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Ultraviolet

2007 Schools Wikipedia Selection. Related subjects: General Physics

   Ultraviolet (UV) light is electromagnetic radiation with a wavelength
   shorter than that of visible light, but longer than soft X-rays. It can
   be subdivided into near UV (380–200 nm wavelength; abbrev. NUV), far or
   vacuum UV (200–10 nm; abbrev. FUV or VUV), and extreme UV (1–31 nm;
   abbrev. EUV or XUV).

Origin of term

   The name means "beyond violet" (from Latin ultra, "beyond"), violet
   being the colour of the shortest wavelengths of visible light.

Discovery

   Soon after infrared radiation had been discovered, the German physicist
   Johann Wilhelm Ritter began to look for radiation at the opposite end
   of the spectrum, at the short wavelengths beyond violet. In 1801 he
   used silver chloride, a light-sensitive chemical, to show that there
   was a type of invisible light beyond violet, which he called chemical
   rays. At that time, many scientists, including Ritter, concluded that
   light was composed of three separate components: an oxidising or
   calorific component (infrared), an illuminating component (visible
   light), and a reducing or hydrogenating component (ultraviolet). The
   unity of the different parts of the spectrum was not understood until
   about 1842, with the work of Macedonio Melloni, Alexandre-Edmond
   Becquerel and others. During that time, UV radiation was also called
   "actinic radiation".

Explanation

   When considering the effect of UV radiation on human health and the
   environment, the range of UV wavelengths is often subdivided into UVA
   (400–315 nm), also called Long Wave or "blacklight"; UVB (315–280 nm),
   also called Medium Wave; and UVC (< 280 nm), also called Short Wave or
   "germicidal". See 1 E-7 m for a list of objects of comparable sizes.

   In photolithography, in laser technology, etc., the term deep
   ultraviolet or DUV refers to wavelengths below 300 nm.

   Some of the UV wavelengths are colloquially called black light, as it
   is invisible to the human eye. Some animals, including birds, reptiles,
   and insects such as bees, can see into the near ultraviolet. Many
   fruits, flowers, and seeds stand out more strongly from the background
   in ultraviolet wavelengths as compared to human colour vision.
   Scorpions glow or take on a yellow to green colour under UV
   illumination. Many birds have patterns in their plumage that are
   invisible at usual wavelengths but observable in ultraviolet, and the
   urine of some animals is much easier to spot with ultraviolet.

Safety aspects of UV

   In humans, prolonged exposure to solar UV radiation may result in acute
   and chronic health effects on the skin, eye, and immune system.

   UVC rays are the highest energy, most dangerous type of ultraviolet
   light. Little attention has been given to UVC rays in the past since
   they are filtered out by the atmosphere. However, their use in
   equipment such as pond sterilization units may pose an exposure risk,
   if the lamp is switched on outside of its enclosed pond sterilization
   unit.
   Ultraviolet photons harm the DNA molecules of living organisms in
   different ways. In one common damage event, adjacent bases bond with
   each other, instead of across the "ladder". This makes a bulge, and the
   distorted DNA molecule does not function properly.
   Enlarge
   Ultraviolet photons harm the DNA molecules of living organisms in
   different ways. In one common damage event, adjacent bases bond with
   each other, instead of across the "ladder". This makes a bulge, and the
   distorted DNA molecule does not function properly.

Skin


   Ultraviolet

    Ultraviolet (UV) irradiation present in sunlight is an environmental
     human carcinogen. The toxic effects of UV from natural sunlight and
   therapeutic artificial lamps are a major concern for human health. The
     major acute effects of UV irradiation on normal human skin comprise
       sunburn inflammation (erythema), tanning, and local or systemic
                             immunosuppression.


