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Ultraviolet

Ultraviolet (UV) is an electromagnetic radiation with a wavelength from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays. UV radiation constitutes about 10% of the total light output of the Sun, and is thus present in sunlight. It is also produced by electric arcs and specialized lights, such as mercury-vapor lamps, tanning lamps, and black lights. Although it's not considered an ionizing radiation because its photons lack the energy to ionize atoms, long-wavelength ultraviolet radiation can cause chemical reactions and causes many substances to glow or fluoresce. Consequently, the chemical and biological effects of UV are greater than simple heating effects, and many practical applications of UV radiation derive from its interactions with organic molecules. Suntan, freckling and sunburn are familiar effects of over-exposure, along with higher risk of skin cancer. Living things on dry land would be severely damaged by ultraviolet radiation from the Sun if most of it were not filtered out by the Earth's atmosphere. More-energetic, shorter-wavelength "extreme" UV below 121 nm ionizes air so strongly that it is absorbed before it reaches the ground. Ultraviolet is also responsible for the formation of bone-strengthening vitamin D in most land vertebrates, including humans. The UV spectrum thus has effects both beneficial and harmful to human health. Ultraviolet rays are invisible to most humans, although insects, birds, and some mammals can see near-UV.

Visibility

Ultraviolet rays are invisible to most humans: the lens in a human eye ordinarily filters out UVB frequencies or higher, and humans lack color receptor adaptations for ultraviolet rays. Under some conditions, children and young adults can see ultraviolet down to wavelengths of about 310 nm, and people with aphakia (missing lens) or replacement lens can also see some UV wavelengths. Near-UV radiation is visible to insects, some mammals, and birds. Small birds have a fourth color receptor for ultraviolet rays; this gives birds "true" UV vision.

Discovery

"Ultraviolet" means "beyond violet" (from Latin ultra, "beyond"), violet being the color of the highest frequencies of visible light. Ultraviolet has a higher frequency than violet light. UV radiation was discovered in 1801 when the German physicist Johann Wilhelm Ritter observed that invisible rays just beyond the violet end of the visible spectrum darkened silver chloride-soaked paper more quickly than violet light itself. He called them "oxidizing rays" to emphasize chemical reactivity and to distinguish them from "heat rays", discovered the previous year at the other end of the visible spectrum. The simpler term "chemical rays" was adopted shortly thereafter, and it remained popular throughout the 19th century, although there were those who held that these were an entirely different sort of radiation from light (notably John William Draper, who named them "tithonic rays""On a new Imponderable Substance and on a Class of Chemical Rays analogous to the rays of Dark Heat", J.W. Draper, The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 1842, LXXX, pp.453–461"Description of the Tithonometer", J.W. Draper, The Practical Mechanic and Engineer's Magazine, January 1844, pp.122–127). The terms chemical and heat rays were eventually dropped in favour of ultraviolet and infrared radiation, respectively. In 1878 the sterilizing effect of short-wavelength light by killing bacteria was discovered. By 1903 it was known the most effective wavelengths were around 250 nm. In 1960, the effect of ultraviolet radiation on DNA was established.James Bolton, Christine Colton, The Ultraviolet Disinfection Handbook, American Water Works Association, 2008 , pp. 3–4 The discovery of the ultraviolet radiation below 200 nm, named vacuum ultraviolet because it is strongly absorbed by air, was made in 1893 by the German physicist Victor Schumann.The ozone layer protects humans from this.

