Ultraviolet

False-color image of the Sun's corona as seen in deep ultraviolet by the Extreme ultraviolet Imaging Telescope

Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 10 nm to 400 nm, and energies from 3eV to 124 eV. It is so named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the colour violet.

UV light is found in sunlight and is emitted by electric arcs and specialized lights such as black lights. Classified as non-ionizing radiation, it can cause chemical reactions, and causes many substances to glow or fluoresce. Most people are aware of the effects of UV through the painful condition of sunburn, but the UV spectrum has many other effects, both beneficial and damaging, to human health.

Discovery

The discovery of UV radiation was intimately associated with the observation that silver salts darken 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 were especially effective at darkening silver chloride-soaked paper. He called them "de-oxidizing rays" to emphasize chemical reactivity and to distinguish them from "heat rays" 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 favor of ultraviolet and infrared radiation, respectively.^[1]

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.^[2]

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 that of violet light.

Subtypes

The electromagnetic spectrum of ultraviolet light can be subdivided in a number of ways. The draft ISO standard on determining solar irradiances (ISO-DIS-21348)^[3] describes the following ranges:

In photolithography and laser technology, the term deep ultraviolet or DUV refers to wavelengths below 300 nm. "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, pure nitrogen being commonly used, which avoids the need for a vacuum chamber.

See 1 E-7 m for a list of objects of comparable sizes.

Sources of UV

Natural sources of UV

The sun emits ultraviolet radiation in the UVA, UVB, and UVC bands. The Earth's ozone layer blocks 97-99% of this UV radiation from penetrating through the atmosphere.^[4] 98.7% of the ultraviolet radiation that reaches the Earth's surface is UVA. (Some of the UVB and UVC radiation is responsible for the generation of the ozone layer.) Extremely hot stars emit proportionally more UV radiation than the sun; the star R136a1 has a thermal energy of 4.57 eV, which falls in the near-UV range.

Ordinary glass is partially transparent to UVA but is opaque to shorter wavelengths while 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.^[5][6][7]

The onset of vacuum UV, 200 nm, is defined by the fact that ordinary air is opaque at shorter wavelengths. This opacity is due to the strong absorption of light of these wavelengths by oxygen in the air. Pure nitrogen (less than about 10 ppm oxygen) is transparent to wavelengths in the range of about 150–200 nm. This has wide practical significance now that semiconductor manufacturing processes are using wavelengths shorter than 200 nm. By working in oxygen-free gas, the equipment does not have to be built to withstand the pressure differences required to work in a vacuum. Some other scientific instruments, such as circular dichroism spectrometers, are also commonly nitrogen purged and operate in this spectral region.

Extreme UV 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, while wavelengths shorter than that 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 has been used to make telescopes for solar imaging; it was pioneered by the NIXT and MSSTA sounding rockets in the 1990s; (current examples are SOHO/EIT and TRACE) and for nanolithography (printing of traces and devices on microchips).

"Black light"

A black light, or Wood's light, is a lamp that emits long wave UV radiation and very little visible light. Commonly these are referred to as simply a "UV 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 may be replaced by a deep-bluish-purple glass called Wood's glass, a nickel-oxide–doped glass, which blocks almost all visible light above 400 nanometres. The color of such lamps is often referred to in the trade as "blacklight blue" or "BLB." This is to distinguish these lamps from "bug zapper" blacklight ("BL") lamps that don't have the blue Wood's glass. 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⁺) while 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.

While "black lights" do produce light in the UV range, their spectrum is confined to the longwave UVA region. Unlike UVB and UVC, which are responsible for the direct DNA damage that leads to skin cancer, black light is limited to lower energy, longer waves and does not cause sunburn. However, UVA is capable of causing damage to collagen fibers and destroying vitamins A and D in skin.

A black light may also be formed by simply using Wood's glass 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 it remains a cheaper alternative to the fluorescent method, it is exceptionally inefficient at producing UV light (less than 0.1% of the input power) owing to the black body nature of the incandescent light source. Incandescent UV bulbs, due to their inefficiency, may also become dangerously hot during use. More rarely still, high power (hundreds of watts) mercury vapor black lights that use a UV emitting phosphor and an envelope of Wood's glass can be found. These lamps are used mainly for theatrical and concert displays and also become very hot during normal use.

