Ultraviolet

Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, that is, in the range between 400 nm and 10 nm, corresponding to photon energies from 3 eV to 124 eV. It is so-named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the color violet. These frequencies are invisible to most humans except those with aphakia. Near-UV is visible to a number of insects and birds.

UV light is found in sunlight and is emitted by electric arcs and specialized lights such as mercury lamps and black lights. It can cause chemical reactions, and causes many substances to glow or fluoresce. A large fraction of UV, including all that reaches the surface of the Earth, is classified as non-ionizing radiation. The higher energies of the ultraviolet spectrum from wavelengths about 120 nm to 10 nm ('extreme' ultraviolet) are ionizing, but, due to this effect, these wavelengths are absorbed by nitrogen and even more strongly by dioxygen, and thus have an extremely short path length through air. However, the entire spectrum of ultraviolet radiation has some of the biological features of ionizing radiation: It does far more damage to many molecules in biological systems than is accounted for by simple heating effects (an example is sunburn). These properties derive from the ultraviolet photon's power to alter chemical bonds in molecules, even without having enough energy to ionize atoms.



Although ultraviolet radiation is invisible to the human eye, most people are aware of the effect it has on the skin of fair-skinned people, i.e., the suntan and sunburn. Normal human skin responds to exposure to small doses of this kind of radiation by increasing the amount of protective melanin in the skin's outer layers; too much of this radiation in too short a period of time, however, results in cellular damage from radiation burn. In fact, the damaging effects of short-wavelength and mid-wavelength UV means that life on Earth outside of the deep oceans is possible only because the atmosphere, primarily the ozone layer, filters out the vast majority of this light. A small amount of the shorter wavelength ultraviolet reaches the surface, which causes sunburn, long-term skin damage, and skin cancer. Ultraviolet is also responsible for the formation of vitamin D in organisms that make this vitamin (including humans). The UV spectrum thus has many effects, both beneficial and damaging, to human health.

Discovery
The discovery of UV radiation was associated with the observation that silver salts darkened when exposed to sunlight. In 1801, the German physicist Johann Wilhelm Ritter made the hallmark observation 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. The terms chemical and heat rays were eventually dropped in favour of ultraviolet and infrared radiation, respectively.

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.

Origin of the term
The name means "beyond violet" (from Latin ultra, "beyond"), violet being the color of the shortest wavelengths of visible light. UV light has a shorter wavelength than violet light.

Subtypes
The electromagnetic spectrum of ultraviolet light can be subdivided in a number of ways. The ISO standard on determining solar irradiances (ISO-21348) describes the following ranges:

Vacuum UV is so-named because it is absorbed strongly by air, and is, therefore, used in a vacuum. In the long-wave limit of this region, roughly 150 – 200 nm, the principal absorber is the oxygen in air. Work in this region can be performed in an oxygen-free atmosphere (commonly pure nitrogen), avoiding the need for a vacuum chamber.

Natural sources and filters


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. For example, the star R136a1 has a thermal energy of 4.57 eV, which falls in the near-UV range (such stars appear blue-white rather than violet).

Atmospheric filtering
Sunlight in space at the top of Earth's atmosphere, at a solar constant output of about 1366 W/m2, is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet light, for a total ultraviolet power of about 140 W/m2 in vacuum. However, at ground level total sunlight power decreases to about 1000–1100 W/m2, and by energy fractions, is composed of 44% visible light, 3% ultraviolet (with the Sun at its zenith), and the remainder infrared. Thus, sunlight's composition at the zenith at ground level, per square meter, is about 527 W infrared radiation, 445 W visible light, and 32 W UV.

Since with the Sun at zenith the Earth's air and ozone layer allows passage of a total of 32 watts/m2 (ground UV power) out of a vacuum value of about 140 watts/m2 (i.e., 23%) of Sun's UV light, this is equivalent to a minimal atmospheric blockage of 77% of the Sun's UV. However, most of the Sun's UV that is blocked by Earth's atmosphere lies in the shorter UV wavelengths. The figure rises to 97–99% of the Sun's UV radiation at the average mixture of other Sun angles encountered through the day.

The Sun's emission in the lowest UV bands, the UVA, UVB, and UVC bands, are of interest, as these are the UV bands commonly encountered from artificial sources on Earth. The shorter bands of UVC, as well as even more energetic radiation as produced by the Sun, 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 UVB and part of UVC, since the shortest wavelengths of UVC (and those even shorter) are blocked by ordinary air. Of the ultraviolet radiation that reaches the Earth's surface, up to 95% is UVA (the very longest wavelength), depending on cloud cover and atmospheric conditions.

Glass filtering
Ordinary glass is partially transparent to UVA but is opaque to shorter wavelengths, whereas silica or 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.

"Black lights"
A black light is a lamp that emits long-wave UVA radiation and little visible light. Fluorescent black light lamps are constructed in the same fashion as normal fluorescent lights, except they use a phosphor on the inner tube surface, which emits UVA light instead of visible white light. BLB type lamps use filtering glass that is deep-bluish-purple optical filter that blocks almost all visible light above 400 nanometres. The color of such lamps is often referred to in the lighting industry as "blacklight blue" or "BLB", to distinguish them from UV lamps used in "bug zapper" insect traps, that do not have the optical filter coating. These are designated "blacklight" ("BL") lamps. The phosphor typically used for a near 368 to 371 nanometre emission peak is either europium-doped strontium fluoroborate (SrB4O7F:Eu2+) or europium-doped strontium borate (SrB4O7:Eu2+), whereas the phosphor used to produce a peak around 350 to 353 nanometres is lead-doped barium silicate (BaSi2O5:Pb+). "Blacklight Blue" lamps peak at 365 nm.

