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Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light but longer than X-rays (from 400 nm to 10 nm in wavelength). Though usually invisible, under some conditions children and young adults can see ultraviolet down to wavelengths of about 310 nm, and people with aphakia (missing lens) can also see some UV wavelengths. Near-UV is visible to a number of insects and birds.
UV radiation is present in sunlight, and is produced by electric arcs and specialized lights such as mercury-vapor lamps, tanning lamps, and black lights. Although lacking the energy to ionize atoms, ultraviolet radiation can cause chemical reactions, and causes many substances to glow or fluoresce. Consequently, biological effects of UV are greater than simple heating effects, and many practical applications of UV radiation derive from its interactions with organic molecules.
Suntan and sunburn are familiar effects of over-exposure. Living things on dry land would be severely damaged by ultraviolet radiation from the sun if most of it were not filtered out by the earth's atmosphere, particularly the ozone layer More-energetic, shorter-wavelength "extreme" UV below 121 nm ionizes air so strongly that it is absorbed before it reaches the ground. Ultraviolet is also responsible for the formation of bone-strengthening vitamin D in most land vertebrates, including humans. The UV spectrum thus has effects both beneficial and harmful to human health.
- 1 Discovery
- 2 Subtypes
- 3 Solar ultraviolet
- 4 Blockers and absorbers
- 5 Artificial sources
- 6 Human health-related effects
- 7 Degradation of polymers, pigments and dyes
- 8 Applications
- 8.1 Photography
- 8.2 Electrical and electronics industry
- 8.3 Fluorescent dye uses
- 8.4 Analytic uses
- 8.5 Material science uses
- 8.6 Polymers
- 8.7 Biology-related uses
- 9 Evolutionary significance
- 10 See also
- 11 References
- 12 Further reading
"Ultraviolet" means "beyond violet" (from Latin ultra, "beyond"), violet being the color of the highest frequencies of visible light. Ultraviolet light has a higher frequency than violet light.
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, although there were those who held that these were an entirely different sort of radiation from light (notably Professor J.W. Draper, M.D.), who named them "tithonic rays". The terms chemical and heat rays were eventually dropped in favour of ultraviolet and infrared radiation, respectively.
|Energy per photon
|Notes / alternative names|
|Ultraviolet A||UVA||400 – 315 nm||3.10 – 3.94 eV||long wave, black light, not absorbed by the ozone layer|
|Ultraviolet B||UVB||315 – 280 nm||3.94 – 4.43 eV||medium wave, mostly absorbed by the ozone layer|
|Ultraviolet C||UVC||280 – 100 nm||4.43 – 12.4 eV||short wave, germicidal, completely absorbed by the ozone layer and atmosphere|
|Near Ultraviolet||NUV||400 – 300 nm||3.10 – 4.13 eV||visible to birds, insects and fish|
|Middle Ultraviolet||MUV||300 – 200 nm||4.13 – 6.20 eV|
|Far Ultraviolet||FUV||200 – 122 nm||6.20 – 10.16 eV|
|Hydrogen Lyman-alpha||H Lyman-α||122 – 121 nm||10.16 – 10.25 eV||spectral line at 121.6 nm, 10.20 eV|
|Vacuum Ultraviolet||VUV||200 – 10 nm||6.20 – 124 eV||strongly absorbed by atmospheric oxygen, though 150–200 nm wavelengths can propagate through nitrogen|
|Extreme Ultraviolet||EUV||121 – 10 nm||10.25 – 124 eV||ionizing radiation, completely absorbed by the atmosphere|
Different detectors are used for the different UV bands. Silicon detectors are used across the spectrum, and the US NIST has characterized simple silicon diodes that work with visible light too. 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. Spectrometers and radiometers are made for measurement of UV radiation.
People cannot perceive UV light directly since the lens of the human 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 or VUV wavelengths (shorter than 200 nm) are strongly absorbed by molecular oxygen in the air, though the longer wavelengths of about 150–200 nm can propagate through nitrogen. Scientific instruments can therefore utilize this spectral range by operating in an oxygen-free atmosphere (commonly pure nitrogen), without the need for costly vacuum chambers. Significant examples include 193 nm photolithography equipment (for semiconductor manufacturing), and circular dichroism spectrometers.