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.

Ultraviolet light can also be generated by some light-emitting diodes.

Ultraviolet fluorescent lamps

Fluorescent lamps without a phosphorescent coating to convert UV to visible light, emit ultraviolet light peaking at 254 nm due to the peak emission of the mercury within the bulb. With the addition of a suitable phosphorescent coating, they can be modified to produce a UVA, UVB, or visible light spectrum (all fluorescent tubes used for domestic and commercial lighting are mercury (Hg) UV emission bulbs at heart).

Such low pressure mercury lamps are used extensively for disinfection, and in standard form have an optimum operating temperature of approx 30 degrees Celsius. Use of a mercury amalgam allows operating temperature to rise to 100 degrees Celsius, and UVC emission to approx double or triple.

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 approx 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 inerted UV curing environments. Power densities approaching 3,000 mW/cm² (30 kW/m²) are now possible and this, coupled with recent developments by photoinitiator and resin formulators make the expansion of LED-cured UV materials likely.

Ultraviolet lasers

UV laser diodes and UV solid-state lasers can be manufactured to emit light in the ultraviolet range. 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). 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.^[8]

Gas-discharge lamps

Argon and deuterium lamps are often used as stable sources, either windowless or with various windows such as magnesium fluoride.^[9]

Detecting and measuring UV radiation

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^[10] 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 the different part of the UV spectrum. Ultraviolet light can be detected by suitable photodiodes and photocathodes, which can be tailored to be sensitive in different parts of the UV spectrum. Sensitive ultraviolet photomultipliers are available.

Near UV

Between 200-400 nm, a variety of detector options exist.

Vacuum UV

Technology for VUV instrumentation has been largely driven by solar physics for many decades and more recently some lithographic applications. While optics can be used to remove unwanted visible light that contaminates the VUV, generally 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 LYRA (see also Marchywka Effect).

Human health-related effects of UV radiation

Beneficial effects

Vitamin D

UVB exposure induces the production of vitamin D in the skin. 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.^[11]

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 the melanin production is increased, which leads to a long-lasting tan. This tan occurs with a 2-day lag phase after irradiation, but it is much less harmful and is longer lasting than the one obtained from UVA.

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.^[12][13]

Harmful effects

An overexposure to UVB radiation can cause sunburn and some forms of skin cancer. In humans, prolonged exposure to solar UV radiation may result in acute and chronic health effects on the skin, eye, and immune system.^[14] 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.^[15]

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 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.

Skin

UVA, UVB and UVC can all damage collagen fibers and thereby accelerate aging of the skin. Both UVA and UVB destroy vitamin A in skin, which may cause further damage.^[17] In the past, UVA was considered less harmful, but today it is known that it can contribute to skin cancer via indirect DNA damage (free radicals and reactive oxygen species). It penetrates deeply but it does not cause sunburn. UVA does not damage DNA directly like UVB and UVC, but it can generate highly reactive chemical intermediates, such as hydroxyl and oxygen radicals, which in turn can damage DNA. Because it does not cause reddening of the skin (erythema) it cannot be measured in SPF testing. There is no good clinical measurement for blockage of UVA radiation, but it is important that sunscreen block both UVA and UVB. Some scientists blame the absence of UVA filters in sunscreens for the higher melanoma-risk that was found for sunscreen users.^[18]

The reddening of the skin due to the action of sunlight depends both on the amount of sunlight and on the sensitivity of the skin ("erythemal action spectrum") over the UV spectrum.

UVB light can cause direct DNA damage. The radiation excites DNA molecules in skin cells, causing aberrant covalent bonds to form between adjacent cytosine bases, producing a dimer. When DNA polymerase comes along to replicate this strand of DNA, it reads the dimer as "AA" and not the original "CC". This causes the DNA replication mechanism to add a "TT" on the growing strand. This is a mutation, which can result in cancerous growths and is known as a "classical C-T mutation". The mutations that are 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 bacteria cultures. This cancer connection is one reason for concern about ozone depletion and the ozone hole. UVB causes some damage to collagen but at a very much slower rate than UVA.

