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ultraviolet radiation,invisible electromagnetic radiationelectromagnetic radiation,
energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an electric field.
..... Click the link for more information. between visible violet light and X rays; it ranges in wavelength from about 400 to 4 nanometers and in frequency from about 1015 to 1017 hertz. It is a component (less than 5%) of the sun's radiation and is also produced artificially in arc lamps, e.g., in the mercury arc lamp.
The ultraviolet radiation in sunlight is divided into three bands: UVA (320–400 nanometers), which can cause skin damage and may cause melanomatous skin cancerskin cancer,
malignant tumor of the skin. The most common types of skin cancer are basal cell carcinoma, squamous cell carcinoma, and melanoma. Rarer forms include mycosis fungoides (a type of lymphoma) and Kaposi's sarcoma.
..... Click the link for more information. ; UVB (280–320 nanometers), stronger radiation that increases in the summer and is a common cause of sunburnsunburn,
inflammation of the skin caused by actinic rays from the sun or artificial sources. Moderate exposure to ultraviolet radiation is followed by a red blush, but severe exposure may result in blisters, pain, and constitutional symptoms.
..... Click the link for more information. and most common skin cancer; and UVC (below 280 nanometers), the strongest and potentially most harmful form. Much UVB and most UVC radiation is absorbed by the ozone layerozone layer
region of the stratosphere containing relatively high concentrations of ozone, located at altitudes of 12–30 mi (19–48 km) above the earth's surface.
..... Click the link for more information. of the atmosphere before it can reach the earth's surface; the depletion of this layer is increasing the amount of ultraviolet radiation that can pass through it. The radiation that does pass through is largely absorbed by ordinary window glass or impurities in the air (e.g., water, dust, and smoke) or is screened by clothing.
The National Weather Service's daily UV index predicts how long it would take a light-skinned American to get a sunburn if exposed, unprotected, to the noonday sun, given the geographical location and the local weather. It ranges from 1 (about 60 minutes before the skin will burn) to a high of 10 (about 10 minutes before the skin will burn).
A small amount of sunlight is necessary for good health. Vitamin D is produced by the action of ultraviolet radiation on ergosterol, a substance present in the human skin and in some lower organisms (e.g., yeast), and treatment or prevention of ricketsrickets
, bone disease caused by a deficiency of vitamin D or calcium. Essential in regulating calcium and phosphorus absorption by the body, vitamin D can be formed in the skin by ultraviolet rays contained in sunlight; it can also be consumed in such
..... Click the link for more information. often includes exposure of the body to natural or artificial ultraviolet light. The radiation also kills germs; it is widely used to sterilize rooms, exposed body tissues, blood plasma, and vaccines.
Ultraviolet radiation can be detected by the fluorescencefluorescence
, luminescence in which light of a visible color is emitted from a substance under stimulation or excitation by light or other forms of electromagnetic radiation or by certain other means.
..... Click the link for more information. it induces in certain substances. It may also be detected by its photographic and ionizing effects. The long-wavelength, "soft" ultraviolet radiation, lying just outside the visible spectrum, is often referred to as black light; low intensity sources of this radiation are often used in mineral prospecting and in conjunction with bright-colored fluorescent pigments to produce unusual lighting effects.
See L. R. Koller, Ultraviolet Radiation (2d ed. 1965).
Electromagnetic radiation in the wavelength range 4–400 nanometers. The ultraviolet region begins at the short wavelength (violet) limit of visibility and extends to the wavelength of long x-rays. It is loosely divided into the near (400–300 nm), far (300–200 nm), and extreme (below 200 nm) ultraviolet regions (see illustration). In the extreme ultraviolet, strong absorption of the radiation by air requires the use of evacuated apparatus; hence this region is called the vacuum ultraviolet. Important phenomena associated with ultraviolet radiation include biological effects and applications, the generation of fluorescence, and chemical analysis through characteristic absorption or fluorescence.
Sources of ultraviolet radiation include the Sun (although much solar ultraviolet radiation is absorbed in the atmosphere); arcs of elements such as carbon, hydrogen, and mercury; and incandescent bodies.
ultraviolet radiationElectromagnetic radiation lying in the wavelength range beyond the Earth's atmospheric absorption at about 320 nm to the hydrogen Lyman limit at 91.2 nm (see hydrogen spectrum). The gap between the X-ray and UV wavebands is the XUV region. Long-wavelength ultraviolet radiation, i.e. with wavelengths up to 350 nm, near that of light, is often called near-ultraviolet with far-ultraviolet (FUV) being applied to short wavelengths, i.e. 91.2 nm up to about 200 nm.