A black light may also be formed, very inefficiently, by simply using Wood's glass, a deep bluish-purple nickel oxide doped glass that filters out all light besides UV, instead of clear glass as the envelope for a common incandescent bulb. This was the method used to create the very first black light sources. Though cheaper than fluorescent UV lamps, only 0.1% of the input power is converted to usable radiation, as the incandescent light radiates as a black body with very little emission in the UV range. Incandescent bulbs used to generate significant UV, due to their inefficiency, may become dangerously hot. High-power mercury-vapor black lights that use a UV-emitting phosphor and an envelope of Wood's glass are also made, in ratings up to 1 kW, used mainly for theatrical and concert displays.

Some UV fluorescent bulbs specifically designed to attract insects use the same near-UV emitting phosphor as normal blacklights, but use plain glass instead of the more expensive Wood's glass. Plain glass blocks less of the visible mercury emission spectrum, making them appear light-blue to the naked eye. These lamps are referred to as "blacklight" or "BL" in most lighting catalogs.

Short wave ultraviolet lamps
Lamps that emit shortwave UV light are also made. Fluorescent lamps without an internal phosphor coating to convert UV to visible light emit ultraviolet light with two peaks in the UV-C band at 253.7 nm and 185 nm due to the peak emission of the mercury within the lamp. Eighty-five to 90% of the UV produced by these lamps is at 253.7 nm, whereas only five to ten percent is at 185 nm. The quartz tube passes the 253 nm radiation but has impurities that block the 185 nm wavelength. These "germicidal" lamps are used extensively for disinfection of surfaces in laboratories and food processing industries, and for disinfecting water supplies.

Standard bulbs have an optimum operating temperature of about 40 degrees Celsius. Use of a mercury amalgam allows operating temperature to rise to 100 degrees Celsius, and UVC emission to about double or triple per unit of light-arc length. These low-pressure lamps have a typical efficiency of approximately thirty to forty percent, meaning that for every 100 watts of electricity consumed by the lamp, they will produce approximately 30–40 watts of total UV output. UVA/UVB emitting bulbs also sold for other special purposes, such as reptile-keeping.

Gas-discharge lamps
Specialized UV gas-discharge lamps are sold, containing a variety of different gases, to produce UV light at particular spectral lines for scientific purposes. Argon and deuterium lamps are often used as stable sources, either windowless or with various windows such as magnesium fluoride. These are often the light sources in UV spectroscopy equipment for chemical analysis.

The excimer lamp, a new UV light source developed within the last two decades, is seeing increasing use in scientific fields. It has the advantages of high-intensity, broadband radiation (no spectral lines) and operation at a variety of wavelength bands into the vacuum ultraviolet.

Ultraviolet LEDs
Light-emitting diodes (LEDs) can be manufactured to emit light in the ultraviolet range, although practical LED arrays are very limited below 365 nm. 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.

Ultraviolet lasers
UV gas lasers, laser diodes and UV solid-state lasers can be manufactured to emit light in the ultraviolet range.

The nitrogen gas laser uses electronic excitation of nitrogen molecules to emit a beam that is mostly UV. The strongest lines are at 337.1 nm wavelength in the ultraviolet. Other lines have been reported at 357.6 nm, also ultraviolet. (This laser also emits weaker lines in blue, red, and infrared)

Another type of high power gas laser is the excimer laser. They are widely-spread lasers emitting in ultraviolet and even vacuum ultraviolet wavelength rage.

Direct UV-emitting laser diodes are available at 375 nm. UV diode lasers have been demonstrated using Ce:LiSAF crystals (cerium doped with lithium strontium aluminum fluoride), a process developed in the 1990s at Lawrence Livermore National Laboratory. Wavelengths shorter than 325 nm are commercially generated from diodes in solid-state modules that use frequency doubling or tripling diode-pumped solid state DPSS technology. Wavelengths available include 262, 266, 349, 351, 355, and 375 nm. Ultraviolet lasers have applications in industry (laser engraving), medicine (dermatology and keratectomy), secure communications, and computing (optical storage).

Detection and measurement
Ultraviolet detection and measurement technology can vary with the part of the spectrum under consideration. While some silicon detectors are used across the spectrum, and in fact the US NIST has characterized simple silicon diodes that work with visible light too, many specializations are possible for different applications. Many approaches seek to adapt visible light-sensing technologies, but these can suffer from unwanted response to visible light and various instabilities. A variety of solid-state and vacuum devices have been explored for use in different parts of the UV spectrum. Ultraviolet light 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.

Another way to measure uv photons is with a spectrometer. For info on spectrometers that can detect uv, see Ultraviolet–visible spectroscopy.

Another way to measure uv photon flux intensity is with a uv radiometer.