   Ultraviolet


   — Matsumura, Y. & Ananthaswamy H. N. , Toxicology and Applied
   Pharmacology (2004)

   UVA, UVB and UVC can all damage collagen fibers and thereby accelerate
   aging of the skin. In general, UVA is the least harmful, but can
   contribute to the aging of skin, DNA damage and possibly skin cancer.
   It penetrates deeply and does not cause sunburn. Because it does not
   cause reddening of the skin (erythema) it cannot be measured in the SPF
   testing. There is no good clinical measurement of the blocking of UVA
   radiation, but it is important that sunscreen block both UVA and UVB.

   UVA light is also known as "dark-light" and, because of its longer
   wavelength, can penetrate most windows. It also penetrates deeper into
   the skin than UVB light and is thought to be a prime cause of wrinkles.
   The reddening of the skin due to the action of sunlight depends both on
   the amount of sunlight as well as the sensitivity of the skin
   ("erythemal action spectrum") over the UV spectrum.
   Enlarge
   The reddening of the skin due to the action of sunlight depends both on
   the amount of sunlight as well as the sensitivity of the skin
   ("erythemal action spectrum") over the UV spectrum.

   UVB light can cause skin cancer. The radiation excites DNA molecules in
   skin cells, causing covalent bonds to form between adjacent thymine
   bases, producing thymidine dimers. Thymidine dimers do not base pair
   normally, which can cause distortion of the DNA helix, stalled
   replication, gaps, and misincorporation. These can lead to mutations,
   which can result in cancerous growths. The mutagenicity of UV radiation
   can be easily observed in bacteria cultures. This cancer connection is
   one reason for concern about ozone depletion and the ozone hole.

   As a defense against UV radiation, the body tans when exposed to
   moderate (depending on skin type) levels of radiation by releasing the
   brown pigment melanin. This helps to block UV penetration and prevent
   damage to the vulnerable skin tissues deeper down. Suntan lotion that
   partly blocks UV is widely available (often referred to as "sun block"
   or " sunscreen"). Most of these products contain an "SPF rating" that
   describes the amount of protection given. This protection, however,
   applies only to UVB rays responsible for sunburn and not to UVA rays
   that penetrate more deeply into the skin and may also be responsible
   for causing cancer and wrinkles. Some sunscreen lotion now includes
   compounds such as titanium dioxide which helps protect against UVA
   rays. Other UVA blocking compounds found in sunscreen include zinc
   oxide and avobenzone. There are also naturally occurring compounds
   found in rainforest plants that have been known to protect the skin
   from UV radiation damage, such as the fern Polypodium leucotomos.

   What to look for in sunscreen:

   UVB protection: Padimate O, Homosalate, Octisalate ( octyl salicylate),
   Octinoxate ( octyl methoxycinnamate)
   UVA protection: Avobenzone
   UVA/UVB protection: Octocrylene, titanium dioxide, zinc oxide, Mexoryl
   ( ecamsule)

   Another means to block UV is sun protective clothing. This is clothing
   that has a "UPF rating" that describes the protection given against
   both UVA and UVB.

Eye

   High intensities of UVB light are hazardous to the eyes, and exposure
   can cause welder's flash ( photokeratitis or arc eye) and may lead to
   cataracts, pterygium , and pinguecula formation.

   Protective eyewear is beneficial to those who are working with or those
   who might be exposed to ultraviolet radiation, particularly short wave
   UV. Given that light may reach the eye from the sides, full coverage
   eye protection is usually warranted if there is an increased risk of
   exposure, as in high altitude mountaineering. Mountaineers are exposed
   to higher than ordinary levels of UV radiation, both because there is
   less atmospheric filtering and because of reflection from snow and ice.

   Ordinary, untreated eyeglasses give some protection. Most plastic
   lenses give more protection than glass lenses, because, as noted above,
   glass is transparent to UVA and the common acrlyic plastic used for
   lenses is less so. Some plastic lens materials, such as polycarbonate,
   inherently block most UV. There are protective treatments available for
   eyeglass lenses that need it which will give better protection. But
   even a treatment that completely blocks UV will not protect the eye
   from light that arrives around the lens. To convince yourself of the
   potential dangers of stray UV light, cover your lenses with something
   opaque, like aluminium foil, stand next to a bright light, and consider
   how much light you see, despite the complete blockage of the lenses.
   Most intraocular lenses help to protect the retina by absorbing UV
   radiation.

