Ultraviolet radiation , UV for short , UV radiation , UV light or black light , is electromagnetic radiation in the optical frequency range ( light ) with shorter wavelengths than light that is visible to humans. Beyond the UV radiation, the X-rays follow.
"Ultraviolet" means "beyond purple"; Violet is the color stimulus of the shortest visible wavelength. In the case of black light lamps , the accompanying proportion of visible radiation is largely suppressed by a filter, so that essentially only fluorescent substances shine in a scene irradiated with them .
The discovery of UV radiation followed from the first experiments with the blackening of silver salts in sunlight. In 1801, the German physicist Johann Wilhelm Ritter in Jena made the observation that rays beyond the violet end in the visible spectrum were very effective in blackening silver chloride paper. He initially called the rays “de-oxidizing rays” in order to emphasize their chemical effectiveness and to distinguish them from the infrared “heat rays” at the other end of the spectrum. Until the 19th century, UV was referred to as "chemical radiation". The terms "infrared radiation" and "ultraviolet radiation" are now used to characterize the two types of radiation.
At the beginning of the 20th century, the healing effects of artificial UV radiation were discovered. For example, the Austrian doctor Gustav Kaiser (1871–1954), who had dealt with electrotherapeutic studies in Würzburg, reported to the general assembly of the Society of Doctors in Vienna in February 1902 about the self-experiment with a UV light bulb, which he used to recover of a wound that does not want to heal. According to the present report, a seriously ill tuberculous patient is said to have been healed in four weeks using the “blue light”. Encouraged by these successes, Kaiser extended his experiments with a hollow lens to include skin diseases, and he also achieved favorable results. From this he concluded that UV radiation has a germicidal effect.
Spectrum and names
Traditionally, the UV range extends from 380 to 100 nm and is divided into the sub-ranges UV-A, UV-B and UV-C with a view to the transmission of atmospheric gases, see table. The “non-round” limits have the following meaning: 380 nm is the sensitivity limit of the human eye. From around 315 nm, ozone absorbs so strongly that light hardly penetrates the ozone layer. From 280 nm, the normal, diatomic oxygen is sufficient for complete absorption within the atmosphere. From around 200 nm, the absorption by oxygen becomes so strong that it is also disturbing at normal laboratory distances; in addition, photolysis and ozone formation begin. On the other hand, flushing the beam path with protective gas or evacuation helps , from which the term "vacuum ultraviolet" (VUV) for this area goes back. The UV-C range traditionally (and according to DIN 5031-7) ends somewhat arbitrarily at 100 nm due to experimental problems (transmission of refractive optics). Light of this wavelength is already absorbed in the ionosphere .
For applications above the ozone layer, i.e. for aeronomy and astronomy , the division into UV-A, -B and -C is not relevant. A distinction is made here between near (NUV), medium (MUV), far (FUV) and extreme UV (EUV) between the limits 400, 300, 200, 100 and 10 nm. The World Health Organization (WHO) also leaves the UV range at its own 400 nm begin.
Sources of ultraviolet radiation
In the case of thermal radiation , the proportion of UV radiation is determined by Planck's law of radiation and Wien's law of displacement . Excited electrons can generate UV radiation if their energy is above 3.3 eV . The same applies to a small extent to the temperature of the filament of incandescent lamps , which is why halogen incandescent lamps in particular also emit some ultraviolet radiation.
Ultraviolet radiation is contained in the short-wave portion of solar radiation . Because of the absorption in the earth's atmosphere (especially in the ozone layer ) UV-A and little UV-B radiation with a wavelength below 300 nm penetrates to the earth's surface. Certain gases, especially CFCs , act on the ozone bond through the sun's UV and shift the equilibrium in the ozone layer, the result is the ozone hole , whereby the UV-B exposure of the earth's surface increases.
Other cosmic objects such as pulsars , highly excited gas masses and most fixed stars also emit UV radiation. Polar light also contains ultraviolet radiation. Natural terrestrial sources of ultraviolet are thunderstorm lightning and Elmsfeuer .
Ultraviolet radiation arises from the following artificial sources:
- Mercury vapor lamps : medium, high and ultra-high pressure lamps
- Quartz lamps (low pressure mercury vapor lamps)
Black light lamps for UV-A
- Incandescent lamps with filter or mercury vapor low pressure lamps with filter and fluorescent material for decoration purposes, disco,
- mineralogical investigations,
- Checking banknotes with a UV tester . The visible part of the radiation is filtered out so that only the fluorescence effects caused by UV are visible
- Excimer laser
- UV light emitting diodes (recently also diode lasers )
- UV cold cathode tubes (UV-CCL)
There are other man-made sources for which the ultraviolet emission is secondary:
- Gas discharge lamps (also daylight lamps and full spectrum tubes ; here the UV emission from daylight lamps is harmless to health and desirable)
- Arc welding
- Corona discharge , corona treatment (see also ionizer )
- all processes in which ionized gases or very high temperatures occur (e.g. laser material processing, ion sources , spark gaps )
- to a small extent halogen bulbs without protective glass
Intense UV sources
- The arc welding so that must protect welders and bystanders eyes and skin an intense ultraviolet source.