Near- and medium-UV
Between 200 and 400 nm, a variety of detector options exist. Photographic film detects near UV coming from blue sky as "violet" as far as the glass optics of cameras will permit, which is usually to about 350 nm. For outdoor film photography, in fact, slightly yellow UV filters are often standard equipment in order to prevent unwanted bluing and overexposure by UV light that the eye does not see (these filters are also convenient lens scratch protectors). For photography only in the near UV, special filters may be used. For UV with wavelengths shorter than 350 nm, special quartz lens systems must be used, which do not absorb the radiation.

Digital cameras use sensors that are usually sensitive to UV, but some have internal filters that block it, in order to present images in truer color as they would be seen by the eye. Some of these systems may be adapted by removing the internal UV filter, and adding an external visible light filter. Others have no internal filter and can be used unmodified for near-UV photography, with only use of an external visible light filter. A few systems are designed for use in the UV. (See ultraviolet photography).

People cannot perceive UV light directly since the lens of the eye blocks most light 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 light, and people lacking a lens (a condition known as aphakia) perceive near-UV light as whitish-blue or whitish-violet.

Vacuum UV
Vacuum UV or VUV (wavelengths shorter than 200 nm) is blocked by air but can propagate through a vacuum. These wavelengths are strongly absorbed by molecular oxygen in the air. Pure nitrogen (with less than about 10 ppm oxygen) is transparent to wavelengths in the range of about 150 – 200 nm. This has practical significance, since semiconductor manufacturing processes have been using wavelengths shorter than 200 nm. By working in oxygen-free gas, the equipment does not have to be built to withstand vacuum. Some other scientific instruments that operate in this spectral region, such as circular dichroism spectrometers, are also commonly nitrogen-purged.

Technology for VUV instrumentation was largely driven by solar astronomy physics for many decades, but more recently some photolithography applications for semiconductors have been developed in this range. 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. Recently, a diamond-based device flew on the solar observation satellite LYRA (see also Marchywka Effect).

Ultraviolet with wavelengths less than 125 nm has photon energies of more than 10 eV and is formally considered to be ionizing radiation, causing ionization even to pure nitrogen gas. This behavior causes the formally ionizing part of ultraviolet to be completely absorbed by the Earth's atmosphere.

Extreme UV
Extreme UV (EUV) is characterized by a transition in the physics of interaction with matter: Wavelengths longer than about 30 nm interact mainly with the chemical valence electrons of matter, whereas shorter wavelengths interact mainly with inner-shell electrons and nuclei. The long end of the EUV/XUV spectrum is set by a prominent He+ spectral line at 30.4 nm. XUV is strongly absorbed by most known materials, but it is possible to synthesize multilayer optics that reflect up to about 50% of XUV radiation at normal incidence. This technology, which was pioneered by the NIXT and MSSTA sounding rockets in the 1990s, has been used to make telescopes for solar imaging (current examples are SOHO/EIT and TRACE), and equipment for nanolithography (printing of very small-scale traces and devices on microchips).

Human health-related effects
The health effects ultraviolet radiation has on human health has implications on weighting the risks and benefits of sun exposure, but is also implicated in issues such as fluorescent lamps and health.

Vitamin D
UVB exposure induces the production of vitamin D in the skin at a rate of up to 1,000 IUs per minute. The majority of positive health effects are related to this vitamin. It has regulatory roles in calcium metabolism (which is vital for normal functioning of the nervous system, as well as for bone growth and maintenance of bone density), immunity, cell proliferation, insulin secretion, and blood pressure.

Exposure to sunlight for extended periods of time does not normally cause vitamin D toxicity.

Within about 20 minutes of ultraviolet exposure in light-skinned individuals (3–6 times longer for pigmented skin), the concentrations of vitamin D precursors produced in the skin reach an equilibrium, and any further vitamin D that is produced is degraded.

Aesthetics
Too little UVB radiation may lead to a lack of vitamin D. Too much UVB radiation may lead to direct DNA damage, sunburn, and skin cancer. An appropriate amount of UVB (which varies according to skin color) leads to a limited amount of direct DNA damage. This is recognized and repaired by the body, then melanin production is increased, which leads to a long-lasting tan. This tan occurs with a 2-day lag phase after irradiation.

Medical applications
Ultraviolet radiation has other medical applications, in the treatment of skin conditions such as psoriasis and vitiligo. UVA radiation has been much used in conjunction with psoralens (PUVA treatment) for psoriasis, although this treatment is less used now because the combination produces dramatic increases in skin cancer, and because treatment with UVB radiation by itself is more effective. In cases of psoriasis and vitiligo, UV light with wavelength of 311 nm is most effective.

Harmful effects
An overexposure to UVB radiation can cause sunburn and some forms of skin cancer. However, the most deadly form – malignant melanoma – is mostly caused by the indirect DNA damage (free radicals and oxidative stress). This can be seen from the absence of a UV-signature mutation in 92% of all melanoma. In humans, prolonged exposure to solar UV radiation may result in acute and chronic health effects on the skin, eye, and immune system. Moreover, UVC can cause adverse effects that can variously be mutagenic or carcinogenic.

UVC rays are the highest energy, most dangerous type of ultraviolet light.

On 13 April 2011, the International Agency for Research on Cancer of the World Health Organization classified all categories and wavelengths of ultraviolet radiation as a Group 1 carcinogen. This is the highest-level designation for carcinogens and means "There is enough evidence to conclude that it can cause cancer in humans".



