Everything about Ultraviolet Radiation totally explained
Ultraviolet (
UV) light is
electromagnetic radiation with a
wavelength shorter than that of
visible light, but longer than soft
X-rays. It is so named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the color
violet.
UV light is typically found as part of the radiation received by the Earth from the Sun. Most humans are aware of the effects of UV through the painful condition of
sunburn. The UV spectrum has many other effects, including both beneficial and damaging changes 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 their 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.
Origin of 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) describes the following ranges:
| Name |
Abbreviation |
Wavelength range in nanometers |
Energy per photon |
| Ultraviolet A, long wave, or black light |
UVA |
400 nm - 315 nm |
3.10 - 3.94 eV |
| Near |
NUV |
400 nm - 300 nm |
3.10 - 4.13 eV |
| Ultraviolet B or medium wave |
UVB |
315 nm - 280 nm |
3.94 - 4.43 eV |
| Middle |
MUV |
300 nm - 200 nm |
4.13 - 6.20 eV |
| Ultraviolet C, short wave, or germicidal |
UVC |
280 nm - 100 nm |
4.43 - 12.4 eV |
| Far |
FUV |
200 nm - 122 nm |
6.20 - 10.2 eV |
| Vacuum |
VUV |
200 nm - 10 nm |
6.20 - 124 eV |
| Extreme |
EUV |
121 nm - 10 nm |
10.2 - 124 eV |
In
photolithography, in
laser technology, etc., the term deep ultraviolet or DUV refers to wavelengths below 300 nm. "Vacuum UV" is so named because it's 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.
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 nanometers. 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 nanometer 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 nanometers 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 doesn't cause sunburn. However, UVA is capable of causing damage to collagen fibers and destroying vitamin A 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's exceptionally inefficient at producing UV light (a mere few lumens per watt) 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 can be found which use a UV emitting phosphor and an envelope of Wood's glass. 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 for use in bug zappers 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.
Natural sources of UV
The
Sun emits ultraviolet radiation in the UVA, UVB, and UVC bands, but because of absorption in the
atmosphere's ozone layer, 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.)
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.
The onset of vacuum UV, 200 nm, is defined by the fact that ordinary air is opaque below this wavelength. 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 doesn't 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 30.4nm. XUV is strongly absorbed by most known materials, but it's 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).
Human health-related effects of UV radiation
Beneficial effects
The Earth's atmosphere blocks UV radiation from penetrating through the atmosphere by 98.7%. A positive effect of UVB exposure is that it induces the production of
vitamin D in the skin. It has been estimated that tens of thousands of premature deaths occur in the United States annually from a range of cancers due to vitamin D deficiency. Another effect of vitamin D deficiency is
osteomalacia (the adult equivalent of rickets), which can result in bone pain, difficulty in weight bearing and sometimes fractures. Other studies show most people get adequate Vitamin D through food and incidental exposure.
Too little UVB radiation leads to a lack of Vitamin D. Too much UVB radiation leads to
direct DNA damages and sunburn. An appropriate amount of UVB (What is appropriate depends on your skin colour) 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's much less harmful and long lasting than the one obtained from UVA.
Ultraviolet radiation has other medical applications, in the treatment of skin conditions such as
psoriasis and
vitiligo. UVA radiation can be used in conjunction with psoralens (
PUVA treatment). UVB radiation is
rarely used in conjunction with
psoralens. 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. In humans, prolonged exposure to solar UV radiation may result in acute and chronic
health effects on the
skin, eye, and
immune system. 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.
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're 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.
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.
In the past UVA was considered less harmful, but today it's known, that it can contribute to skin cancer via the
indirect DNA damage (free radicals and reactive oxygen species). It penetrates deeply but it doesn't cause
sunburn.
UVA doesn't 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 doesn't cause reddening of the skin (erythema) it can't be measured in the
SPF testing. There is no good clinical measurement of the blocking of UVA radiation, but it's 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.
UVB light can cause
direct DNA damage. The radiation
excites DNA molecules in skin cells, causing
covalent bonds to form between adjacent
thymine bases, producing thymidine dimers. Thymidine dimers don't base pair normally, which can cause distortion of the DNA helix, stalled replication, gaps, and misincorporation. These can lead to
mutations, which can result in
cancerous growths. The mutations that are caused by the direct DNA damage carry a UV signature mutation. 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 body tans when exposed to moderate (depending on
skin type) levels of radiation. UVA gives a quick tan that last for days by oxidizing melanin that was already present and it 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 purpose of melanin is to block UV radiation and to dissipate the energy as harmless heat. Thereby the potential damage to the skin tissues is prevented. The photochemical properties of melanin make it an excellent
photoprotectant. However, sunscreen chemicals can not dissipate the energy of the excited state as efficiently as melanin and therefore the penetration of sunscreen ingredients into the lower layers of the skin is increasing the amount of
free radicals and
ROS.