Subtypes

The electromagnetic spectrum of ultraviolet radiation (UVR), defined most broadly as 10–400 nanometers, can be subdivided into a number of ranges recommended by the ISO standard ISO-21348: A variety of solid-state and vacuum devices have been explored for use in different parts of the UV spectrum. Many approaches seek to adapt visible light-sensing devices, but these can suffer from unwanted response to visible light and various instabilities. Ultraviolet can be detected by suitable photodiodes and photocathodes, which can be tailored to be sensitive to different parts of the UV spectrum. Sensitive ultraviolet photomultipliers are available. Spectrometers and radiometers are made for measurement of UV radiation. Silicon detectors are used across the spectrum. People cannot perceive UV directly, since the lens of the human eye blocks most radiation in the wavelength range of 300–400 nm; shorter wavelengths are blocked by the cornea. Nevertheless, the photoreceptors of the retina are sensitive to near-UV, and people lacking a lens (a condition known as aphakia) perceive near-UV as whitish-blue or whitish-violet. Vacuum UV, or VUV, wavelengths (shorter than 200 nm) are strongly absorbed by molecular oxygen in the air, though the longer wavelengths of about 150–200 nm can propagate through nitrogen. Scientific instruments can therefore utilize this spectral range by operating in an oxygen-free atmosphere (commonly pure nitrogen), without the need for costly vacuum chambers. Significant examples include 193 nm photolithography equipment (for semiconductor manufacturing) and circular dichroism spectrometers. Technology for VUV instrumentation was largely driven by solar astronomy for many decades. While optics can be used to remove unwanted visible light that contaminates the VUV, in general, detectors can be limited by their response to non-VUV radiation, and the development of "solar-blind" devices has been an important area of research. Wide-gap solid-state devices or vacuum devices with high-cutoff photocathodes can be attractive compared to silicon diodes. Extreme UV (EUV or sometimes XUV) is characterized by a transition in the physics of interaction with matter. Wavelengths longer than about 30 nm interact mainly with the outer valence electrons of atoms, while wavelengths shorter than that interact mainly with inner-shell electrons and nuclei. The long end of the EUV spectrum is set by a prominent He+ spectral line at 30.4 nm. EUV is strongly absorbed by most known materials, but it is possible to synthesize multilayer optics that reflect up to about 50% of EUV radiation at normal incidence. This technology was pioneered by the NIXT and MSSTA sounding rockets in the 1990s, and has been used to make telescopes for solar imaging.

Solar ultraviolet

Very hot objects emit UV radiation (see black-body radiation). The Sun emits ultraviolet radiation at all wavelengths, including the extreme ultraviolet where it crosses into X-rays at 10 nm. Extremely hot stars emit proportionally more UV radiation than the Sun. Sunlight in space at the top of Earth's atmosphere (see solar constant) is composed of about 50% infrared light, 40% visible light, and 10% ultraviolet light, for a total intensity of about 1400 W/m2 in vacuum. However, at ground level sunlight is 44% visible light, 3% ultraviolet (with the Sun at its zenith), and the remainder infrared. Thus, the atmosphere blocks about 77% of the Sun's UV, almost entirely in the shorter UV wavelengths, when the Sun is highest in the sky (zenith). Of the ultraviolet radiation that reaches the Earth's surface, more than 95% is the longer wavelengths of UVA, with the small remainder UVB. There is essentially no UVC. The fraction of UVB which remains in UV radiation after passing through the atmosphere is heavily dependent on cloud cover and atmospheric conditions. Thick clouds block UVB effectively, but in "partly cloudy" days, patches of blue sky showing between clouds are also sources of (scattered) UVA and UVB, which are produced by Rayleigh scattering in the same way as the visible blue light from those parts of the sky. UV-B also plays a major role in plant development as it affects most of the plant hormones. The shorter bands of UVC, as well as even more-energetic UV radiation produced by the Sun, are absorbed by oxygen and generate the ozone in the ozone layer when single oxygen atoms produced by UV photolysis of dioxygen react with more dioxygen. The ozone layer is especially important in blocking most UVB and the remaining part of UVC not already blocked by ordinary oxygen in air.

Blockers and absorbers

Ultraviolet absorbers are molecules used in organic materials ( polymers, paints, etc.) to absorb UV radiation to reduce the UV degradation (photo-oxidation) of a material. The absorbers can themselves degrade over time, so monitoring of absorber levels in weathered materials is necessary. In sunscreen, ingredients that absorb UVA/UVB rays, such as avobenzone, oxybenzone and octyl methoxycinnamate, are organic chemical absorbers or "blockers". They are contrasted with inorganic absorbers/"blockers" of UV radiation such as titanium dioxide and zinc oxide. For clothing, the Ultraviolet Protection Factor (UPF) represents the ratio of sunburn-causing UV without and with the protection of the fabric, similar to SPF ( Sun Protection Factor) ratings for sunscreen. Standard summer fabrics have UPF of approximately 6, which means that about 20% of UV will pass through. Suspended nanoparticles in stained glass prevent UV rays from causing chemical reactions that change image colors. A set of stained glass color reference chips is planned to be used to calibrate the color cameras for the 2019 ESA Mars rover mission, since they will remain unfaded by the high level of UV present at the surface of Mars. Common soda lime glass is partially transparent to UVA but is opaque to shorter wavelengths, whereas fused quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths. Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm. Wood's glass is a nickel-bearing form of glass with a deep blue-purple color that blocks most visible light and passes ultraviolet.