Applications of UV

Black lights

   A bird appears on every Visa credit card when held under a UV light
   source.
   Enlarge
   A bird appears on every Visa credit card when held under a UV light
   source.

   A black light is a lamp that emits long wave UV radiation and very
   little visible light. Fluorescent black lights are typically made in
   the same fashion as normal fluorescent lights except that only one
   phosphor is used and the normally clear glass envelope of the bulb is
   replaced by a deep bluish purple glass called Wood's glass.

   To thwart counterfeiters, sensitive documents (e.g. credit cards,
   driver's licenses, passports) may also include a UV watermark that can
   only be seen when viewed under a UV-emitting light. Passports issued by
   most countries usually contain UV sensitive inks and security threads.
   Visa stamps and stickers such as those issued by Ukraine contain large
   and detailed seals invisible to the naked eye under normal lights, but
   strongly visible under UV illimunation. Passports issued by the United
   States have the UV sensitive threads on the last page of the passport
   along with the barcode.

Fluorescent lamps

   Fluorescent lamps produce UV radiation by ionising low-pressure mercury
   vapour. A phosphorescent coating on the inside of the tubes absorbs the
   UV and converts it to visible light.

   The main mercury emission wavelength is in the UVC range. Unshielded
   exposure of the skin or eyes to mercury arc lamps that do not have a
   conversion phosphor is quite dangerous.

   The light from a mercury lamp is predominantly at discrete wavelengths.
   Other practical UV sources with more continuous emission spectra
   include xenon arc lamps (commonly used as sunlight simulators),
   deuterium arc lamps, mercury-xenon arc lamps, metal-halide arc lamps,
   and tungsten-halogen incandescent lamps.

Astronomy

   Aurora at Jupiter's north pole as seen in ultraviolet light by the
   Hubble Space Telescope.
   Enlarge
   Aurora at Jupiter's north pole as seen in ultraviolet light by the
   Hubble Space Telescope.

   In astronomy, very hot objects preferentially emit UV radiation (see
   Wien's law). However, the same ozone layer that protects us causes
   difficulties for astronomers observing from the Earth, so most UV
   observations are made from space. (see UV astronomy, space observatory)

Pest control

   Ultraviolet fly traps are used for the elimination of various small
   flying insects. They are attracted to the UV light and are killed using
   an electrical shock or trapped once they come into contact with the
   device.

Spectrophotometry

   UV/VIS spectroscopy is widely used as a technique in chemistry, for
   analysis of chemical structure, most notably conjugated systems. UV
   radiation is often used in visible spectrophotometry to determine the
   existence of fluorescence a given sample.

Analyzing minerals

   A collection of mineral samples brilliantly fluorescing at various
   wavelengths as seen while being irradiated by UV light.
   Enlarge
   A collection of mineral samples brilliantly fluorescing at various
   wavelengths as seen while being irradiated by UV light.

   Ultraviolet lamps are also used in analyzing minerals, gems, and in
   other detective work including authentication of various collectibles.
   Materials may look the same under visible light, but fluoresce to
   different degrees under ultraviolet light; or may fluoresce differently
   under short wave ultraviolet versus long wave ultraviolet. UV
   fluorescent dyes are used in many applications (for example,
   biochemistry and forensics). The fluorescent protein Green Fluorescent
   Protein (GFP) is often used in genetics as a marker. Many substances,
   proteins for instance, have significant light absorption bands in the
   ultraviolet that are of use and interest in biochemistry and related
   fields. UV-capable spectrophotometers are common in such laboratories.