Sunscreen prevents the
direct DNA damage which 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
FDA 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.
Cantaloupe extract, rich in the compound
superoxide dismutase (SOD), can be bound with
gliadin to form
glisodin, an orally-effective protectant against UVB radiation. There are also naturally occurring compounds found in rainforest plants that have been known to protect the skin from UV radiation damage, such as the fern
Phlebodium aureum.
Sunscreen safety debate
The majority of doctors recommend patients protect themselves from UV radiation using sunscreen. These recommendations exist despite the ample evidence in the scientific literature that the sunscreen chemicals are harmful if they're illuminated while in contact with living cells. It is undisputed in the scientific literature, that sunscreen chemicals in contact with living tissue are harmful. However, the proponents of sunscreen claim that the amount of sunscreen which penetrates through the
stratum corneum isn't large enough to cause damage. The Authors Chatelain et al. criticize (page 34):
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 that was published by Kerry Hanson et al. the amount of harmful
ROS had been 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. (See the
sunscreen controversy article for a full discussion of this issue.)
Eye
High intensities of UVB light are hazardous to the eyes, and exposure can cause
welder's flash (
photokeratitis or
arc eye) and may lead to
cataracts,
pterygium, and
pinguecula formation.
Protective eyewear is beneficial to those who are working with or those who might be exposed to ultraviolet radiation, particularly short wave UV. Given that light may reach the eye from the sides, full coverage eye protection is usually warranted if there's an increased risk of exposure, as in high altitude mountaineering. Mountaineers are exposed to higher than ordinary levels of UV radiation, both because there's 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 won't 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 used externally and so exposed to
sunlight. The problem appears as discoloration or fading,
cracking and sometimes, total product disintegration if cracking has proceeded far enough. The rate of attack increases with exposure time and sunlight intensity.
It is 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.
In addition, many
pigments and
dyes absorb UV and change colour, so
paintings and
textiles may need extra protection both from sunlight and
fluorescent lamps, 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 shielding.
Blockers and absorbers
Ultraviolet Light Absorbers (UVAs) are molecules used in organic materials (
polymers,
paints, etc.) to absorb UV light in order 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 which 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
Security
To help thwart
counterfeiters, sensitive documents (for example
credit cards,
driver's licenses,
passports) may also include a UV watermark that can only be seen when viewed under a UV-emitting light. Passports issued by most countries usually contain UV sensitive inks and security threads.
Visa stamps and stickers on passports of visitors contain large and detailed seals invisible to the
naked eye under normal lights, but strongly visible under UV illumination. Passports issued by many nations have UV sensitive watermarks on all pages of the passport. Currencies of various countries'
banknotes have an image, as well as many multicolored fibers, that are visible only under ultraviolet light.
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 don't 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
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.)
Biological surveys and pest control
Some animals, including
birds,
reptiles, and
insects such as
bees, can see into the near ultraviolet. Many fruits, flowers, and seeds stand out more strongly from the background in ultraviolet wavelengths as compared to human color vision.
Scorpions glow or take on a yellow to green color under UV illumination. 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.
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 would eventually attract to itself the flying insect.
Ultraviolet traps 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 existence of fluorescence in a given sample.
Analyzing minerals
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.
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 many others. UVB phototherapy doesn't 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 regimes involve short exposure to UVB rays 3 to 5 times a week at a hospital or clinic, and for the best results, up to 30 or more sessions may be required.
Side effects may include itching and redness of the skin due to UVB exposure, and possibly sunburn, if patients don't minimize exposure to natural UV rays during treatment days.
Photolithography
Ultraviolet radiation is used for very fine resolution
photolithography, a procedure where a chemical known as a photoresist is exposed to UV radiation which has passed through a mask. The light allows chemical reactions to take place in the photoresist, and after development (a step that either removes the exposed or unexposed photoresist), a geometric pattern which is determined by the mask remains on the sample. Further steps may then be taken to "etch" away parts of the sample with no photoresist remaining.
UV radiation is used extensively in the electronics industry because photolithography is used in the manufacture of
semiconductors,
integrated circuit components and
printed circuit boards.
Checking electrical insulation
A new application of UV is to detect
corona discharge (often simply called "corona") on electrical apparatus. Degradation of insulation of electrical apparatus or pollution causes corona, wherein a strong electric field ionizes the air and excites nitrogen molecules, causing the emission of ultraviolet radiation. The corona degrades the insulation level of the apparatus. Corona produces
ozone and to a lesser extent
nitrogen oxide which may subsequently react with water in the air to form
nitrous acid and
nitric acid vapour in the surrounding air.