Technology for VUV instrumentation was largely driven by solar astronomy for many decades. While optics can be used to remove unwanted visible light that contaminates the VUV, in general, detectors can be limited by their response to non-VUV radiation, and the development of "solar-blind" devices has been an important area of research. Wide-gap solid-state devices or vacuum devices with high-cutoff photocathodes can be attractive compared to silicon diodes.
Extreme UV (EUV or sometimes XUV) is characterized by a transition in the physics of interaction with matter. Wavelengths longer than about 30 nm interact mainly with the outer valence electrons of atoms, while wavelengths shorter than that interact mainly with inner shell electrons and nuclei. The long end of the EUV spectrum is set by a prominent He+ spectral line at 30.4 nm. EUV is strongly absorbed by most known materials, but it is possible to synthesize multilayer optics that reflect up to about 50% of EUV radiation at normal incidence. This technology, which was pioneered by the NIXT and MSSTA sounding rockets in the 1990s, has been used to make telescopes for solar imaging such as SOHO/EIT and TRACE), and equipment for extreme ultraviolet lithography used in the manufacture of integrated circuits.
Very hot objects emit UV radiation (see Black-body radiation). The Sun emits ultraviolet radiation at all wavelengths, including the extreme ultraviolet where it crosses into X-rays at 10 nm. Extremely hot stars emit proportionally more UV radiation than the Sun. Sunlight in space at the top of Earth's atmosphere is composed 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 sunlight is 44% visible light, 3% ultraviolet (with the Sun at its zenith), and the remainder infrared.
The atmosphere blocks 77% of the Sun's UV, mostly in the shorter UV wavelengths, when the Sun is highest in the sky (zenith). 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 shorter bands of UVC, as well as even more-energetic radiation 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 shorter wavelengths are blocked by ordinary air. Of the ultraviolet radiation that reaches the Earth's surface, up to 95% is the longer wavelengths of UVA, depending on cloud cover and atmospheric conditions.
Blockers and absorbers
Ultraviolet light absorbers are molecules used in organic materials (polymers, paints, etc.) to absorb UV light to reduce the UV degradation (photo-oxidation) of a material. The absorbers can themselves degrade over time, so monitoring of absorber levels in weathered materials is necessary.
In sunscreen, ingredients that absorb UVA/UVB rays, such as avobenzone, oxybenzone and octyl methoxycinnamate, are known as absorbers or chemical "blockers". They are contrasted with physical "blockers" of UV radiation such as titanium dioxide and zinc oxide.
Suspended nanoparticles in stained glass prevent UV light from causing chemical reactions that change image colors. A set of stained glass color reference chips is planned to be used to calibrate the color cameras for the 2019 ESA Mars rover mission, since they will remain unfaded by the high level of UV present at the surface of Mars.
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.
Wood's glass is a nickel-bearing form of glass with a deep blue-purple color that blocks most visible light and passes ultraviolet light.
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.
A black light lamp emits long-wave UVA radiation and little visible light. Fluorescent black light lamps use a phosphor on the inner tube surface, which emits UVA light instead of visible light. Some lamps use a deep-bluish-purple Wood's glass optical filter that blocks almost all visible light with wavelengths longer that 400 nanometres.  Others use plain glass instead of the more expensive Wood's glass, so they appear light-blue to the eye when operating. A black light may also be formed, very inefficiently, by using a layer of Wood's glass in the envelope for an incandescent bulb. Though cheaper than fluorescent UV lamps, only 0.1% of the input power is converted to usable radiation, as the incandescent light radiates very little emission in the UV range. Mercury-vapor black lights in ratings up to 1 kW with UV-emitting phosphor and an envelope of Wood's glass are used for theatrical and concert displays. UVA/UVB emitting bulbs are also sold for other special purposes, such as tanning lamps and reptile-keeping.
