Encyclopedia
Ultraviolet light is
electromagnetic radiation with a
wavelength shorter than that of visible light, but longer than soft
X-rays. It can be subdivided into
near UV ,
far or
vacuum UV , and
extreme UV .
When considering the effect of UV radiation on human health and the environment, the range of UV wavelengths is often subdivided into
UVA , also called Long Wave or "blacklight";
UVB , also called Medium Wave; and
UVC , also called Short Wave or "germicidal". See 1 E-7 m for a list of objects of comparable sizes.
In
photolithography, in
laser technology, etc., the term
deep ultraviolet or
DUV refers to wavelengths below 300 nm.
The name means "beyond violet" , violet being the
color of the shortest wavelengths of visible light. Some of the UV wavelengths are 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 of some animals is much easier to spot with ultraviolet.
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.
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 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 .
Discovery
Soon after
infrared radiation had been discovered, the German physicist Johann Wilhelm Ritter began to look for radiation at the opposite end of the spectrum, at the short wavelengths beyond violet. In 1801 he used
silver chloride, a light-sensitive chemical, to show that there was a type of invisible light beyond violet, which he called chemical rays. At that time, many scientists, including Ritter, concluded that light was composed of three separate components: an oxidising or calorific component , an illuminating component , and a reducing or hydrogenating component . The unity of the different parts of the spectrum was not understood until about 1842, with the work of Macedonio Melloni,
Alexandre-Edmond Becquerel and others. During that time, UV radiation was also called "actinic radiation".
Health concerns and protection
In humans, prolonged exposure to solar UV radiation may result in acute and chronic health effects on the
skin, eye, and
immune system.
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.
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 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 "dark-light" and, because of its longer wavelength, can penetrate most windows. It also penetrates deeper into the skin than UVB light and is thought to be a prime cause of
wrinkles.
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 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 that partly blocks UV 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 Polypodium leucotomos.
What to look for in sunscreen:UVB protection: Padimate O,
Homosalate, Octisalate , Octinoxate
UVA protection: AvobenzoneUVA/UVB protection: Octocrylene,
titanium dioxide,
zinc oxide, Mexoryl
Eye
High intensities of UVB light are hazardous to the eyes, and exposure can cause
welder's flash 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 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 acrlyic 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. To convince yourself of the potential dangers of stray UV light, cover your lenses with something opaque, like aluminum foil, stand next to a bright light, and consider how much light you see, despite the complete blockage of the lenses. Most intraocular lenses help to protect the
retina by absorbing UV radiation.
Beneficial effects
A positive effect of UV light is that it induces the production of
vitamin D in the skin. claims tens of thousands of premature deaths occur in the US annually from cancer due to insufficient UVB exposures . Another effect of vitamin D deficiency is osteomalacia , which can result in bone pain, difficulty in weight bearing and sometimes fractures.
Ultraviolet radiation has other medical applications, in the treatment of skin conditions such as
psoriasis and
vitiligo. UVB and UVA radiation can be used, in conjunction with psoralens .
Most effective in case of psoriasis and vitiligo is UV light with wavelength of 311 nm.
Uses
Black lights
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 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 such as those issued by Ukraine contain large and detailed seals invisible to the naked eye under normal lights, but strongly visible under UV illimunation. Passports issued by the United States have the UV sensitive threads on the last page of the passport along with the barcode.
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 , 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 . However, the same ozone layer that protects us causes difficulties for astronomers observing from the Earth, so most UV observations are made from space.
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 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. UV fluorescent
dyes are used in many applications . The fluorescent
protein Green Fluorescent Protein 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.
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 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 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 which coincides very well with one of the two peaks of the germicidal effectiveness curve . 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 . 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 . Since microorganisms can be shielded from ultraviolet light in small cracks and other shaded areas, however, 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. 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 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 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 turns out 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 .
Fire detection
Ultraviolet detectors generally use either a solid-state device, such as one based on
silicon carbide or
aluminum 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 , 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 adhesives and coatings
Certain
adhesives and coatings are formulated with photoinitiators. When exposed to the correct dose and intensity in the required band of UV light, polymerisation 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, paper finishes in offset printing, and dental fillings.
Deterring Substance Abuse in Public Restrooms
UV lights have been installed in some parts of the world in public restrooms 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.
Erasing EPROM modules
Some
EPROM modules are erased by exposure to UV radiation. These modules often have a transparent glass 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.
See also
References
External links