Artificial sources

"Black lights"

A black light lamp emits long-wave UVA radiation and little visible light. Fluorescent black light lamps work similarly to other fluorescent lamps, but use a phosphor on the inner tube surface which emits UVA radiation instead of visible light. Some lamps use a deep-bluish-purple Wood's glass optical filter that blocks almost all visible light with wavelengths longer than 400 nanometres. Others use plain glass instead of the more expensive Wood's glass, so they appear light-blue to the eye when operating. A black light may also be formed, very inefficiently, by using a layer of Wood's glass in the envelope for an incandescent bulb. Though cheaper than fluorescent UV lamps, only 0.1% of the input power is emitted as usable ultraviolet radiation. Mercury-vapor black lights in ratings up to 1 kW with UV-emitting phosphor and an envelope of Wood's glass are used for theatrical and concert displays. Black lights are used in applications in which extraneous visible light must be minimized; mainly to observe fluorescence, the colored glow that many substances give off when exposed to UV light. UVA/UVB emitting bulbs are also sold for other special purposes, such as tanning lamps and reptile-keeping.

Short-wave ultraviolet lamps

A shortwave UV lamp can be made using a fluorescent lamp tube with no phosphor coating. These lamps emit ultraviolet light with two peaks in the UVC band at 253.7 nm and 185 nm due to the mercury within the lamp, as well as some visible light. From 85% to 90% of the UV produced by these lamps is at 253.7 nm, whereas only 5–10% is at 185 nm. The fused quartz glass tube passes the 253 nm radiation but blocks the 185 nm wavelength. Such tubes have two or three times the UVC power of a regular fluorescent lamp tube. These low-pressure lamps have a typical efficiency of approximately 30–40%, meaning that for every 100 watts of electricity consumed by the lamp, they will produce approximately 30–40 watts of total UV output. These "germicidal" lamps are used extensively for disinfection of surfaces in laboratories and food-processing industries, and for disinfecting water supplies.

Gas-discharge lamps

Specialized UV gas-discharge lamps containing different gases produce UV radiation at particular spectral lines for scientific purposes. Argon and deuterium arc lamps are often used as stable sources, either windowless or with various windows such as magnesium fluoride. These are often the emitting sources in UV spectroscopy equipment for chemical analysis. Other UV sources with more continuous emission spectra include xenon arc lamps (commonly used as sunlight simulators), deuterium arc lamps, mercury-xenon arc lamps, and metal-halide arc lamps. The excimer lamp, a UV source developed within the last two decades, is seeing increasing use in scientific fields. It has the advantages of high-intensity, high efficiency, and operation at a variety of wavelength bands into the vacuum ultraviolet.

Ultraviolet LEDs

Light-emitting diodes (LEDs) can be manufactured to emit radiation in the ultraviolet range. LED efficiency at 365 nm is about 5–8%, whereas efficiency at 395 nm is closer to 20%, and power outputs at these longer UV wavelengths are also better. Such LED arrays are beginning to be used for UV curing applications, and are already successful in digital print applications and inert UV curing environments. Power densities approaching 3 W/cm2 (30 kW/m2) are now possible, and this, coupled with recent developments by photoinitiator and resin formulators, makes the expansion of LED-cured UV materials likely. UVC LEDs are beginning to be used in disinfection and as line sources to replace deuterium lamps in liquid chromatography instruments.

Ultraviolet lasers

Gas lasers, laser diodes and solid-state lasers can be manufactured to emit ultraviolet rays, and lasers are available which cover the entire UV range. The nitrogen gas laser uses electronic excitation of nitrogen molecules to emit a beam that is mostly UV. The strongest ultraviolet lines are at 337.1 nm and 357.6.6 nm, wavelength. Another type of high power gas laser is the excimer laser. They are widely used lasers emitting in ultraviolet and vacuum ultraviolet wavelength ranges. Presently, UV argon-fluoride (ArF) excimer lasers operating at 193 nm are routinely used in integrated circuit production by photolithography. The current wavelength limit of production of coherent UV is about 126 nm, characteristic of the Ar2* excimer laser. Direct UV-emitting laser diodes are available at 375 nm. UV diode lasers have been demonstrated using Ce:LiSAF crystals ( cerium- doped lithium strontium aluminum fluoride), a process developed in the 1990s at Lawrence Livermore National Laboratory.{{cite web |last = Marshall |first = Chris |title = A simple, reliable ultraviolet laser: the Ce:LiSAF |publisher = Lawrence Livermore National Laboratory |year = 1996 |url = https://www.llnl.gov/str/Marshall.html |accessdate = 2008-01-11 |deadurl = no |archiveurl = https://web.archive.org/web/20080920155324/https://www.llnl.gov/str/Marshall.html |archivedate = 20 September 2008 |df = dmy-all }} Wavelengths shorter than 325 nm are commercially generated in diode-pumped solid-state lasers. Ultraviolet lasers can also be made by applying frequency conversion to lower-frequency lasers. Ultraviolet lasers have applications in industry ( laser engraving), medicine ( dermatology, and keratectomy), chemistry ( MALDI), free air secure communications, computing ( optical storage) and manufacture of integrated circuits.

