(Redirected from Ultraviolet ray)
An ultraviolet photograph of the Earth taken from the Moon by
Apollo 16 astronauts.
'Ultraviolet' ('UV') light is
electromagnetic radiation with a
wavelength shorter than that of
visible light, but longer than soft
X-rays.
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.
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 "deoxidizing 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
[1].
Subtypes
The part of the electromagnetic spectrum which ultraviolet light covers can be further subdivided in several different overlapping ways:
| Name | Abbreviation | Wavelength range in nanometres |
|---|
| 'Near' | NUV | 400 nm - 200 nm |
| UVA, long wave, or black light | | 400 nm - 320 nm |
| UVB or medium wave | | 320 nm - 280 nm |
| UVC, short wave, or germicidal | | Below 280 nm |
| 'Far or vacuum' | FUV, VUV | 200 nm - 10 nm |
| 'Extreme or deep' | EUV, XUV | 31 nm - 1 nm |
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 is absorbed by
air.
See
1 E-7 m for a list of objects of comparable sizes.
Black light
Main articles: Black light
Ultraviolet is colloquially called 'black light', as it is invisible to the human
eye. 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.
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, 99% of the ultraviolet radiation that reaches the Earth's surface is UVA. (Some of the UVB and UVC light 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.
[2][3][4]
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 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 30.4nm. 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).
Human Health Related Effects of UV Radiation
Beneficial Effects of UV radiation
A positive effect of UVB exposure is that it induces the production of
vitamin D in the skin. It has been estimated
[5] 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.
6
Many countries have
fortified certain foods with Vitamin D to prevent deficiency. Eating fortified foods or taking a
dietary supplement pill is usually preferred to UVB exposure, due to the increased risk of skin cancer from UV radiation.
[6]
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 of UV Radiation
In humans, prolonged exposure to solar UV radiation may result in acute and chronic
health effects on the
skin,
eye, and
immune system[7].
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 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. In general, UVA is the least harmful, but can contribute to the aging of skin, DNA damage and possibly skin cancer. It penetrates deeply and does not cause
sunburn. Because it does not cause reddening of the skin (erythema) it cannot be measured in the
SPF testing. There is no good clinical measurement of the blocking of UVA radiation, but it is important that
sunscreen block both UVA and UVB.
UVA light is also known as "black light" and, because of its longer wavelength, can penetrate many windows. It also penetrates deeper into the skin than UVB light and is thought to be a prime cause of
wrinkles.
The reddening of the skin due to the action of sunlight depends both on the amount of sunlight as well as the sensitivity of the skin ("erythemal action spectrum") over the UV spectrum.
UVB light can cause
skin cancer. The radiation
excites DNA molecules in skin cells, causing
covalent bonds to form between adjacent
thymine bases, producing thymidine dimers. Thymidine dimers do not 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
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.
As a defense against UV radiation, the body tans when exposed to moderate (depending on
skin type) levels of radiation by releasing the brown pigment
melanin. This helps to block UV penetration and prevent damage to the vulnerable skin tissues deeper down.
Suntan lotion, often referred to as "sun block" or "sunscreen", partly blocks UV and is widely available. Most of these products contain an
SPF rating that describes the amount of protection given. This protection, however, applies only to UVB rays responsible for sunburn and not to UVA rays that penetrate more deeply into the skin and may also be responsible for causing cancer and wrinkles. Some sunscreen lotion now includes compounds such as
titanium dioxide which helps protect against UVA rays. Other UVA blocking compounds found in sunscreen include
zinc oxide and
avobenzone. 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''.
'What to look for in sunscreen:'
'UVB protection:'
Padimate O,
Homosalate, Octisalate (
octyl salicylate), Octinoxate (
octyl methoxycinnamate)
'UVA protection:'
Avobenzone
'UVA/UVB protection:'
Octocrylene,
titanium dioxide,
zinc oxide, Mexoryl (
ecamsule)
Another means to block UV is
sun protective clothing. This is clothing that has a "UPF rating" that describes the protection given against both UVA and UVB.
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 [5] [5], 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 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.
Other Effects of UV Radiation
Many
polymers used in consumer products are degraded by UV light, and need addition of
UV stabilizers to inhibit attack. Products include thermoplastics, such as
polypropylene and
polyethylene as well as speciality fibres like
aramids. UV absorption leads to chain degradation and loss of 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 lamps.
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 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
Black lights
A bird appears on every Visa credit card when held under a UV light source.
A
black light is a lamp that emits long wave UV radiation and very little visible 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 is replaced by a deep bluish purple glass called
Wood's glass.
To thwart
counterfeiters, sensitive documents (e.g.
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 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''.)
Pest control
Ultraviolet fly traps are used for the elimination of various small flying insects. They are attracted to the UV light and are killed using an electrical shock or trapped once they come into contact with the device.
Spectrophotometry
UV/VIS spectroscopy is widely used as a technique in
chemistry, for analysis of
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
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.
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, proteins for instance, 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 310nm narrowband UVB range, is an effective long-term treatment for many skin conditions like
psoriasis,
vitiligo,
eczema, and many others. UVB phototherapy does not require additional medications or topical preparations for the therapeutic benefit; only the light exposure is needed. However, phototherapy can be effective when used in conjunction with certain topical treatments such as anthralin, coal tar, and Vitamin A and D derivatives, or systemic treatments such as methotrexate and soriatane.
[10]
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 do not 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
[11] 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
[12].
Sterilization
Main articles: Ultraviolet Germicidal Irradiation
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 was more commonly used in wastewater treatment applications but is finding increased usage in drinking water treatment. A process named SODIS
[1] has been extensively researched in Switzerland and proven ideal to treat small quantities of water. Contaminated water is filled into transparent plastic bottles and exposed to full sunlight for six hours. The sunlight is treating the contaminated water through two synergetic mechanisms: Radiation in the spectrum of UV-A (wavelength 320-400nm) and increased water temperature. If the water temperatures rises above 50°C, the disinfection process is three times faster.
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 (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 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
[5]. It has been found that
protists are able to survive high UV-C doses but are sterilized at low doses.
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 remove 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 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.
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 focussed 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 focussed 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.
[14] 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
A female sunbather
describes a darkening of the skin (especially of fair-skinned individuals) 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.
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 will be greater.
Reading completely 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. 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.
Evolutionary significance
Evolution of early reproductive
proteins and
enzymes is attributed in modern models of
evolutionary theory to ultraviolet light. Ultraviolet 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 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 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.
[5]
See also
★
UV index
★
High energy visible light
★
Sun tanning
★
Black light
★
Wood's lamp
★
UV Stabilizers in plastics
Further Reading
★
★
References
1.
Photochem Photobiol, 2002, 76, 561-579
2. Soda Lime Glass Transmission Curve
3. B270-Superwite Glass Transmission Curve
4. Selected Float Glass Transmission Curve
5.
6. The Science of Sun Protection, Talk of the Nation Science Friday, 24 June 2005. Vitamin D pills recommended over sun exposure, but most people in Australia and Canada get enough Vitamin D by incidental exposure, studies show.
7. Health effects of UV radiation
8.
9.
10. http://www.psoriasis.org/treatment/psoriasis/phototherapy/uvb.php
11. Deep UV Photoresists
12. Corona - The Daytime UV Inspection Magazine
13.
14. Public toilets' lighting has wrong effect from Coventry Telegraph
15.
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