. B. |title=Einstein's proposal of the photon concept—a translation of the Annalen der Physik paper of 1905 |journal=American Journal of Physics |date=1965 |volume=33 |issue=5 |pages=367–374 |bibcode=1965AmJPh..33..367A |doi=10.1119/1.1971542 |s2cid=27091754 }} Available at [https://web.archive.org/web/20141121114532/http://www.esfm2005.ipn.mx/ESFM_Images/paper1.pdf Wayback Machine].</ref>
* 1905–1908: [[Biography:William Coblentz|William Coblentz]] published infrared spectra in units of wavelength (micrometers) for several chemical compounds in ''Investigations of Infra-Red Spectra''.<ref>{{Cite book |url=https://books.google.com/books?id=qVUoHbyKbDsC&q=coblentz,+william+weber&pg=PP7 |title=Investigations of Infra-red Spectra: Part I, II |last=Coblentz |first=William Weber |date=1905 |publisher=Carnegie institution of Washington }}</ref><ref>{{Cite book |url=https://archive.org/details/investigationsi01coblgoog |title=Investigations of Infra-red Spectra: Part III, IV |last=Coblentz |first=William Weber |date=1905 |publisher=Washington, D.C., Carnegie institution of Washington |others=University of Michigan }}</ref><ref>{{Cite book |url=https://archive.org/details/investigationsof03coblrich |title=Investigations of Infra-red Spectra: Part V, VI, VII |last=Coblentz |first=William Weber |date=August 1905 |publisher=Washington, D.C. : Carnegie Institution of Washington |others=University of California Libraries }}</ref>
* 1917: [[Biography:Theodore Case|Theodore Case]] developed the thallous sulfide detector, which helped produce the first [[Physics:Infrared search and track|infrared search and track]] device able to detect aircraft at a range of one mile (1.6&nbsp;km).
* 1935: Lead salts – early missile guidance in [[Social:World War II|World War II]].
* 1938: Yeou Ta predicted that the pyroelectric effect could be used to detect infrared radiation.<ref>{{cite book|url=https://books.google.com/books?id=OTXABAAAQBAJ&q=yeou+ta+1938&pg=PA406|title=Waste Energy Harvesting: Mechanical and Thermal Energies|page=406|publisher=Springer Science & Business Media|year=2014|isbn=9783642546341|access-date=2020-01-07}}</ref>
* 1945: The [[Physics:Zielgerät 1229|Zielgerät 1229]] "Vampir" infrared weapon system was introduced as the first portable infrared device for military applications.
* 1952: [[Biography:Heinrich Welker|Heinrich Welker]] grew synthetic [[Chemistry:Indium antimonide|InSb]] crystals.
* 1950s and 1960s: Nomenclature and radiometric units defined by Fred Nicodemenus, G.&nbsp;J. Zissis and R. Clark; [[Biography:Robert Clark Jones|Robert Clark Jones]] defined ''D''*.
* 1958: W.&nbsp;D. Lawson (Royal Radar Establishment in Malvern) discovered IR detection properties of [[Physics:Mercury cadmium telluride|Mercury cadmium telluride]] (HgCdTe).<ref name=Reine>{{cite journal|url=https://link.springer.com/content/pdf/10.1007/s11664-015-3737-1.pdf|title=Interview with Paul W. Kruse on the Early History of HgCdTe (1980)|author=Marion B. Reine|year=2015|access-date=2020-01-07|doi=10.1007/s11664-015-3737-1|s2cid=95341284}}</ref>
* 1958: Falcon and [[Engineering:AIM-9 Sidewinder|Sidewinder]] missiles were developed using infrared technology.
* 1960s: Paul Kruse and his colleagues at [[Company:Honeywell|Honeywell Research Center]] demonstrate the use of HgCdTe as an effective [[Chemistry:Chemical compound|compound]] for infrared detection.<ref name=Reine/>
* 1962: J. Cooper demonstrated pyroelectric detection.<ref>{{cite journal|title=A fast-response pyroelectric thermal detector
|year=1962|author=J Cooper|journal = Journal of Scientific Instruments|volume = 39|issue = 9|pages = 467–472|doi=10.1088/0950-7671/39/9/308|bibcode = 1962JScI...39..467C}}</ref>
* 1964: W.&nbsp;G. Evans discovered infrared thermoreceptors in a pyrophile beetle.<ref name=Evans/>
* 1965: First IR handbook; first commercial imagers (Barnes, Agema (now part of FLIR Systems Inc.)); Richard Hudson's landmark text; F4 TRAM FLIR by [[Company:Hughes Aircraft Company|Hughes]]; phenomenology pioneered by Fred Simmons and A.&nbsp;T. Stair; U.S. Army's night vision lab formed (now Night Vision and Electronic Sensors Directorate (NVESD)), and Rachets develops detection, recognition and identification modeling there.
* 1970: [[Biography:Willard Boyle|Willard Boyle]] and [[Biography:George E. Smith|George E. Smith]] proposed CCD at [[Organization:Bell Labs|Bell Labs]] for picture phone.
* 1973: Common module program started by NVESD.<ref>{{cite web|url=https://c5isr.ccdc.army.mil/inside_c5isr_center/nvesd/history/|title=History of Army Night Vision|publisher=C5ISR Center|access-date=2020-01-07}}{{Dead link|date=November 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>
* 1978: Infrared imaging astronomy came of age, observatories planned, [[Organization:NASA Infrared Telescope Facility|IRTF]] on Mauna Kea opened; 32&nbsp;×&nbsp;32 and 64&nbsp;×&nbsp;64 arrays produced using InSb, HgCdTe and other materials.
* 2013: On 14 February, researchers developed a neural implant that gives [[Biology:Rat|rat]]s the ability to sense infrared light, which for the first time provides living creatures with new abilities, instead of simply replacing or augmenting existing abilities.<ref>{{cite magazine |url=https://www.wired.co.uk/news/archive/2013-02/14/implant-gives-rats-sixth-sense-for-infrared-light |title=Implant gives rats sixth sense for infrared light |magazine=Wired UK |date=14 February 2013 |access-date=14 February 2013 }}</ref>