Photolithography

   Ultraviolet radiation is used for very fine resolution
   photolithography, a procedure where a chemical known as a photoresist
   is exposed to UV radiation which has passed through a mask. The light
   allows chemical reactions to take place in the photoresist, and after
   development (a step that either removes the exposed or unexposed
   photoresist), a geometric pattern which is determined by the mask
   remains on the sample. Further steps may then be taken to "etch" away
   parts of the sample with no photoresist remaining.

   UV radiation is used extensively in the electronics industry because
   photolithography is used in the manufacture of semiconductors,
   integrated circuit components and printed circuit boards.

Checking electrical insulation

   A new application of UV is to detect corona discharge (often simply
   called "corona") on electrical apparatus. Degradation of insulation of
   electrical apparatus or pollution causes corona, wherein a strong
   electric field ionizes the air and excites nitrogen molecules, causing
   the emission of ultraviolet radiation. The corona degrades the
   insulation level of the apparatus. Corona produces ozone and to a
   lesser extent nitrogen oxide which may subsequently react with water in
   the air to form nitrous acid and nitric acid vapour in the surrounding
   air.

Sterilization

   A low pressure mercury vapor discharge tube floods the inside of a hood
   with shortwave UV light when not in use, sterilizing microbiological
   contaminants from irradiated surfaces.
   Enlarge
   A low pressure mercury vapor discharge tube floods the inside of a hood
   with shortwave UV light when not in use, sterilizing microbiological
   contaminants from irradiated surfaces.

   Ultraviolet lamps are used to sterilize workspaces and tools used in
   biology laboratories and medical facilities. Commercially-available low
   pressure mercury-vapor lamps emit about 86% of their light at 254
   nanometers (nm) which coincides very well with one of the two peaks of
   the germicidal effectiveness curve (i.e., effectiveness for UV
   absorption by DNA). One of these peaks is at about 265 nm and the other
   is at about 185 nm. Although 185 nm is better absorbed by DNA, the
   quartz glass used in commercially-available lamps, as well as
   environmental media such as water, are more opaque to 185 nm than 254
   nm (C. von Sonntag et al., 1992). UV light at these germicidal
   wavelengths causes adjacent thymine molecules on DNA to dimerize, if
   enough of these defects accumulate on a microorganism's DNA its
   replication is inhibited, thereby rendering it harmless (even though
   the organism may not be killed outright). Since microorganisms can be
   shielded from ultraviolet light in small cracks and other shaded areas,
   however, these lamps are used only as a supplement to other
   sterilization techniques.

Disinfecting drinking water

   UV radiation can be an effective viricide and bactericide. Disinfection
   using UV radiation was more commonly used in wastewater treatment
   applications but is finding increased usage in drinking water
   treatment. A process named SODIS has been extensively researched in
   Switzerland and proven ideal to treat small quantities of water.
   Contaminated water is filled into transparent plastic bottles and
   exposed to full sunlight for six hours. The sunlight is treating the
   contaminated water through two synergetic mechanisms: Radiation in the
   spectrum of UV-A (wavelength 320-400nm) and increased water
   temperature. If the water temperatures raises above 50°C, the
   disinfection process is three times faster. It used to be thought that
   UV disinfection was more effective for bacteria and viruses, which have
   more exposed genetic material, than for larger pathogens which have
   outer coatings or that form cyst states (e.g., Giardia) that shield
   their DNA from the UV light. However, it was recently discovered that
   ultraviolet radiation can be somewhat effective for treating the
   microorganism Cryptosporidium. The findings resulted in two US patents
   and the use of UV radiation as a viable method to treat drinking water.
   Giardia in turn has been shown to be very susceptible to UV-C when the
   tests were based on infectivity rather than excystation. It turns out
   that protists are able to survive high UV-C doses but are sterilized at
   low doses.