Tunable vacuum ultraviolet (VUV) via sum and difference frequency mixing

The vacuum ultraviolet (VUV) band (100–200 nm) can be generated by non-linear 4 wave mixing in gases by sum or difference frequency mixing of 2 or more longer wavelength lasers. The generation is generally done in gasses (e.g. krypton, hydrogen which are two-photon resonant near 193 nm) or metal vapors (e.g. magnesium). By making one of the lasers tunable, the VUV can be tuned. If one of the lasers is resonant with a transition in the gas or vapor then the VUV production is intensified. However, resonances also generate wavelength dispersion, and thus the phase matching can limit the tunable range of the 4 wave mixing. Difference frequency mixing (lambda1 + lambda2 − lambda3) has an advantage over sum frequency mixing because the phase matching can provide greater tuning. In particular, difference frequency mixing two photons of an ArF (193 nm) excimer laser with a tunable visible or near IR laser in hydrogen or krypton provides resonantly enhanced tunable VUV covering from 100 nm to 200 nm. Practically, the lack of suitable gas/vapor cell window materials above the lithium fluoride cut-off wavelength limit the tuning range to longer than about 110 nm. Tunable VUV wavelengths down to 75 nm was achieved using window-free configurations.

Plasma and synchrotron sources of extreme UV

Lasers have been used to indirectly generate non-coherent extreme UV (EUV) radiation at 13.5 nm for extreme ultraviolet lithography. The EUV is not emitted by the laser, but rather by electron transitions in an extremely hot tin or xenon plasma, which is excited by an excimer laser. This technique does not require a synchrotron, yet can produce UV at the edge of the X-ray spectrum. Synchrotron light sources can also produce all wavelengths of UV, including those at the boundary of the UV and X-ray spectra at 10 nm.

Human health-related effects

The impact of ultraviolet radiation on human health has implications for the risks and benefits of sun exposure and is also implicated in issues such as fluorescent lamps and health. Getting too much sun exposure can be harmful, but in moderation is beneficial.

Beneficial effects

The benefits of UV can outweigh manageable risks. UV light causes the body to produce vitamin D, which is essential for life. The human body needs some UV radiation in order for one to maintain adequate vitamin D levels. "The known health effects of UV, Ultraviolet radiation and the INTERSUN Programme" , World Health Organization.

Vitamin D

Reasonable exposure to ultraviolet radiation from the sun can be a good source of vitamin D. One minimal erythemal dose of sunlight UV radiation provides the equivalent of about 20,000 IU of vitamin D2, taken as an oral supplement. If an adult's arms and legs are exposed to a half minimal erythemal UV radiation, it is the same as taking 3,000 IU of vitamin D3 through an oral supplement. This exposure of 10–15 minutes, on a frequency of two to three times per week will cause the adult's skin to produce enough vitamin D. It is not necessary to expose the face to the UV, as facial skin provides little vitamin D3. Individuals whose metabolism makes taking oral vitamin D ineffective are able, through exposure to an ultraviolet lamp that emits UV-B radiation, to achieve a 25 (OH) D blood level. Three benefits of UV exposure are production of vitamin D, improvement in mood, and increased energy. UVB induces production of vitamin D in the skin at rates of up to 1,000 IUs per minute. This vitamin helps to regulate calcium metabolism (vital for the nervous system and bone health), immunity, cell proliferation, insulin secretion, and blood pressure. In third-world countries, foods fortified with vitamin D are "practically nonexistent." Most people in the world depend on the sun to get vitamin D. There are not many foods that naturally have vitamin D. Examples are cod liver oil and oily fish. If people cannot get sunlight, then they will need 1,000 IU of vitamin D per day to stay healthy. A person would have to eat oily fish three or four times per week in order to get enough vitamin D from that food source alone. People with higher levels of vitamin D tend to have lower rates of diabetes, heart disease, and stroke and tend to have lower blood pressure. However, it has been found that vitamin D supplementation does not improve cardiovascular health or metabolism, so the link with vitamin D must be in part indirect. People who get more sun are generally healthier, and also have higher vitamin D levels. It has been found that ultraviolet radiation (even UVA) produces nitric oxide (NO) in the skin, and nitric oxide can lower blood pressure. High blood pressure increases the risk of stroke and heart disease. Although long-term exposure to ultraviolet contributes to non-melanoma skin cancers that are rarely fatal, it has been found in a Danish study that those who get these cancers were less likely to die during the study, and were much less likely to have a heart attack, than those who did not have these cancers. People in certain situations, such as people with intellectual disabilities and neurodevelopmental disorders who stay inside most of the time have low vitamin D levels. Getting enough vitamin D can help stave off "autoimmune diseases, cardiovascular disease, many types of cancer, dementia, types 1 and 2 diabetes mellitus, and respiratory tract infections." Fetuses and children who do not get enough vitamin D can suffer from "growth retardation and skeletal deformities."