Sterilization
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 (for example, 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 gallon per day ultraviolet drinking water disinfection facility. 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.
It used to be thought that UV disinfection was more effective for bacteria and viruses, which have more exposed genetic material, than for larger pathogens which have outer coatings or that form cyst states (for example,
Giardia) that shield their DNA from the UV light. However, it was recently discovered that ultraviolet radiation can be somewhat effective for treating the microorganism
Cryptosporidium. The findings resulted in two
US patents
and the use of UV radiation as a viable method to treat drinking
water. Giardia in turn has been shown to be very susceptible to UV-C when the tests were based on infectivity rather than excystation. It has been found that
protists are able to survive high UV-C doses but are sterilized at low doses.
A process named SODIS
(External Link) has been extensively researched in Switzerland and has proven ideal to treat small quantities of water 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: Radiation in the spectrum of UV-A (wavelength 320-400 nm) 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).
Fire detection
Ultraviolet detectors generally use either a solid-state device, such as one based on
silicon carbide or
aluminium nitride, or a gas-filled tube as the sensing element. UV detectors which are sensitive to UV light in any part of the spectrum respond to irradiation by
sunlight and
artificial light. A burning hydrogen flame, for instance, radiates strongly in the 185 to 260 nanometer range and only very weakly in the
IR region, while a
coal fire emits very weakly in the UV band yet very strongly at IR wavelengths; thus a fire detector which operates using both UV and IR detectors is more reliable than one with a UV detector alone. Virtually all fires emit some
radiation in the UVB band, while the
Sun's radiation at this band is absorbed by the
Earth's atmosphere. The result is that the UV detector is "solar blind", meaning it won't cause an alarm in response to radiation from the Sun, so it can easily be used both indoors and outdoors.
UV detectors are sensitive to most fires, including
hydrocarbons,
metals,
sulfur,
hydrogen,
hydrazine, and
ammonia.
Arc welding, electrical arcs,
lightning,
X-rays used in nondestructive metal testing equipment (though this is highly unlikely), and radioactive materials can produce levels that will activate a UV detection system. The presence of UV-absorbing gases and vapors will attenuate the UV radiation from a fire, adversely affecting the ability of the detector to detect flames. Likewise, the presence of an oil mist in the air or an oil film on the detector window will have the same effect.
Curing of 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.
An industry has developed around the manufacture of
UV lamps sourced for UV curing applictions. 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) based bulbs are used which can be energised with electric arc or microwaves. Lower power fluorescent lamps 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.
Radtech is a trade association dedicated to the promotion of this technology.
Deterring substance abuse in public places
UV lights have been installed in some parts of the world in public restrooms, and on public transport, for the purpose of deterring substance abuse. The blue color of these lights, combined with the fluorescence of the skin, make it harder for intravenous drug users to find a vein. The efficacy of these lights for that purpose has been questioned, with some suggesting that drug users simply find a vein outside the public restroom and mark the spot with a marker for accessibility when inside the restroom. There is currently no published evidence supporting the idea of a deterrent effect.
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's 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 - this causes the higher amount of melanoma that had been found repeatedly in sunscreen users compared to non-users.
Erasing EPROM modules
Some
EPROM (electronically programmable read-only memory) modules are erased by exposure to UV radiation. These modules often have a transparent glass (
quartz) window on the top of the chip that allows the UV radiation in. These have been largely superseded by
EEPROM and
flash memory chips in most devices.
Preparing low surface energy polymers
UV radiation is useful in preparing low surface energy polymers for adhesives. Polymers exposed to UV light will oxidize thus raising the surface energy of the polymer. Once the surface energy of the polymer has been raised, the bond between the adhesive and the polymer won't be smaller.
Reading otherwise illegible papyruses
Using multi-spectral imaging it's possible to read illegible
papyruses, such as the burned papyruses of the
Villa of the Papyri or of
Oxyrhynchus. The technique involves taking pictures of the illegible papyruses using different filters in the infrared or ultraviolet range, finely tuned to capture certain wavelengths of light. Thus, the optimum spectral portion can be found for distinguishing ink from paper on the papyrus surface.
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.
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 which reproductive enzymes can't 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 which verified the genetic material and broke up
thymine dimer bonds, known as
excision repair enzymes. Many enzymes and proteins involved in modern
mitosis and
meiosis are extremely similar to excision repair enzymes, and are believed to be evolved modifications of the enzymes originally used to overcome UV light.
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