Skin
UVA, UVB, and UVC can all damage collagen fibers and, therefore, accelerate aging of the skin. Both UVA and UVB destroy vitamin A in skin, which may cause further damage. In the past, UVA was considered not harmful or less harmful, but today it is known it can contribute to skin cancer via indirect DNA damage (free radicals and reactive oxygen species). UVA can generate highly reactive chemical intermediates, such as hydroxyl and oxygen radicals, which in turn can damage DNA. The DNA damage caused indirectly to skin by UVA consists mostly of single-strand breaks in DNA, while the damage caused by UVB includes direct formation of thymine dimers or other pyrimidine dimers, and double-strand DNA breakage. UVA is immunosuppressive for the entire body (accounting for a large part of the immunosuppressive effects of sunlight exposure), and UVA is mutagenic for basal cell keratinocytes in skin.

Because UVA does not cause reddening of the skin (erythema), it is not measured in the usual types of SPF testing. There is no good clinical measurement for blockage of UVA radiation, but it is important for sunscreen to block both UVA and UVB. Some scientists blame the absence of UVA filters in sunscreens for the higher melanoma risk found for sunscreen users.



UVB light can cause direct DNA damage. As noted above, UVB radiation excites DNA molecules in skin cells, causing aberrant covalent bonds to form between adjacent Pyrimidine bases, producing a dimer. Most UV-induced pyrimidine dimers in DNA are removed by the process known as nucleotide excision repair that employs about 30 different proteins. Those pyrimidine dimers that escape this repair process can induce a form of programmed cell death called apoptosis or can cause DNA replication errors leading to mutation.

When DNA polymerase comes along to replicate a strand of DNA with an unrepaired pyrimidine dimer, it reads a CC dimer as AA and not the original CC. This causes the DNA replication mechanism to add a TT on the growing strand. This mutation can result in cancerous growths, and is known as a "classical C-T mutation". The mutations caused by the direct DNA damage carry a UV signature mutation that is commonly seen in skin cancers. The mutagenicity of UV radiation can be easily observed in bacterial cultures. This cancer connection is one reason for concern about ozone depletion and the ozone hole. Individuals with an inherited defect in one of the proteins necessary for nucleotide excision repair may suffer from a condition called xeroderma pigmentosum that is characterized by extreme sun-sensitivity and by a high incidence of skin cancers. (Also see DNA repair-deficiency disorder.

As a defense against UV radiation, the type and amount of the brown pigment melanin in the skin increases when exposed to moderate (depending on skin type) levels of radiation; this is commonly known as a sun tan. The purpose of melanin is to absorb UV radiation and dissipate the energy as harmless heat, blocking the UV from damaging skin tissue. UVA gives a quick tan that lasts for days by oxidizing melanin that was already present, and it triggers the release of the melanin from melanocytes. However, because this process does not increase the total amount of melanin, a UVA-produced tan is largely cosmetic and does not protect against either sunburn or UVB-produced DNA damage or cancer.

By contrast, UVB yields a slower tan that requires roughly two days to develop, because the mechanism of UVB tanning is to stimulate the body to produce more melanin. However, the production of melanin by UV, called melanogenesis, requires direct DNA damage by UVB to initiate. The photochemical properties of melanin make it an excellent photoprotectant from both UVA and UVB. Older and more widespread sunscreen chemicals cannot dissipate the energy of the excited state as efficiently as melanin, and, therefore, the penetration of these sunscreen ingredients into the lower layers of the skin may increase the amount of free radicals and reactive oxygen species (ROS). In recent years, improved filtering substances have come into use in commercial sunscreen lotions that do not significantly degrade or lose their capacity to protect the skin as the exposure time increases (photostable substances).

Sunscreen prevents the direct DNA damage that causes sunburn, by blocking of UVB. As such, most of these products contain an SPF rating that indicates how well they block UVB as a measure of their effectiveness (SPF is, therefore, also called UVB-PF, for "UVB protection factor"). This rating, however, offers no data about protection against UVA, exposure to which does not lead to sunburn but is still harmful, since it causes indirect UV DNA damage and is also (along with UVB and UVC) considered carcinogenic. In the US, the Food and Drug Administration is considering adding a star rating system to show UVA protection (also known as UVA-PF). A similar system is already used in some European countries. Some sunscreen lotions now include compounds such as titanium dioxide, which helps protect against UVA rays. Other UVA blocking compounds found in sunscreen include zinc oxide and avobenzone.

Sunscreen safety debate


Medical organizations recommend patients protect themselves from UV radiation by using sunscreen. Five sunscreen ingredients have been shown to protect mice against skin tumors (see sunscreen).

However, some sunscreen chemicals produce potentially harmful substances if they are illuminated while in contact with living cells. The amount of sunscreen that penetrates through the stratum corneum may be large enough to cause damage. In one study of sunscreens, the authors write:

"The question whether UV filters acts on or in the skin has so far not been fully answered. Despite the fact that an answer would be a key to improve formulations of sun protection products, many publications carefully avoid addressing this question."

In an experiment by Hanson et al. published in 2006, the amount of harmful reactive oxygen species (ROS) was measured in untreated and in sunscreen treated skin. In the first 20 minutes, the film of sunscreen had a protective effect and the amount of ROS was smaller. After 60 minutes, however, the amount of absorbed sunscreen was so high, the amount of ROS was higher in the sunscreen treated skin than in the untreated skin.