Short wave ultraviolet lamps
A shortwave UV lamp can be made using a fluorescent lamp tube with no phosphor coating. These lamps emit ultraviolet light with two peaks in the UVC band at 253.7 nm and 185 nm due to the mercury within the lamp. 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 blocks the 185 nm wavelength. Such tubes have two or three times the UVC power of a regular fluorescent lamp tube. These low-pressure lamps have a typical efficiency of approximately 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. These "germicidal" lamps are used extensively for disinfection of surfaces in laboratories and food processing industries, and for disinfecting water supplies.
Specialized UV gas-discharge lamps containing different gases 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 UV light source developed within the last two decades, is seeing increasing use in scientific fields. It has the advantages of high-intensity, high efficiency, and operation at a variety of wavelength bands into the vacuum ultraviolet.
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.
Gas lasers, laser diodes and 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. Another type of high power gas laser is the excimer laser. They are widely-spread lasers emitting in ultraviolet and vacuum ultraviolet wavelength ranges.
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 can also be made by applying frequency conversion to lower-frequency lasers.
Ultraviolet lasers have applications in industry (laser engraving), medicine (dermatology, and keratectomy), chemistry (MALDI), free air secure communications, and computing (optical storage). Presently, UV argon-fluoride (ArF) excimer lasers operating at 193 nm are routinely used in integrated circuit production by photolithography. Recently, extreme ultraviolet lasers at 13.5 nm (bordering on X-ray wavelengths at 10 nm) have been employed experimentally in extreme ultraviolet lithography in producing the smallest features (10 nm and less) of the latest generation of integrated circuits. These are hoped to be in production by 2016.
UVB induces production of vitamin D in the skin at rates of up to 1,000 IUs per minute. The majority of UV's positive health effects are related to this vitamin, which helps to regulate calcium metabolism (vital for the nervous system and bone health), immunity, cell proliferation, insulin secretion, and blood pressure.
The amount of the brown pigment melanin in the skin increases after exposure to UV radiation at moderate levels depending on skin type; this is commonly known as a sun tan. Melanin is an excellent photoprotectant that absorbs both UVB and UVA radiation and dissipates the energy as harmless heat, protecting the skin against both direct and indirect DNA damage.
In humans, excessive exposure to UV radiation can result in acute and chronic harmful effects on the skin, eye, and immune system. Overexposure to UVB radiation can cause sunburn and some forms of skin cancer. The World Health Organization (WHO) classified all categories and wavelengths of ultraviolet radiation as a Group 1 carcinogen. The WHO-standard Ultraviolet index is a widely publicized measurement of the strength of UV wavelengths that cause sunburn on human skin.
UVB light can cause direct DNA damage. The mutagenicity of UV radiation can be observed in bacterial cultures. This cancer connection is one reason for concern about ozone depletion and the ozone hole.
Medical organizations recommend that patients protect themselves from UV radiation by using sunscreen. Five sunscreen ingredients have been shown to protect mice against skin tumors. However, some sunscreen chemicals produce potentially harmful substances if they are illuminated while in contact with living cells.
Aggravation of certain skin conditions
Ultraviolet radiation can aggravate several skin conditions and diseases, including:
- Systemic lupus erythematosus
- Sjögren’s syndrome
- Sinear Usher syndrome
- Darier’s disease
- Kindler-Weary syndrome
Unshielded exposure of the skin or eyes to mercury arc lamps that do not have a conversion phosphor is quite dangerous. High intensities of UVB light are hazardous to the eyes, and can cause welder's flash or arc eye (photokeratitis and 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 different wavelengths at different rates, a pattern known as the absorption spectrum. If too much UV light is absorbed, the cornea, the lens, and the retina can be damaged.
Protective eyewear is beneficial to those exposed to ultraviolet radiation, in particular short-wave UV. Since light can reach the eyes 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. Protective coating is available for eyeglass lenses that need it, but even a coating 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.
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 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.