Skin conditions

UV rays also treat certain skin conditions. Modern phototherapy has been used to successfully treat psoriasis, eczema, jaundice, vitiligo, atopic dermatitis, and localized scleroderma. "Health effects of ultraviolet radiation" . Government of Canada.

Cardiovascular and hypertension

Worldwide, one billion people suffer from hypertension. In the U.S., half of the 146 million hypertensive patients don't have their blood pressure under control. In hypertension patients who suffer from vitamin D deficiency, UVB radiation (but not UVA) lowered blood pressure. Modern pharmaceutical therapy has resulted in an overall reduction in hypertension, particularly in countries with high GDP per capita. A review of blood pressure statistics before these pharmaceuticals were available shows a coherent correlation between high blood pressure and higher latitude. Seasons of the year also impact high blood pressure; BP is lower in the summer months in high latitudes than it is in the winter, when there is less sunlight. Individuals with more sun exposure synthesize more active vitamin D (1,25 di-hydroxy cholecalciferol) from diet or ultraviolet radiation exposure. A combination of lower ultraviolet radiation with insufficient vitamin D in a diet leads to vitamin D deficiency. Individuals whose vitamin D ranks in the lowest quartile have double the all-cause mortality of those who rank in the highest quartile. They are also more likely to suffer from cardiovascular disease, hypertension and organ cancer. Medical trials have demonstrated that vitamin D supplements do not prevent or treat hypertension or cardiovascular disease, although they can help in skeletal metabolism. Epidemiological and observational studies show indications that exposure to ultraviolet radiation, particularly sunlight, might reduce all-cause mortality and can help reduce cardiovascular disease and hypertension. One hundred years of scientific data has demonstrated that the effect of ultraviolet radiation on human skin is carcinogenic. There is a lack of evidence that this carcinogenic effect, like risks such as smoking or alcohol, is responsible for higher mortality. There are significant archives of studies demonstrating that ultraviolet radiation from sunlight provides measurable health benefits, independent of vitamin D.

Serotonin

Vitamin D promotes the creation of serotonin. The production of serotonin is in direct proportion to the degree of bright sunlight the body receives. Conversely, serotonin levels decrease when sunlight is at its lowest levels, as in autumn and winter.Korb, Alex, "Boosting Your Serotonin Activity" . Psychology Today, 17 November 2011. Changes in serotonin levels affect how humans act relative to mood and behavior. Measured serotonin is much higher among those who die in summer, rather than winter.Lambert, G.W. et al., "Effect of sunlight and season on serotonin turnover in the brain" , Lancet, 7 December 2002, pp 1840–42 Serotonin is a monoamine neurotransmitter that is thought to provide sensations of happiness, well being and serenity to human beings.Young, Simon. "How to increase serotonin in the human brain without drugs" . The Journal of Psychiatry and Neuroscience, November 2007, pp 394–399 It is thought that serotonin affects a plethora of human bodily functions from anxiety and mood to bowel function to bone density to sexuality. Its importance in human activity continues to be a source of much scientific examination and experimentation.McIntosh, James and Webberley, Helen. "Serotonin: Facts, What Does Serotonin Do?" Medical News Today, 29 April 2016

Melanin

The amount of the brown pigment melanin in the skin increases after exposure to UV radiation at moderate levels depending on skin type; this is commonly known as a sun tan. Melanin is an excellent photoprotectant that absorbs both UVB and UVA radiation and dissipates the energy as harmless heat, protecting the skin against both direct and indirect DNA damage. "There is no doubt that a little sunlight is good for you!" – World Health Organization