Such effects can be avoided by using newer generations of filter substances or combinations that maintain their UV protective properties even after several hours of solar exposure. Sunscreen products containing photostable filters like drometrizole trisiloxane, bisoctrizole, or bemotrizinol have been available for many years throughout the world, but are not yet available in the U.S., whereas another high-quality filter, ecamsule, has also been available in the U.S. since 2006.

Aggravation of skin diseases
Ultraviolet radiation causes aggravation of several skin conditions and diseases, including:
 * Systemic lupus erythematosus
 * Sjögren’s syndrome
 * Sinear Usher syndrome
 * Rosacea
 * Dermatomyositis
 * Darier’s disease
 * Kindler-Weary syndrome.

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.

UV light is absorbed by molecules known as chromophores, which are present in the eye cells and tissues. Chromophores absorb light energy from the various wavelengths at different rates – a pattern known as absorption spectrum. If too much UV light is absorbed, eye structures such as the cornea, the lens, and the retina can be damaged.

Protective eyewear is beneficial to those working with or exposed to ultraviolet radiation, in particular 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 acrylic 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.

Degradation of polymers, pigments and dyes
Many polymers used in consumer products are degraded by UV light, and need addition of UV absorbers to inhibit attack, especially if the products are exposed to sunlight. The problem appears as discoloration or fading, cracking, and, sometimes, total product disintegration if cracking has proceeded sufficiently. The rate of attack increases with exposure time and sunlight intensity.

It is known as UV degradation, and is one form of polymer degradation. Sensitive polymers include thermoplastics, such as polypropylene, polyethylene, and poly(methyl methacrylate) as well as speciality fibers like aramids. UV absorption leads to chain degradation and loss of strength at sensitive points in the chain structure. They include tertiary carbon atoms, which in polypropylene occur in every repeat unit. Aramid rope must be shielded with a sheath of thermoplastic if it is to retain its strength. The impact of UV on polymers is used in nanotechnology, transplantology, X-ray lithography, and other fields for modification of properties (roughness, hydrophobicity) of polymer surfaces. For example, a poly(methyl methacrylate) surface can be smoothed by vacuum ultraviolet (VUV).



In addition, many pigments and dyes absorb UV and change colour, so paintings and textiles may need extra protection both from sunlight and fluorescent bulbs, two common sources of UV radiation. Old and antique paintings such as watercolour paintings, for example, usually must be placed away from direct sunlight. Common window glass provides some protection by absorbing some of the harmful UV, but valuable artifacts need extra shielding. Many museums place black curtains over watercolour paintings and ancient textiles, for example. Since watercolours can have very low pigment levels, they need extra protection from UV light. Tinted glasses, such as sunglasses also provide protection from UV rays.

Blockers and absorbers
Ultraviolet Light Absorbers (UVAs) are molecules used in organic materials (polymers, paints, etc.) to absorb UV light to reduce the UV degradation (photo-oxidation) of a material. A number of different UVAs with different absorption properties exist. UVAs can disappear over time, so monitoring of UVA levels in weathered materials is necessary.

In sunscreen, ingredients that absorb UVA/UVB rays, such as avobenzone and octyl methoxycinnamate, are known as absorbers. They are contrasted with physical "blockers" of UV radiation such as titanium dioxide and zinc oxide. (See sunscreen for a more complete list.)

UV absorption is also important in the behaviors of many animals. For example, in the Colias eurytheme butterfly, males rely on visual cues to locate and identify females. Instead of using chemical stimuli to find mates, males are attracted to the ultraviolet-absorbing color of female hind wings.

Suspended nanoparticles in stained glass prevent UV light from causing chemical reactions that change image colors. For this reason, stained glass is being used to capture true color images of mars for the 2019 ESA mars rover mission.

Applications
Because of its ability to cause chemical reactions and excite fluorescence in materials, ultraviolet light has a huge number of useful applications in modern society, listed below. The following table gives some of the uses of specific wavelength bands in the UV spectrum
 * 13.5 nm: Extreme ultraviolet lithography
 * 30–200 nm: Photoionization, ultraviolet photoelectron spectroscopy
 * 230–365 nm: UV-ID, label tracking, barcodes
 * 230–400 nm: Optical sensors, various instrumentation
 * 240–280 nm: Disinfection, decontamination of surfaces and water (DNA absorption has a peak at 260 nm)
 * 200–400 nm: Forensic analysis, drug detection
 * 270–360 nm: Protein analysis, DNA sequencing, drug discovery
 * 280–400 nm: Medical imaging of cells
 * 300–320 nm: Light therapy in medicine
 * 300–365 nm: Curing of polymers and printer inks
 * 300–400 nm: Solid-state lighting
 * 350–370 nm: Bug zappers (flies are most attracted to light at 365 nm)

Astronomy


In astronomy, very hot objects preferentially emit UV radiation (see Wien's law). Because the ozone layer blocks many UV frequencies from reaching telescopes on the surface of the Earth, most UV observations are made from space. (See UV astronomy, space observatory.)

Fire detection
In general, ultraviolet detectors 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 that 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, whereas a coal fire emits very weakly in the UV band yet very strongly at IR wavelengths; thus, a fire detector that 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 UVC band, whereas 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.

Checking high-voltage electrical insulation by corona discharge detection
An application of UV is to detect corona discharge (often called "corona") on electrical apparatus. Degradation of insulation in 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.

