Editing Infrared (section)

==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 [[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 carry a large amount of energy. 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 |last=Sliney |first=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=[[Journal of the Optical Society of America]] | volume=66 |issue=4 |pages=339–341 |bibcode=1976JOSA...66..339S |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.}}</ref><ref name="LynchLivingston2001">{{Cite book |last=Lynch |first=David K. |url=https://books.google.com/books?id=4Abp5FdhskAC&pg=PA231 |title=Color and Light in Nature |last2=Livingston |first2=William Charles |publisher=Cambridge University Press |year=2001 |isbn=978-0-521-77504-5 |edition=2nd |location=Cambridge, UK |page=231 |quote=Limits of the eye's overall range of sensitivity extends from about 310 to 1,050 nanometers |access-date=12 October 2013 |archive-url=https://web.archive.org/web/20240529134755/https://books.google.com/books?id=4Abp5FdhskAC&pg=PA231#v=onepage&q&f=false |archive-date=29 May 2024 |url-status=live}}</ref><ref name="Saidman1933">{{Cite journal |last=Saidman |first=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 |url=http://visualiseur.bnf.fr/ark:/12148/bpt6k3148d |url-status=live |journal=[[Comptes rendus de l'Académie des sciences]] | language=fr |volume=196 |pages=1537–9 |archive-url=https://web.archive.org/web/20131024092515/http://visualiseur.bnf.fr/ark:/12148/bpt6k3148d |archive-date=24 October 2013 |access-date=3 July 2014}}</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 |isbn=978-1-4020-9252-7 |pages=21–22 |bibcode=2009uodm.book.....B}}</ref><ref name="RP-photonics">{{Cite web |title=Infrared Light |url=https://www.rp-photonics.com/infrared_light.html |url-status=live |archive-url=https://web.archive.org/web/20210801132547/https://www.rp-photonics.com/infrared_light.html |archive-date=1 August 2021 |access-date=20 July 2021 |website=RP Photonics Encyclopedia |publisher=RP Photonics}}</ref><ref>{{Cite web |date=2024-09-16 |title=Definition of NEAR-INFRARED |url=https://www.merriam-webster.com/dictionary/near-infrared |access-date=2024-10-02 |website=www.merriam-webster.com |language=en}}</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 [[Wien's displacement law]].<ref>{{Cite web |title=Peaks of Blackbody Radiation Intensity |url=http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/wien3.html |url-status=live |archive-url=https://web.archive.org/web/20110318195600/http://hyperphysics.phy-astr.gsu.edu/Hbase/quantum/wien3.html |archive-date=18 March 2011 |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;[[meV]]
| {{convert|3864|–|2070|K|C|lk=on|disp=br()}}
| Goes up to the wavelength of the first [[water absorption]] band, and commonly used in [[fiber optic]] telecommunication because of low attenuation losses in the SiO<sub>2</sub> glass ([[silica]]) medium. [[Image intensifier]]s are sensitive to this area of the spectrum; examples include [[night vision]] devices such as night vision goggles. [[Near-infrared spectroscopy]] is another common application.
|-
! Short-wavelength infrared
| SWIR, IR-B ''DIN''
| 1.4–3&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 [[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 |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 |url-status=live |archive-url=https://web.archive.org/web/20240922080442/https://www.rdworldonline.com/ |archive-date=September 22, 2024 |access-date=September 8, 2012 |work=[[R&D Magazine]] | at=rdmag.com}}</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 [[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 or moon or an infrared illuminator. This region is also called the "thermal 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 [[far-infrared laser]] and [[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 |title=Wavelength considerations |url=http://info.tuwien.ac.at/iflt/safety/section1/1_1_1.htm |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 |access-date=2007-10-18 |publisher=Instituts für Umform- und Hochleistungs}}</ref><ref>{{Cite web |last=CIE (International Commission on Illumination) |title=infrared radiation IR radiation IRR |url=https://cie.co.at/eilvterm/17-21-004 |url-status=live |archive-url=https://web.archive.org/web/20240922080552/https://cie.co.at/eilvterm/17-21-004 |archive-date=22 September 2024 |access-date=18 October 2022 |website=17-21-004}}</ref>
{| class="wikitable"
|-
! Abbreviation
! Wavelength
! Frequency
|-
| IR-A || {{val|780|–|1400|u=nm}}    || {{val|215|–|384|u=THz}}
|-
| IR-B || {{val|1400|–|3000|u=nm}}    || {{val|100|–|215|u=THz}}
|-
| IR-C || {{val|3|–|1000|u=μm}}       || {{val|0.3|–|100|u=THz}}
|}

=== ISO 20473 scheme ===

[[ISO]] 20473 specifies the following scheme:<ref name="ISO 20473">{{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 |title=Near, Mid and Far-Infrared |url=http://www.ipac.caltech.edu/Outreach/Edu/Regions/irregions.html |url-status=dead |archive-url=https://archive.today/20120529/http://www.ipac.caltech.edu/Outreach/Edu/Regions/irregions.html |archive-date=2012-05-29 |access-date=2007-04-04 |publisher=NASA IPAC}}</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;" | {{val|0.7|–|2.5|u=μm}}
|-
| align="left" | Mid-infrared
| style="text-align:center;" | MIR
| style="text-align:center;" | {{val|3|–|25|u=μm}}
|-
| align="left" | Far-infrared
| style="text-align:center;" | FIR
| style="text-align:center;" | above {{val|25|u=μm}}
|}

These divisions are not precise and can vary depending on the publication. The three regions are used for observation of different temperature ranges,<ref>{{Cite web |title=Near, Mid and Far-Infrared |url=https://www.icc.dur.ac.uk/~tt/Lectures/Galaxies/Images/Infrared/Regions/irregions.html |url-status=live |archive-url=https://web.archive.org/web/20240328203215/https://www.icc.dur.ac.uk/~tt/Lectures/Galaxies/Images/Infrared/Regions/irregions.html |archive-date=2024-03-28 |access-date=2024-03-28 |website=www.icc.dur.ac.uk}}</ref> and hence different environments in space.

The most common photometric system used in astronomy allocates capital [[Jhk|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 [[Infrared window|atmospheric windows]] and appear, for instance, in the titles of many [[Academic paper|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 [[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 [[indium antimonide]], InSb and [[mercury cadmium telluride]], HgCdTe, and partially by [[lead selenide]], PbSe).
* Long-wave infrared: 8 to 12, or 7 to 14&nbsp;μm (this is the atmospheric window covered by HgCdTe and [[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 [[silicon]] detectors are sensitive to about 1,050&nbsp;nm, while [[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 [[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 |last=Griffin |first=Donald R. |last2=Hubbard |first2=Ruth |last3=Wald |first3=George |year=1947 |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 |bibcode=1947JOSA...37..546G |doi=10.1364/JOSA.37.000546 |pmid=20256359}}</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 |title=Optical Fiber Communication: From Transmission to Networking |journal=IEEE Communications Magazine |volume=40 |issue=5 |pages=138–147 |doi=10.1109/MCOM.2002.1006983 |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
|-
| [[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 [[telecommunications network]]s. The S and L bands are based on less well established technology, and are not as widely deployed.