Harmful effects

In humans, excessive exposure to UV radiation can result in acute and chronic harmful effects on the eye's dioptric system and retina. The risk is elevated at high altitudes and people living in high latitude countries where snow covers the ground right into early summer and sun positions even at zenith are low, are particularly at risk. Skin, the circadian and immune systems can also be affected. molecules of living organisms in different ways. In one common damage event, adjacent thymine bases bond with each other, instead of across the "ladder". This " thymine dimer" makes a bulge, and the distorted DNA molecule does not function properly.]] ) is the product of the sunlight spectrum (radiation intensity) and the erythemal action spectrum (skin sensitivity) across the range of UV wavelengths. Sunburn production per milliwatt is increased by almost a factor of 100 between the near UVB wavelengths of 315–295 nm]] The differential effects of various wavelengths of light on the human cornea and skin are sometimes called the "erythemal action spectrum.". The action spectrum shows that UVA does not cause immediate reaction, but rather UV begins to cause photokeratitis and skin redness (with Caucasians more sensitive) at wavelengths starting near the beginning of the UVB band at 315 nm, and rapidly increasing to 300 nm. The skin and eyes are most sensitive to damage by UV at 265–275 nm, which is in the lower UVC band. At still shorter wavelengths of UV, damage continues to happen, but the overt effects are not as great with so little penetrating the atmosphere. The WHO-standard ultraviolet index is a widely publicized measurement of total strength of UV wavelengths that cause sunburn on human skin, by weighting UV exposure for action spectrum effects at a given time and location. This standard shows that most sunburn happens due to UV at wavelengths near the boundary of the UVA and UVB bands. Bioolympics discover UV reaction index to detect the leak of UV light.

Skin damage

Overexposure to UVB radiation not only can cause sunburn but also some forms of skin cancer. However, the degree of redness and eye irritation (which are largely not caused by UVA) do not predict the long-term effects of UV, although they do mirror the direct damage of DNA by ultraviolet. All bands of UV radiation damage collagen fibers and accelerate aging of the skin. Both UVA and UVB destroy vitamin A in skin, which may cause further damage.

See also

References

Further reading

  • {{Cite journal
| last = Hu | first = S | last2 = Ma | first2 = F | last3 = Collado-Mesa | first3 = F | last4 = Kirsner | first4 = R. S. | title = UV radiation, latitude, and melanoma in US Hispanics and blacks | journal = Arch. Dermatol. | volume = 140 | issue = 7 | pages = 819–824 |date=July 2004 | url = http://archderm.jamanetwork.com/article.aspx?articleid=480678 | doi = 10.1001/archderm.140.7.819 | id = | pmid = 15262692 }}
  • {{Cite journal
| last = Strauss | first = CEM | last2 = Funk | first2 = DJ | title = Broadly tunable difference-frequency generation of VUV using two-photon resonances in H2 and Kr | journal = Optics Lett. | volume = 16 | issue = 15 | page = 1192 |date=1991 | url = https://www.osapublishing.org/ol/abstract.cfm?uri=ol-16-15-1192 | doi=10.1364/ol.16.001192 | bibcode = 1991OptL...16.1192S }}
  • {{Cite journal
| last = Hockberger | first = Philip E. | title = A History of Ultraviolet Photobiology for Humans, Animals and Microorganisms | journal =Photochemisty and Photobiology | volume =76 | issue =6 | pages =561–569 | year =2002 | url =http://www.bioone.org/doi/abs/10.1562/0031-8655(2002)076%3C0561%3AAHOUPF%3E2.0.CO%3B2 | doi = 10.1562/0031-8655(2002)0760561AHOUPF2.0.CO2 | format = – Scholar search | pmid=12511035}}
  • {{Cite book
| last = Allen | first = Jeannie | title = Ultraviolet Radiation: How it Affects Life on Earth | url = http://earthobservatory.nasa.gov/Features/UVB/ |date=6 September 2001 | series = Earth Observatory | publisher = NASA, USA }}

External links

"green air" © 2007 - Ingo Malchow, Webdesign Neustrelitz
This article based upon the http://en.wikipedia.org/wiki/Ultraviolet, the free encyclopaedia Wikipedia and is licensed under the GNU Free Documentation License.
Further informations available on the list of authors and history: http://en.wikipedia.org/w/index.php?title=Ultraviolet&action=history
presented by: Ingo Malchow, Mirower Bogen 22, 17235 Neustrelitz, Germany