Fluorescent lamps
Fluorescent lamps produce UV radiation by ionizing 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.

Lasers
Ultraviolet lasers have applications in industry (laser engraving), medicine (dermatology, and keratectomy), chemistry (MALDI), free air secure communications, and computing (optical storage). They can be made by applying frequency conversion to lower-frequency lasers, or from Ce:LiSAF crystals (cerium doped with lithium strontium aluminum fluoride), a process developed in the 1990s at Lawrence Livermore National Laboratory.

Fluorescent optical brighteners
Colorless fluorescent dyes that emit blue light under UV are added as optical brighteners to a number of white-colored products, from white paper to white fabrics and other textiles as textile finishing agents. These ubiquitous dyes are the reason for the bright-blue fluorescence of many papers and fabrics under UV. The extra blue light emitted by these agents counteracts yellow tints that may be present, and causes the colors and whites to appear whiter or (if colored) more brightly and purely colored.

UV fluorescent dyes that glow in the primary color of paints, papers and textiles, also are used to enhance the color of these materials.

Paints
Paints that contain dyes that glow under UV are used in a number of art and esthetic applications.

Security
To help prevent counterfeiters, sensitive documents (e.g., credit cards, driver's licenses, passports) may also include a UV watermark that is visible only under ultraviolet light. Passports issued by most countries usually contain "UV-sensitive" (which means UV fluorescent) inks and security threads. These emit characteristic visible light of a particular color when activated by UV. Visa stamps and stickers on passports of visitors contain large detailed seals made of such inks, that are invisible under normal light, but strongly visible under UV illumination. Many passports have UV-sensitive (fluorescent) watermarks on all pages. Currencies of various countries' banknotes have an image, as well as many multicolor fibers, that are visible only under ultraviolet light.

Some brands of pepper spray will leave an invisible chemical (UV dye) that is not easily washed off on a pepper-sprayed attacker, which would help police identify the attacker later.

Forensics
UV is an investigative tool at the crime scene helpful in locating and identifying bodily fluids such as semen, blood, and saliva. For example, ejaculated fluids or saliva can be detected by high-power UV light sources, irrespective of the structure or colour of the surface the fluid is deposited upon. UV-Vis microspectroscopy is also used to analyze trace evidence, such as textile fibers and paint chips, as well as questioned documents.

Authentication
In other detective work including authentication of various collectibles and art, and detecting counterfeit currency even absent of UV-fluorescent marker dyes (for use of such dyes, see "security" section above). Even unmarked 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.

Reading otherwise-illegible papyri and manuscripts
Using multi-spectral imaging it is possible to read illegible papyrus, such as the burned papyri of the Villa of the Papyri or of Oxyrhynchus, or the Archimedes palimpsest. The technique involves taking pictures of the illegible document 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. Simple NUV sources can be used to highlight faded iron-based ink on vellum.

Chemical markers
UV fluorescent dyes are used in many applications (for example, biochemistry and forensics). The Green Fluorescent Protein (GFP) is often used in genetics as a marker. Many substances, such as proteins, 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.

Sanitary compliance
Ultraviolet light aids in the detection of organic material deposits that remain on surfaces where periodic cleaning and sanitizing may not have been properly accomplished. The phenyl and indole chemical moieties in proteins absorb UV, and are made visible by blocking the fluorescence of the material beneath them—often UV brighteners in fabrics. Detergents are easily detected using UV inspection. In "ABS" or alkylbenzenesulfonate detergents, the substituted benzine absorbs UV. Phosphate detergents with a phenyl moiety also absorb.

Pet urine deposits in carpeting or other hard surfaces can be detected for accurate treatment and removal of mineral traces and the odor-causing bacteria that feed on proteins in urine. Many hospitality industries use UV lamps to inspect for unsanitary bedding to determine life-cycle for mattress restoration, as well as general performance of the cleaning staff. A perennial news feature for many television news organizations involves an investigative reporter's using a similar device to reveal unsanitary conditions in hotels, public toilets, hand rails, and such.

Spectrophotometry
UV/VIS spectroscopy is widely used as a technique in chemistry to analyze chemical structure, the most notable one being conjugated systems. UV radiation is often used to excite a given sample where the fluorescent emission is measured with a spectrofluorometer. In biological research, UV light is used for quantification of nucleic acids or proteins.

Analyzing minerals


Ultraviolet lamps are also used in analyzing minerals and 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.

Photolithography
Ultraviolet radiation is used for very fine resolution photolithography, a procedure wherein a chemical called a photoresist is exposed to UV radiation that has passed through a mask. The light causes chemical reactions to occur in the photoresist, and, after development (a step that removes either the exposed or the unexposed photoresist), a pattern determined by the mask remains on the sample. Steps may then be taken to "etch" away, deposit on or otherwise modify areas of the sample where no photoresist remains.

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.