(also UV radiation, ultraviolet light), invisible electromagnetic radiation that occupies the spectral region between visible light and X rays, which corresponds to the wavelength (λ) range 400-10 nanometers (nm). The ultraviolet region is divided into the near-ultraviolet (400-200 nm) and the far-, or vacuum-, ultraviolet (200-10 nm) regions; the vacuum ultraviolet is so called because the ultraviolet radiation in this region is strongly absorbed by air and is studied with evacuated spectroscopic instruments.
Near-ultraviolet radiation was discovered in 1801 by the German scientist J. Ritter and in 1802 by the British scientist W. Wollaston from its photochemical effect on silver chloride. Vacuum-ultraviolet radiation was observed by the German scientist V. Schumann with an evacuated fluorite-prism spectrograph, which he built between 1885 and 1903, and gelatinless photographic plates. He was able to detect short-wavelength radiation down to 130 nm. The American scientist T. Lyman built the first evacuated spectrograph with a concave diffraction grating and, in 1924, detected ultraviolet radiation with a wavelength as short as 25 nm. By 1927, the entire region between the vacuum ultraviolet and X rays had been studied.
Depending on the nature of the ultraviolet radiation source, an ultraviolet spectrum may be a line, continuous, or band spectrum (seeSPECTRUM, OPTICAL). The ultraviolet radiation from atoms, ions, or light molecules, such as H2, has a line spectrum. Spectra of heavy molecules are characterized by bands attributable to electronic-vibrational-rotational transitions of the molecules (seeMOLECULAR SPECTRA). A continuous spectrum results from the deceleration or recombination of electrons (see).
Optical properties of matter. The optical properties of substances in the ultraviolet region of the spectrum differ considerably from their optical properties in the visible region. A characteristic difference is a decrease in the transmittance, or an increase in the absorptance, of most solids that are transparent in the visible region. For example, ordinary glass is opaque at λ < 320 nm; at shorter wavelengths, only uviol glass, sapphire, magnesium fluoride, quartz, fluorite, lithium fluoride, and certain other substances are transparent. Among solids, lithium fluoride has the farthest transmission cutoff, namely, 105 nm. For λ < 105 nm, practically no material is transparent.
The most transparent gaseous substances are the inert gases; their transmission cutoff is determined by their ionization’poten-tial. Helium has the shortest-wavelength transmission cutoff, namely, 50.4 nm. Air is practically opaque at λ < 185 nm because of absorption by oxygen.
The reflection coefficient of all materials, including metals, decreases as the wavelength of the radiation decreases. For example, the reflection coefficient of freshly deposited aluminum, which is one of the best materials for mirror coatings in the visible region of the spectrum, decreases sharply at λ < 90 nm (Figure 1). The reflection of light by aluminum is also reduced substantially as a result of surface oxidation. Coatings based on lithium fluoride or magnesium fluoride are used to protect aluminum surfaces from oxidation. In the region λ < 80 nm, several materials, such as gold, platinum, radium, and tungsten, have reflection coefficients of 10-30 percent, but for λ < 40 nm their reflection coefficients are reduced to 1 percent or less.
Sources of ultraviolet radiation. The radiation from solids heated to 3000°K contains an appreciable fraction of continuous ultraviolet radiation, whose intensity increases with increasing temperature. More intense ultraviolet radiation is emitted by a gas-discharge plasma. Depending on the discharge conditions and the plasma material, both continuous and line spectra may be emitted. Mercury, hydrogen, xenon, and other types of discharge lamps are manufactured for various uses of ultraviolet radiation. The window or the entire bulb of the lamp is made of a material that is transparent to ultraviolet radiation, most often quartz.
Any high-temperature plasma is a strong ultraviolet source. Examples of such plasmas include spark-discharge plasmas, arc-discharge plasmas, and plasmas produced when intense laser radiation is focused in a gas or on a solid surface. Intense continuous ultraviolet radiation is emitted by electrons accelerated in a synchrotron; such radiation is called synchrotron radiation. Lasers have been developed for the ultraviolet region of the spectrum. The hydrogen laser has the shortest wavelength, namely, 109.8 nm.