Food Processing

   As consumer demand for fresh and "fresh like" food products increases,
   the demand for nonthermal methods of food processing is likewise on the
   rise. In addition, public awareness regarding the dangers of food
   poisoning is also raising demand for improved food processing methods.
   Ultraviolet radiation is used in several food processes to remove
   unwanted microorganisms. UV light can be used to pasteurize fruit
   juices by flowing the juice over a high intensity ultraviolet light
   source. The effectiveness of such a process depends on the UV
   absorbance of the juice (see Beer's law).

Fire detection

   Ultraviolet detectors generally use either a solid-state device, such
   as one based on silicon carbide or aluminium nitride, or a gas-filled
   tube as the sensing element. UV detectors which are sensitive to UV
   light in any part of the spectrum respond to irradiation by sunlight
   and artificial light. A burning hydrogen flame, for instance, radiates
   strongly in the 185 to 260 nanometer range and only very weakly in the
   IR region, while a coal fire emits very weakly in the UV band yet very
   strongly at IR wavelengths; thus a fire detector which operates using
   both UV and IR detectors is more reliable than one with a UV detector
   alone. Virtually all fires emit some radiation in the UVB band, while
   the Sun's radiation at this band is absorbed by the Earth's atmosphere.
   The result is that the UV detector is "solar blind", meaning it will
   not cause an alarm in response to radiation from the Sun, so it can
   easily be used both indoors and outdoors.

   UV detectors are sensitive to most fires, including hydrocarbons,
   metals, sulfur, hydrogen, hydrazine, and ammonia. Arc welding,
   electrical arcs, lightning, X-rays used in nondestructive metal testing
   equipment (though this is highly unlikely), and radioactive materials
   can produce levels that will activate a UV detection system. The
   presence of UV-absorbing gases and vapors will attenuate the UV
   radiation from a fire, adversely affecting the ability of the detector
   to detect flames. Likewise, the presence of an oil mist in the air or
   an oil film on the detector window will have the same effect.

Curing of adhesives and coatings

   Certain adhesives and coatings are formulated with photoinitiators.
   When exposed to the correct dose and intensity in the required band of
   UV light, polymerisation occurs, and so the adhesives harden or cure.
   Usually, this reaction is very quick, a matter of a few seconds.
   Applications include glass and plastic bonding, optical fibre coatings,
   the coating of flooring, paper finishes in offset printing, and dental
   fillings.

Deterring Substance Abuse in Public Places

   UV lights have been installed in some parts of the world in public
   restrooms, and on public transport, for the purpose of deterring
   substance abuse. The blue colour of these lights, combined with the
   fluorescence of the skin, make it harder for intravenous drug users to
   find a vein. The efficacy of these lights for that purpose has been
   questioned, with some suggesting that drug users simply find a vein
   outside the public restroom and mark the spot with a marker for
   accessibility when inside the restroom. There is currently no published
   evidence supporting the idea of a deterrent effect.

Erasing EPROM modules

   Some EPROM (electronically programmable read-only memory) modules are
   erased by exposure to UV radiation. These modules often have a
   transparent glass (quartz) window on the top of the chip that allows
   the UV radiation in. These have been largely superseded by EEPROM and
   flash memory chips in most devices.

Preparing Low Surface Energy Polymers

   UV radiation is useful in preparing low surface energy polymers for
   adhesives. Polymers exposed to UV light will oxidize thus raising the
   surface energy of the polymer. Once the surface energy of the polymer
   has been raised, the bond between the adhesive and the polymer will be
   greater.

Reading completely illegible papyruses

   Using multi-spectral imaging it is possible to read illegible
   papyruses, such as the burned papyruses of the Villa of the Papyri or
   of Oxyrhynchus. The technique involves taking pictures of the illegible
   papyruses using different filters in the infrared or ultraviolet range,
   finely tuned to capture certain wavelengths of light. Thus, the optimum
   spectral portion can be found for distinguishing ink from paper on the
   papyrus surface.

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