 * Photolithography processes (Processes used to fabricate electronic integrated circuits) especially make use of Extreme Ultraviolet radiations. For example, the microprocessor manufacturing processes implemented by major companies such as Intel Corporation, AMD, Qualcomm make use of EUV light pencil to draw electronic circuits on silicon wafers at subatomic scales. Latest microprocessor devices manufactured in this way have their onchip integrated circuitry of 22 nm size (latest process technology by Intel as of 2012). Other integrated chip manufacturing processes help fabricate electronic chips of standard sizes of 32 nm, 45 nm, 65 nm. Going forward, the thickness of electronic circuits on these chips would further come down to 14 nm and then to thickness range of 7 nm, 5 nm and 4 nm. Reducing the thickness of circuits on silicon wafer chips provide advantages of low power usage, lesser heating, and faster response time, along with providing faster circuitry on smaller form factors (miniaturization). All this becomes possible using EUV-based photolithographic processes.

Curing of electronic potting resins
Electronic components that require clear transparency for light to exit or enter (photo voltaic panels and sensors) can be potted using acrylic resins that are cured using UV light energy. The advantages are low VOC emissions and rapid curing.

Curing of inks, adhesives, varnishes and coatings
Certain inks, coatings, and adhesives are formulated with photoinitiators and resins. When exposed to the correct energy and irradiance in the required band of UV light, polymerization 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 fiber coatings, the coating of flooring, UV Coating and paper finishes in offset printing, dental fillings, and decorative finger nail "gels".

An industry has developed around the manufacture of UV sources for UV curing applications. This includes UV lamps, UV LEDs, and Excimer Flash lamps. Fast processes such as flexo or offset printing require high-intensity light focused via reflectors onto a moving substrate and medium; and high-pressure Hg (mercury) or Fe (iron, doped)-based bulbs are used, which can be energized with electric arc or microwaves. Lower-power sources (fluorescent lamps, LED) can be used for static applications, and, in some cases, small high-pressure lamps can have light focused and transmitted to the work area via liquid-filled or fiber-optic light guides.

Erasing EPROM modules
Some EPROM (erasable 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 is stronger.

UV solar cells and UV degradation of solar cells
Japan's National Institute of Advanced Industrial Science and Technology (AIST) has succeeded in developing a transparent solar cell that uses ultraviolet light to generate electricity but allows visible light to pass through it. Most conventional solar cells use visible and infrared light to generate electricity. In contrast, the innovative new solar cell uses ultraviolet radiation. Used to replace conventional window glass, the installation surface area could be large, leading to potential uses that take advantage of the combined functions of power generation, lighting, and temperature control.

Also PEDOT-PSS solar cells is an ultraviolet (UV) light-selective and -sensitive photovoltaic cell easily fabricated.

On the other hand, a nanocrystalline layer of Cu2O in the construction of photovoltaic cells increases their ability to utilize UV radiations for photocurrent generation.

Nondestructive testing
UV light of a specified spectrum and intensity is used to stimulate fluorescent dyes so as to highlight defects in a broad range of materials. These dyes may be carried into surface-breaking defects by capillary action (liquid penetrant inspection) or they may be bound to ferrite particles caught in magnetic leakage fields in ferrous materials (magnetic particle inspection).

Postage stamps
Postage stamps are tagged with a phosphor that glows under UV light (the U.S. uses short wave UV) to permit automatic detection of the stamp and facing of the letter.

Air purification
Using a catalytic reaction from titanium dioxide and UV light exposure, a strong oxidative effect occurs on any organic objects that pass through the media, converting otherwise-irritating pathogens, pollens, and mold spores into harmless inert byproducts. The cleansing mechanism of UV is a photochemical process. The contaminants that pollute the indoor environment are almost entirely based upon organic or carbon-based compounds. These compounds break down when exposed to high-intensity UV at 240 to 280 nm. Short-wave ultraviolet light can destroy DNA in living microorganisms and break down organic material found in indoor air. UVC's effectiveness is directly related to intensity and exposure time.

UV light has also been shown (by KJ Scott et al) as effective in reducing gaseous contaminants such as carbon monoxide and VOCs. Scott and his colleagues demonstrated that the correct mixture of UV lamps radiating at 184 and 254 nm can remove low concentrations of hydrocarbons and carbon monoxide, if the lamps are held in a radiation chamber (a box or drum) and the air is recycled between the room and the reaction chamber. This arrangement prevents the introduction of ozone into the treated air. Likewise, air may be treated by passing by a single UV source operating at 184 nm and subsequent catalysis with iron pentaoxide. The iron oxides remove the ozone produced by the UV lamp.

Microbial sterilization


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). However, since microorganisms can be shielded from ultraviolet light in small cracks and other shaded areas, 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 is commonly used in wastewater treatment applications and is finding an increased usage in drinking water treatment. Many bottlers of spring water use UV disinfection equipment to sterilize their water. Solar water disinfection is the process of using PET bottles and sunlight to disinfect water. Ultraviolet germicidal irradiation is the generic process to inactivate microorganisms in water, air, medical environments, etc.

New York City has approved the construction of a 2.2 billion US gallon per day (535,000 m3/hr) ultraviolet drinking water disinfection facility which was due to be online in 2012. There are also several facilities under construction and several in operation that treat waste water with several stages of filters, hydrogen peroxide, and UV light to bring the water up to drinking standards. One such facility exists in Orange County, California, which is designed to treat wastewater and convert it into high-quality water for Indirect Potable Reuse. NASA has examined the use of this technology, using titanium dioxide as catalyst, for breaking down harmful products in spacecraft waste water.

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 that 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 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 has been found that protists are able to survive high UV-C doses but are sterilized at low doses.