As a defense against UV radiation, the 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 triggers the release of the melanin from melanocytes. UVB yields a tan that takes roughly 2 days to develop because it stimulates the body to produce more melanin. The photochemical properties of melanin make it an excellent photoprotectant. Older and more widespread sunscreen chemicals can not 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).^[19] In recent years, improved filtering substances have come into use in commercial sunscreen lotions that don't significantly degrade or lose their capacity to protect the skin as the exposure time increases (photostable substances).^[20]

Sunscreen prevents the direct DNA damage that causes sunburn. Most of these products contain an SPF rating to show how well they block UVB rays. The SPF rating, however, offers no data about UVA protection. In the US, the Food and Drug Administration is considering adding a star rating system to show UVA protection. 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 that patients protect themselves from UV radiation 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.^[21][22][23] The amount of sunscreen that penetrates through the stratum corneum may or may not be large enough to cause damage. In one study of sunscreens, the authors write:^[24]

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. that was 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 number of ROS species was smaller. After 60 minutes, however, the amount of absorbed sunscreen was so high, that the amount of ROS was higher in the sunscreen treated skin than in the untreated skin.^[19]

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.^[20]

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,^[25][26] 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 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 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

UV damaged polypropylene rope (left) and new rope (right)

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 and polyethylene as well as speciality fibres 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.

IR spectrum showing carbonyl absorption due to UV degradation of polyethylene

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 need to 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.

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 exist with different absorption properties. 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.)

Applications of UV

By wavelength:[1]

• 230-400 nm: Optical sensors, various instrumentation
• 230-365 nm: UV-ID, label tracking, barcodes
• 240-280 nm: Disinfection, decontamination of surfaces and water (DNA absorption has a peak at 260 nm)
• 250-300 nm: Forensic analysis, drug detection
• 270-300 nm: Protein analysis, DNA sequencing, drug discovery
• 280-400 nm: Medical imaging of cells
• 300-400 nm: Solid-state lighting
• 300-365 nm: Curing of polymers and printer inks
• 300-320 nm: Light therapy in medicine
• 350-370 nm: Bug zappers (flies are most attracted to light at 365 nm)[2]

Security

A bird appears on many Visa credit cards when held under a UV light source

To help thwart counterfeiters, sensitive documents (e.g. credit cards, driver's licenses, passports) may also include a UV watermark that is visible only under a UV-emitting light. Passports issued by most countries usually contain UV sensitive inks and security threads. Visa stamps and stickers on passports of visitors contain large detailed seals invisible under normal light, but strongly visible under UV illumination. Passports issued by many nations have UV sensitive watermarks on all pages. Currencies of various countries' banknotes have an image, as well as many multicolored 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 them later.^[27]

Forensics

UV is an investigative tool at the crime scene helpful in locating and identifying bodily fluids (semen, blood, bile etc.). E.g., ejaculated fluids or saliva are detected by high power UV, irrespective of the structure or colour of the surface the fluid is deposited upon.^[28]

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.

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.)

Hunting

Hunters can use UV lights to follow the blood trail of a wounded animal.

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 emissor will normally disrupt the navigation process and will eventually attract the flying insect.

Entomologist using a UV light for collecting beetles in the Paraguayan Chaco.

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.

Spectrophotometry

UV/VIS spectroscopy is widely used as a technique in chemistry, to analyze chemical structure, most notably conjugated systems. UV radiation is often used in visible spectrophotometry to determine the fluorescence of a given sample. In biological research, UV light is used for quantification of nucleic acids.

Sanitary compliance

UV lamps including newer LEDs (light emitting diode) aid in the detection of organic mineral deposits that remain on surfaces where periodic cleaning and sanitizing may not be properly accomplished. Both urine and phosphate soaps are easily detected using UV inspection. Pet urine deposits in carpeting or other hard surfaces can be detected for accurate treatment and removal of mineral tracers and the odor causing bacteria that feeds on proteins within. 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 using a similar device to reveal unsanitary conditions in hotels, public toilets, hand rails, and such.

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.

Analyzing minerals

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.

