In physics, ultraviolet (UV) light refers to a non-visible portion of the electromagnetic spectrum ranging from wavelengths of 200 nm to 400nm. The name ultraviolet comes from Latin infra- meaning below, i.e., infrared has a higher frequency than violetin the spectrum.
Various disciplines further subdivide the UV spectrum, but there is no consensus on the divisions. They vary from discipline-to-discipline and even widely within a given discipline. The following table shows a typical set of divisions:
|Near Ultraviolet, Ultraviolet A||UV-A||320 - 400 nm|
|Ultraviolet B||UV-B||290 - 320 nm|
|Ultraviolet C, Vacuum ultraviolet||UV-C, VUV||200 - 290 nm|
|Extreme ultraviolet||EUV||200 - 40 nm|
 Viewing devices
Ultraviolet, as opposed to infrared, is not widely used in night vision devices or other assistance to vision on Earth.
Ultraviolet light, especially in the far and extreme spectra, provide complements to astronomical observations in the visible and radio spectra. Like X-ray telescopes, they must operate in space to avoid atmospheric blocking.
 Biomedical significance
UV-A makes up 99 percent of the ultraviolet light reaching the earth, UV-B making up the remaining 1 percent. Shorter wavelengths are blocked by the ozone layer.
UV-A through -C have applications in treatment of skin disorders; a Woods' Light, generating UV-A, will cause some skin disorders to fluoresce. UV-B, enhanced by UV-A, causes the bulk of sunburn. UV-C is germicidal and used in decontamination and sterilization. All may be used in treating skin disorders.
 Military applications
Most guidance and tracking applications, which used neither radar nor visual wavelengths, used infrared. Increasingly, UV is used in addition to infrared guidance (e.g., FIM-92 Stinger) or for detection of rocket motor launch (e.g., AN/AAR-54), to avoid being decoyed by conventional thermal flares.
 Applications in electronics
In creating integrated circuits, the shorter the wavelength used to create the etching mask, the shorter the wavelength, the higher the resulting component density may be. Extreme ultraviolet, as well as X-rays, are increasingly used in photolithographic creation of masks.
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