Solar water disinfection (SODIS) has been extensively researched in Switzerland and has proven ideal to treat small quantities of water cheaply using natural sunlight. Contaminated water is poured into transparent plastic bottles and exposed to full sunlight for six hours. The sunlight treats the contaminated water through two synergetic mechanisms: UV-A irradiation and increased water temperature. If the water temperatures rises above 50 C, the disinfection process is three times faster.

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 kill 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).

Biological surveys and pest control
Some animals, including birds, reptiles, and insects such as bees, can see near-ultraviolet light. Many fruits, flowers, and seeds stand out more strongly from the background in ultraviolet wavelengths as compared to human color vision. Scorpions glow or take on a yellow to green color under UV illumination, thus assisting in the control of these arachnids. Many birds have patterns in their plumage that are invisible at usual wavelengths but observable in ultraviolet, and the urine and other secretions of some animals, including dogs, cats, and human beings, is much easier to spot with ultraviolet. Urine trails of rodents can be detected by pest control technicians for proper treatment of infested dwellings.

Butterflies use ultraviolet as a communication system for sex recognition and mating behavior.

Many insects use the ultraviolet wavelength emissions from celestial objects as references for flight navigation. A local ultraviolet emitter will normally disrupt the navigation process and will eventually attract the flying insect. Ultraviolet traps called bug zappers are used to eliminate various small flying insects. They are attracted to the UV light, and are killed using an electric shock, or trapped once they come into contact with the device. Different designs of ultraviolet light traps are also used by entomologists for collecting nocturnal insects during faunistic survey studies.

Photochemotherapy
Exposure to UVA light while the skin is hyper-photosensitive by taking psoralens is an effective treatment for psoriasis called PUVA. Due to the potential of psoralens to cause damage to the liver, PUVA may be used only a limited number of times over a patient's lifetime.

Phototherapy
Exposure to UVB light, in particular, the 310 nm narrowband UVB range, is an effective long-term treatment for many skin conditions like psoriasis, vitiligo, eczema, and others. UVB phototherapy does not require additional medications or topical preparations for the therapeutic benefit; only the light exposure is needed. However, phototherapy can be effective when used in conjunction with certain topical treatments such as anthralin, coal tar, and Vitamin A and D derivatives, or systemic treatments such as methotrexate and soriatane.

Typical treatment regimens involve short exposure to UVB rays 3 to 5 times a week at a hospital or clinic, and repeated sessions may be required before results are noticeable. Almost all of the conditions that respond to UVB light are chronic problems, so continuous treatment is required to keep those problems in check. Home UVB systems are common solutions for those whose conditions respond to treatment. Home systems permit patients to treat themselves every other day (the ideal treatment regimen for most) without the frequent, costly trips to the office/clinic and back.

Side-effects may include itching and redness of the skin due to UVB exposure, and possibly sunburn, if patients do not minimize exposure to natural UV rays during treatment days. Cataracts can frequently develop if the eyes are not protected from UVB light exposure. To date, there is no link between an increase in a patient's risk of skin cancer and the proper use of narrow-band UVB phototherapy. "Proper use" is generally defined as reaching the "Sub-Erythemic Dose" (S.E.D.), the maximum amount of UVB your skin can receive without burning. Certain fungal growths under the toenail can be treated using a specific wavelength of UV delivered from a high-power LED (light-emitting diode) and can be safer than traditional systemic drugs.

Note that this is different from phototherapy for physiological neonatal jaundice in infants, which uses blue light, not UV.

Herpetology
Reptiles need long wave UV light for de novo synthesis of vitamin D. Vitamin D is needed to metabolize calcium for bone and egg production. Thus, in a typical reptile enclosure, a fluorescent UV lamp should be available for vitamin D synthesis. This should be combined with the provision of heat for basking, either in the same or by another lamp. Certain reptiles such as Bearded Dragons need both, UVA and UVB light.

Sun tanning
Sun tanning describes a darkening of the skin in a natural physiological response stimulated by exposure to ultraviolet radiation from sunshine (or a sunbed). With excess exposure to the sun, a suntanned area can also develop sunburn. The increased production of melanin is triggered by the direct DNA damage. This kind of damage is recognized by the body and as a defense against UV radiation the skin produces more melanin. Melanin dissipates the UV energy as harmless heat, and therefore it is an excellent photoprotectant. Melanin protects against the direct DNA damage and against the indirect DNA damage. Sunscreen protects only against the direct DNA damage, but increases the indirect DNA damage. Some studies suggest that this may be the cause of the higher incidence of melanoma found in sunscreen users compared to non-users.

Evolutionary significance
Evolution of early reproductive proteins and enzymes is attributed in modern models of evolutionary theory to ultraviolet light. UVB light causes thymine base pairs next to each other in genetic sequences to bond together into thymine dimers, a disruption in the strand that reproductive enzymes cannot copy (see picture above). This leads to frameshifting during genetic replication and protein synthesis, usually killing the organism. As early prokaryotes began to approach the surface of the ancient oceans, before the protective ozone layer had formed, blocking out most wavelengths of UV light, they almost invariably died out. The few that survived had developed enzymes that monitored the genetic material and removed thymine dimers by nucleotide excision repair enzymes. Many enzymes and proteins involved in modern mitosis and meiosis are similar to repair enzymes, and are believed to be evolved modifications of the enzymes originally used to overcome DNA damages caused by UV light.