Because of its ability to cause chemical reactions and excite fluorescence in materials, ultraviolet light has a huge number of useful applications. The following table gives some uses of specific wavelength bands in the UV spectrum
- 13.5 nm: Extreme ultraviolet lithography
- 30–200 nm: Photoionization, ultraviolet photoelectron spectroscopy, standard integrated circuit manufacture by photolithography
- 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)
Photographic film responds to ultraviolet radiation but the glass lenses of cameras usually block radiation shorter than 350 nm. Slightly yellow UV-blocking filters are often used for outdoor photography to prevent unwanted bluing and overexposure by UV light. For photography in the near UV, special filters may be used. Photography with wavelengths shorter than 350 nm requires special quartz lenses which do not absorb the radiation. Digital cameras sensors may have internal filters that block UV to improve color rendition accuracy. Sometimes these internal filters can be removed, or they may be absent, and an external visible-light filter prepares the camera for near-UV photography. A few cameras are designed for use in the UV.
Photography by reflected ultraviolet radiation is useful for medical, scientific, and forensic investigations, in applications as wide spread as detecting bruising of skin, alterations of documents, or restoration work on paintings. Photography of the fluorescence produced by ultraviolet illumination uses visible wavelengths of light.
In ultraviolet astronomy, measurements are used to discern the chemical composition of the interstellar medium, and the temperature and composition of stars. Because the ozone layer blocks many UV frequencies from reaching telescopes on the surface of the Earth, most UV observations are made from space.
Electrical and electronics industry
An application of UV is to detect corona discharge 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.
Some EPROM (erasable programmable read-only memory) modules are erased by exposure to UV radiation. These modules have a transparent (quartz) window on the top of the chip that allows the UV radiation in.
Fluorescent dye uses
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. Blacklight paints that contain dyes that glow under UV are used in a number of art and esthetic applications.
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. Postage stamps are tagged with a phosphor that glows under UV light to permit automatic detection of the stamp and facing of the letter.
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.
In some types of nondestructive testing UV light stimulates fluorescent dyes 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).
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.
Other applications include authentication of various collectibles and art, and detecting counterfeit currency. Even materials not specially marked with UV sensitive dyes may have distinctive fluorescence under UV light, or may fluoresce differently under short-wave versus long-wave ultraviolet.
Enhancing contrast of ink
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.
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.
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.
In pollution control applications, ultraviolet analyzers are used to detect emissions of nitrogen oxides, sulfur compounds, mercury and ammonia, for example in the flue gas of fossil fired power plants. Ultraviolet light can detect thin sheens of spilled oil on water, either by the high reflectivity of oil films at UV wavelengths, fluorescence of compounds in oil, or by absorbing of UV light created by Raman scattering in water.
Material science uses
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.
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. After removal of unwanted 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.
Photolithography is used in the manufacture of semiconductors, integrated circuit components, and printed circuit boards. Photolithography processes used to fabricate electronic integrated circuits presently use 193 nm UV, and are experimentally using 13.5 nm UV for extreme ultraviolet lithography.
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.
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.
The impact of UV on polymers is used for modification of the (roughness and hydrophobicity) of polymer surfaces. For example, a poly(methyl methacrylate) surface can be smoothed by vacuum ultraviolet.
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.
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.
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.
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.
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 (120 °F), the disinfection process is three times faster.
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.
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. 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.
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.
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Ultraviolet radiation is helpful in the treatment of skin conditions such as psoriasis and vitiligo. Exposure to UVA light while the skin is hyper-photosensitive by taking psoralens is an effective treatment for psoriasis. Due to the potential of psoralens to cause damage to the liver, PUVA therapy may be used only a limited number of times over a patient's lifetime.
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.
Reptiles need long-wave UVA light for synthesis of vitamin D, which in turn 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 by the same lamp or another. Certain reptiles such as Bearded Dragons need both UVA and UVB light.
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. This leads to frameshifting during genetic replication and protein synthesis, usually killing the cell. Before formation of the UV-blocking ozone layer, when early prokaryotes approached the surface of the ocean, 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.
- High-energy visible light
- UV stabilizers in plastics
- Weather testing of polymers
- Ultraviolet catastrophe
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