==See also==
{{Columns-list|colwidth=30em|
* [[Astronomy:Black-body radiation|Black-body radiation]]
* [[Physics:Infrared non-destructive testing of materials|Infrared non-destructive testing of materials]]
* [[Physics:Solar cell#Infrared solar cells|Infrared solar cells]]
* [[Engineering:Infrared thermometer|Infrared thermometer]]
* [[People counter]]
* [[Engineering:Index of infrared articles|Index of infrared articles]]
}}

==Notes==
{{notelist-lr}}

==References==
{{Reflist}}

==External links==
* [http://www.omega.com/literature/transactions/volume1/historical1.html Infrared: A Historical Perspective] (Omega Engineering)
* [http://www.irda.org/ Infrared Data Association], a standards organization for infrared data interconnection
* [http://yengal-marumugam.blogspot.com/2011/06/sirc-part-i-basics.html SIRC Protocol ]
* [http://www.ocinside.de/html/modding/usb_ir_receiver/usb_ir_receiver.html How to build a USB infrared receiver to control PC's remotely]
* [https://web.archive.org/web/20060114051647/http://imagers.gsfc.nasa.gov/ems/infrared.html Infrared Waves]: detailed explanation of infrared light. (NASA)
* [https://archive.org/details/philtrans08733349 Herschel's original paper from 1800 announcing the discovery of infrared light]
* [http://www.thethermograpiclibrary.org/index.php/Cat%C3%A9gorie:Library The thermographic's library], collection of thermogram
* [http://colourlex.com/project/ir-reflectography/ Infrared reflectography in analysis of paintings] at ColourLex
* Molly Faries, [http://www.nap.edu/read/11413/chapter/8 Techniques and Applications – Analytical Capabilities of Infrared Reflectography: An Art Historian s Perspective], in Scientific Examination of Art: Modern Techniques in Conservation and Analysis, Sackler NAS Colloquium, 2005

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[[Category:Infrared| ]]
[[Category:Electromagnetic spectrum]]

{{Sourceattribution|Infrared}}