Sunday, June 7, 2009

Electromagnetic spectrum (wave length)

Solar radiation : 0.2 micrometer to 5 micrometer
Terrestrial radiation : 5 to 100 micrometer

note : 1000 nm = 1 micrometer

less than 100 nm : Photo-ionisation band : photo-ionisation of N2, O2, O gives molecular ions and free electron.

100 to 200 nm : photo-dissociation band : Photo dissociation of oxygen gives atomic oxygen.

200 to 280 nm : Ultraviolet-C : Absorption by oxygen molecule

280 to 320 nm : Ultraviolet-B : Absorption by ozone molecule

320 to 400 nm : Ultra violet-A : weak ultra violet

400 to -- nm : violet

-- to 450 nm : indigo

450 to 520 nm : blue

520 to 590 nm : green

590 to -- nm : yellow

-- to 620 nm : orange

620 to 680 nm : red

680 to 1100 nm : near-infrared


CIE division scheme of Infrared

The International Commission on Illumination (CIE) recommended the division of optical radiation into the following three bands:

  • IR-A: 700 nm–1400 nm
  • IR-B: 1400 nm–3000 nm
  • IR-C: 3000 nm–1 mm

A commonly used sub-division scheme is:

  • Near-infrared (NIR, IR-A DIN): 0.75-1.4 µm in wavelength, defined by the water absorption, 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.
  • Short-wavelength infrared (SWIR, IR-B DIN): 1.4-3 µm, water absorption increases significantly at 1,450 nm. The 1,530 to 1,560 nm range is the dominant spectral region for long-distance telecommunications.
  • Mid-wavelength infrared (MWIR, IR-C DIN) also called intermediate infrared (IIR): 3-8 µm. 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 IR signature of the target aircraft, typically the jet engine exhaust plume.
  • Long-wavelength infrared (LWIR, IR-C DIN): 8–15 µm. This is the "thermal imaging" region, in which sensors can obtain a completely passive picture of the outside world based on thermal emissions only and requiring no external light or thermal source such as the sun, moon or infrared illuminator. Forward-looking infrared (FLIR) systems use this area of the spectrum. Sometimes also called the "far infrared."
  • Far infrared (FIR): 15-1,000 µm (see also far infrared laser).

NIR and SWIR is sometimes called "reflected infrared" while 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.

Sensor response division scheme

Plot of atmospheric transmittance in part of the infrared region.

A third scheme divides up the band based on the response of various detectors:[7]

  • Near infrared: from 0.7 to 1.0 micrometers (from the approximate end of the response of the human eye to that of silicon).
  • Short-wave infrared: 1.0 to 3 micrometers (from the cut off of silicon to that of the MWIR atmospheric window. InGaAs covers to about 1.8 micrometers; the less sensitive lead salts cover this region.
  • Mid-wave infrared: 3 to 5 micrometers (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 micrometers: the atmospheric window (Covered by HgCdTe and microbolometers).
  • Very-long wave infrared (VLWIR): 12 to about 30 micrometers, covered by doped silicon.

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