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Regions Within The Infrared
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.
Therefore, the infrared band is often subdivided into smaller sections.
Commonly used sub-division scheme
A commonly used sub-division scheme is:
NIR and SWIR is sometimes called "reflected infrared", whereas MWIR and LWIR is sometimes referred to as "thermal infrared". Due to the nature of the blackbody radiation curves, typical "hot" objects, such as exhaust pipes, often appear brighter in the MW compared to the same object viewed in the LW.
Division name | Abbreviation | Wavelength | Frequency | Photon energy | Temperature | Characteristics |
Near-infrared | NIR, IR-A DIN | 0.75–1.4 µm | 214–400 THz | 886–1653 meV | 3,864–2,070 K | Defined by water absorption,[clarification needed] and commonly used in fiber optic telecommunication because of low attenuation losses in the SiO2 glass (silica) medium. Image intensifiers are sensitive to this area of the spectrum; examples include night vision devices such as night vision goggles. Near-infrared spectroscopy is another common application. |
Short-wavelength infrared | SWIR, IR-B DIN | 1.4–3 µm | 100–214 THz | 413–886 meV | 2,070–966 K | Water absorption increases significantly at 1450 nm. The 1530 to 1560 nm range is the dominant spectral region for long-distance telecommunications. |
Mid-wavelength infrared | MWIR, IR-C DIN; MidIR. Also called intermediate infrared (IIR) | 3–8 µm | 37–100 THz | 155–413 meV | 966–362 K | In guided missile technology the 3–5 µm portion of this band is the atmospheric window in which the homing heads of passive IR 'heat seeking' missiles are designed to work, homing on to the Infrared signature of the target aircraft, typically the jet engine exhaust plume. This region is also known as thermal infrared. |
Long-wavelength infrared | LWIR, IR-C DIN | 8–15 µm | 20–37 THz | 83–155 meV | 362–193 K | 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". |
Far infrared | FIR | 15–1000 µm | 0.3–20 THz | 1.2–83 meV | 193–3 K | (see also far-infrared laser and far infrared) |
CIE division scheme
The International Commission on Illumination (CIE) recommended the division of infrared radiation into the following three bands:
Abbreviation | Wavelength | Frequency |
IR-A | 700 nm – 1400 nm | 215 THz – 430 THz |
IR-B | 1400 nm – 3000 nm | 100 THz – 215 THz |
IR-C | 3000 nm – 1 mm | 300 GHz – 100 THz |
ISO 20473 scheme
ISO 20473 specifies the following scheme:
Designation | Abbreviation | Wavelength |
Near-Infrared | NIR | 0.78–3 µm |
Mid-Infrared | MIR | 3–50 µm |
Far-Infrared | FIR | 50–1000 µm |
Astronomy division scheme
Astronomers typically divide the infrared spectrum as follows:
Designation | Abbreviation | Wavelength |
Near-Infrared | NIR | (0.7–1) to 5 µm |
Mid-Infrared | MIR | 5 to (25–40) µm |
Far-Infrared | FIR | (25–40) to (200–350) µm. |
These divisions are not precise and can vary depending on the publication. The three regions are used for observation of different temperature ranges, and hence different environments in space.The most common photometric system used in astronomy allocates capital letters to different spectral regions according to filters used; I, J, H, and K cover the near-infrared wavelengths; L, M, N, and Q refer to the mid-infrared region. These letters are commonly understood in reference to atmospheric windows and appear, for instance, in the titles of many papers.
Sensor response division scheme
A third scheme divides up the band based on the response of various detectors:
*Near-infrared: from 0.7 to 1.0 µm (from the approximate end of the response of the human eye to that of silicon).
*Short-wave infrared: 1.0 to 3 µm (from the cut-off of silicon to that of the MWIR atmospheric window). InGaAs covers to about 1.8 µm; the less sensitive lead salts cover this region.
*Mid-wave infrared: 3 to 5 µm (defined by the atmospheric window and covered by Indium antimonide [InSb] and HgCdTe and partially by lead selenide [PbSe]).
*Long-wave infrared: 8 to 12, or 7 to 14 µm (this is the atmospheric window covered by HgCdTe and microbolometers).
*Very-long wave infrared (VLWIR) (12 to about 30 µm, covered by doped silicon).
Near-infrared is the region closest in wavelength to the radiation detectable by the human eye. mid- and far-infrared are progressively further from the visible spectrum. Other definitions follow different physical mechanisms (emission peaks, vs. bands, water absorption) and the newest follow technical reasons (the common silicon detectors are sensitive to about 1,050 nm, while InGaAs's sensitivity starts around 950 nm and ends between 1,700 and 2,600 nm, depending on the specific configuration). No international standards for these specifications are currently available.
Information comes from https://en.wikipedia.org/wiki/Infrared
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