Authentication

In other detective work including authentication of various collectibles and art, and detecting counterfeit currency absent of marker dyes. 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.

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.

Photochemotherapy

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

Phototherapy

Exposure to UVB light, particularly the 310 nm narrowband UVB range, is an effective long-term treatment for many skin conditions like psoriasis, vitiligo, eczema, and others.^[29] 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.^[30]

Typical treatment regimes involve short exposure to UVB rays 3 to 5 times a week at a hospital or clinic, and up to 30 or more sessions may be required before results are noticeable. Almost all of the conditions that respond to UVB light are chronic problems, so continual 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 the patient 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. There is no link between an increase in the patient's risk for skin cancer and the proper use of 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.

Photolithography

Ultraviolet radiation is used for very fine resolution photolithography, a procedure where 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 either removes the exposed or unexposed photoresist), a pattern determined by the mask remains on the sample. Steps may then be taken to "etch" away 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^[31] and printed circuit boards.

Checking electrical insulation

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.^[32]

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.

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.

New York City has approved the construction of a 2 billion US gallon per day (7,600,000 m³/d) ultraviolet drinking water disinfection facility.^[33] 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.^[34][35] NASA has examined the use of this technology, using titanium dioxide as catalyst, for breaking down harmful products in spacecraft waste water.^[36]

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.^[37] It has been found that protists are able to survive high UV-C doses but are sterilized at low doses.

Solar water disinfection^[38] (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 (120 °F), 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.^[39] 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 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, while 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, 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.

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(basking), either in the same or by another lamp.

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, and dental fillings. Curing of 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 energised 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 fibre optic light guides.

Deterring substance abuse in public places

UV lights have been installed in many parts of the world in public restrooms, and on public transport, for the purpose of deterring substance abuse. The blue color of these lights, combined with the fluorescence of the skin, make it harder for intravenous drug users to find a vein.^[40] 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.

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.^[41] 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.^[21][22][23] Some studies suggest that this may be the cause of the higher incidence of melanoma found in sunscreen users compared to non-users.^{[42][43][18][44][45]}

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.

Reading otherwise 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, or the Archimedes palimpsest. 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.

Lasers

Ultraviolet lasers have applications in industry (laser engraving), medicine (dermatology and keratectomy), 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.^[46]

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.^[47]

Also PEDOT-PSS solar cells is a ultraviolet (UV) light selective and sensitive photovoltaic cell easily fabricated.^[48]

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.^[49]

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).

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 verified the genetic material and broke up thymine dimer bonds, known as base excision repair enzymes. Many enzymes and proteins involved in modern mitosis and meiosis are similar to excision repair enzymes, and are believed to be evolved modifications of the enzymes originally used to overcome UV light.^[50]

See also

• Black light
• High-energy visible light
• Infrared light
• Polymer degradation
• Risks and benefits of sun exposure
• Sun tanning
• Tanning lamp
• Titanium dioxide
• Ultraviolet blood irradiation
• Ultraviolet light and cancer
• Ultraviolet photography
• UV degradation
• UV index
• UV stabilizers in plastics
• Wood's lamp
• Weather testing of polymers

References

Further reading

• Hu, S; Ma, F; Collado-Mesa, F; Kirsner, R. S. (July 2004). "UV radiation, latitude, and melanoma in US Hispanics and blacks". Arch. Dermatol. 140 (7): 819–824. doi:10.1001/archderm.140.7.819. PMID 15262692. http://archderm.ama-assn.org/cgi/content/full/140/7/819. 
• Hockberger, Philip E. (2002). "A History of Ultraviolet Photobiology for Humans, Animals and Microorganisms" (– ^{Scholar search}). Photochemisty and Photobiology 76 (6): 561–569. doi:10.1562/0031-8655(2002)076<0561:AHOUPF>2.0.CO;2. http://www.bioone.org/archive/0031-8655/76/6/pdf/i0031-8655-76-6-561.pdf. 
• Allen, Jeannie (2001-09-06). Ultraviolet Radiation: How it Affects Life on Earth. Earth Observatory. NASA, USA. http://earthobservatory.nasa.gov/Library/UVB/.