Natural sources of ultraviolet radiation include the sun, stars, nebulas, and other celestial objects. Only the long-wavelength portion (λ > 290 nm) of celestial ultraviolet radiation, however, reaches the earth’s surface. Shorter-wavelength ultraviolet radiation is absorbed by ozone, oxygen, and other components of the atmosphere at heights of 30-200 km above the earth’s surface; such absorption plays a large role in atmospheric processes. In addition to the absorption in the earth’s atmosphere, ultraviolet radiation at wavelengths of 91.2-20 nm from stars and other celestial objects is almost totally absorbed by interstellar hydrogen.
Detectors of ultraviolet radiation. Conventional photographic materials are used to detect ultraviolet radiation at λ > 230 nm. Special photographic emulsions containing little gelatin are sensitive to ultraviolet radiation at shorter wavelengths. Photoelectric detectors are used that rely on the ability of ultraviolet radiation to cause ionization and the photoelectric effect; such detectors include photodiodes, ionization chambers, scintillation counters, and photomultipliers.
A special type of photomultiplier, called the channel multiplier, has also been developed; this device is used to construct microchannel plates. Each stage in a microchannel plate is a channel multiplier with a maximum of 10 micrometers. MicroChannel plates make it possible to obtain a photoelectric display of ultraviolet radiation and to combine the advantages of the photographic and photoelectric methods of radiation detection.
Various luminescent substances that convert ultraviolet radiation to visible light are also used in the study of ultraviolet radiation. The instruments that make ultraviolet images visible are based on luminescence.
Uses of ultraviolet radiation. Ultraviolet emission, absorption, and reflectance spectra are studied in order to determine the electronic structure of atoms, ions, molecules, and solids. The ultraviolet spectra of the sun, stars, and other celestial objects carry information on the physical processes that occur in hot regions of the objects (see and VACUUM SPECTROSCOPY). Photoelectron spectroscopy is based on the photoelectric effect induced by ultraviolet radiation. Ultraviolet radiation can break the chemical bonds in molecules; as a result of this effect, such chemical reactions as oxidation, reduction, decomposition, and polymerization can occur.
Luminescence caused by ultraviolet radiation underlies the operation of fluorescent lamps and luminous paints and is made use of in luminescence analysis and in fluorescent pénétrant inspection. Ultraviolet radiation is used in criminalistics to identify dyes and to establish the authenticity of documents. In art studies, ultraviolet radiation makes it possible to detect invisible signs of restoration in paintings. The capability of many substances for selective absorption of ultraviolet radiation is used in the detection of air pollution and in ultraviolet microscopy.
REFERENCESMeyer, A., and E. Seitz. Ul’trafioletovoe izluchenie. Moscow, 1952. (Translated from German.)
Lazarev, D. N. Ul’trafioletovaia radiatsüa i ee primenenie. Leningrad-Moscow, 1950.
Samson, J. A. R. Techniques of Vacuum Ultraviolet Spectroscopy. New York-London-Sydney .
Zaidel’, A. N., and E. Ia. Shreider. Spektroskopiia vakuumnogo ul’trafioleta. Moscow, 1967.
Stoliarov, K. P. Khimicheskii analiz v ul’trafioletovykh luchakh. Moscow-Leningrad, 1965.
Baker, A., and D. Betteridge. Fotoelektronnaiaspektroskopiia. Moscow, 1975. (Translated from English.) A. N. RIABTSEV
Biological effects of ultraviolet radiation. The effects of ultraviolet radiation on living organisms are associated with the absorption of the radiation by the outer layers of plant tissues or of human and animal skin. Chemical changes in biopolymer molecules are the basis for the biological effects of ultraviolet radiation. Such changes result both from the direct absorption of photons by the molecules and, to a lesser extent, from the radicals of water and other compounds of low molecular weight that form during irradiation.
In man and animals, small doses of ultraviolet radiation have a beneficial effect: the development of D vitamins (seeCALCIFEROLS) is facilitated, and the biological immunity of the organism is enhanced. The typical reaction of the skin to ultraviolet radiation is a specific reddening, called erythema, which usually develops into protective pigmentation (suntan). Ultraviolet radiation with λ = 296.7 nm and λ = 253.7 nm has the greatest erythemal effect. Large doses of ultraviolet radiation can cause eye injury (photo-ophthalmia) and sunburn. In certain cases, frequent and excessive doses can have a carcinogenic effect on the skin.
In plants, ultraviolet radiation alters the activity of enzymes and hormones and affects the formation of pigments, the rate of photosynthesis, and photoperiodism reactions. It has not been determined whether small doses of ultraviolet radiation are useful, let alone necessary, for the germination of seeds, the development of sprouts, and the normal vital activities of higher plants. Large doses of ultraviolet radiation are undoubtedly detrimental to plants, as is evidenced by the protective devices that exist in plants, such as the accumulation of certain pigments and the cellular mechanisms for recovery from injuries.
In microorganisms and in cultured cells of higher animals and plants, ultraviolet radiation has lethal and mutagenic effects; radiation with λ in the range 280-240 nm is the most effective in this respect. The spectrum of the lethal and mutagenic action of ultraviolet radiation usually coincides roughly with the absorption spectrum (Figure 2,a) of the nucleic acids deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). In some cases, the action spectrum is similar to the absorption spectrum of proteins (Figure 2,b). Chemical changes in DNA apparently play a major role in the effects of ultraviolet radiation on cells; when ultraviolet photons are absorbed, the pyrimidine bases (mainly thymine) of which DNA is composed form dimers that inhibit the normal replication of DNA when a cell is ready to divide. This inhibition can lead to the death of cells or to mutations, that is, changes in the hereditary properties of cells. Injury to biological membranes and the disruption of the formation of various components of membranes and cell walls are also definite factors in the lethal effect of ultraviolet radiation on cells.
Most living cells can recover from injuries caused by ultraviolet radiation because they contain repair systems. The ability to recover from such injuries probably developed in the early stages of evolution and played an important role in the survival of primitive organisms exposed to intense ultraviolet radiation from the sun.
Living things differ greatly in their sensitivity to ultraviolet radiation. For example, the dose of ultraviolet radiation that kills 90 percent of the cells is equal to 10,100, or 800 ergs/mm2 for different strains of the bacillus Escherichia coli but is equal to 7,000 ergs/mm2 for the bacterium Micrococcus radiodurans (Figure 3,a and b). The sensitivity of cells to ultraviolet radiation also depends to a large extent on their physiological state and on culture conditions (for example, the temperature and composition of the culture medium) before and after irradiation. The mutations of certain genes greatly affect the sensitivity of cells to ultraviolet radiation. About twenty genes in bacteria and yeasts are known to have mutations that make the organisms more sensitive to ultraviolet radiation. In a number of cases, these genes are responsible for a cell’s recovery from radiation damage. Mutations of other genes disrupt the synthesis of proteins and the building of cell membranes, thereby increasing the radiosensitivity of the nongenetic components of a cell. Mutations that increase the sensitivity to ultraviolet radiation are also known for higher organisms, including man. For example, the hereditary disease xeroderma pigmentosum is caused by mutations of the genes that regulate dark repair.
The genetic consequences of exposing the pollen of higher plants, the cells of plants and animals, and microorganisms to ultraviolet radiation include a higher incidence of gene, chromosome, and plasmid mutations. The frequency of mutation for individual genes exposed to high doses of ultraviolet radiation may increase by a factor of several thousand in comparison with the natural level and may reach several percent. Unlike the genetic effects of ionizing radiation, mutations of genes exposed to ultraviolet radiation occur relatively more often than mutations of chromosomes. As a result of its strong mutagenic effect, ultraviolet radiation is widely employed both in genetic research and in the breeding of plants and of microorganisms used in industry as producers of antibiotics, amino acids, vitamins, and protein biomass. The genetic effects of ultraviolet radiation may have played a significant role in the evolution of living organisms. The use of ultraviolet radiation in medicine is discussed in PHOTOTHERAPY.
REFERENCESSamoilova, K. A. Deistvie ul’trafioletovoi radiatsii na kletku. Leningrad, 1967.
Dubrov, A. P. Genelicheskie i fiziologicheskie effekty deistviia ul’trafioletovoi radiatsii na vysshie rasteniia. Moscow, 1968.
Galanin, N. F. Luchistaia energiia i ee gigienicheskoe znachenie. Leningrad, 1969.
Smith, K., and P. Hanawalt. Molekuliarnaia fotobiologiia. Moscow, 1972. (Translated from English.)
Shul’gin, I. A. Rastenie isolntse. Leningrad, 1973.
Miasnik, M. N. Geneticheskii kontrol’ radiochuvstvitel’nosti bakterii. Moscow, 1974.
V. I. KOROGODIN