- Space travel: Intensive UV-B and UV-C require special materials, especially for the spacesuits and their visors for outdoor use. Solar cells located outside the “UV filter” of the earth's atmosphere are damaged and have a shorter lifespan than on earth.
- With laser and electron beam processing, the UV emission must be observed with regard to occupational safety.
Ultraviolet radiation is not perceived by the human eye because it is completely absorbed by the eye lens beforehand . The transition from violet to ultraviolet is individually fluid. Patients who lost their lenses after accidents or surgery described UV light as a whitish, “milky” blue-violet. The absorbing lens probably protects the retina from damage, as otherwise the relatively long-lived person could go blind. A noticeable change in perception in the border area violet / UV can be noticed after the lens exchange as a result of the operation on the cataract of the lens through an intraocular lens . There also seems to be a connection with visual acuity: animal species whose lenses allow less UV light to penetrate see more clearly and more accurately. Some animals (insects, birds, fish, reptiles) can partially perceive them. According to studies in 2014, the lenses of significantly more animals than previously thought allow ultraviolet light to pass through, including those of dogs and cats. Whether they can really see ultraviolet radiation needs to be researched in further studies.
Below a wavelength of 200 nm, the energy of a radiation quantum is high enough to release electrons from atoms or molecules , i.e. to ionize them . As with gamma and X-rays , short-wave ultraviolet radiation below 200 nm is called ionizing radiation .
Quartz glass (silica glass) is transparent for the entire UV range naturally coming from the sun up to about 250 nm on the earth's surface. Normal glass (soda-lime glass), especially normal window glass , does not allow ultraviolet radiation below 320 nm through. Borosilicate glass (like Jenaer Glas ) allows UV radiation to pass through up to around 290 nm, high borosilicate glass allows UV radiation to pass through up to around 180 nm. Window glass is permeable to UV-A. Radiation below 290 nm is transmitted through natural or synthetic quartz crystals . In the UV-C range between 100 nm and 250 nm, synthetic quartz glass and some borosilicate glasses are transparent. On the other hand, natural quartz and ordinary silica glass do not allow UV radiation below 200 nm to be transmitted due to their titanium content, which is why high-purity quartz glass made of synthetic silicon dioxide is used for the discharge vessels of UV lamps that are supposed to generate such short wavelengths. Such an application is photolithography or the treatment of high-purity water, whereby UV is used to oxidize the dissolved organic carbon compounds. Other uses for this glass are optical elements for the Ar F - excimer laser wavelength (193 nm). However, short-wave, high-intensity ultraviolet clouds glasses and optical components. High purity requirements are therefore placed on optics (for example for excimer lasers). For shorter wavelengths (up to 45 nm), single-crystalline calcium fluoride is used for lenses, prisms or windows .
Because of its short wavelength, ultraviolet is often the excitation wavelength for fluorescence in the visible range. The UV-excited fluorescence radiation can itself be in the ultraviolet range. The external photo effect occurs with ultraviolet on all metal surfaces. It is used in photo multipliers on scintillation detectors to register ultraviolet radiation pulses ( neutrino detector , detection and classification of ionizing radiation ).
UV radiation can break organic bonds. On the one hand, this makes it hostile to life due to the destruction of biogenic substances. Many plastics are damaged by ultraviolet radiation through clouding, embrittlement and decay. Technically, high-energy UV radiation can initiate the crosslinking of monomers in order to produce special polymers.
The splitting of oxygen molecules by short-wave UV radiation below 200 nm into atomic oxygen is of particular importance . The recombination leads to the formation of ozone - a characteristic of the interaction of UV radiation with air due to its odor. A variety of other secondary reactions take place in these processes, such as those that take place in the ozone layer . With these processes in the ozone layer, the earth's surface is protected from hard (short-wave) UV radiation from the sun through absorption reactions, which prevents damage to biological material - including humans - and thus enables the existence of life.
Although ultraviolet radiation is the lowest energy level of ionizing radiation, it can be dangerous to humans and other organisms. UV radiation with a longer wavelength can destroy chemical bonds in organic molecules. Care should be taken when handling sunlight ( sun protection ) and technical UV sources. The excessive use of solariums remains controversial.
The effect of UV radiation can be divided into different areas:
|UV-A||315-380 nm||Long UV waves with less energy than UV have a greater penetration depth into scattering biological tissue and reach the dermis|
|UV-B||280-315 nm||short-wave, high-energy, is more strongly scattered in biological tissue
|UV-C||100-280 nm||Very short-wave, very high-energy, is strongly scattered in biological tissue
Generating ozone below about 242 nm by photolysis of atmospheric oxygen .
UV-C radiation (especially the emission line of mercury vapor at 253.652 nm that can be excited at low vapor pressure with high yield (30-40% of the applied electrical power)) is used in physical disinfection technology (see also mercury vapor lamps ). While at 280 nm (absorption maximum of most proteins ) the incorporated amino acid tryptophan absorbs the ultraviolet radiation, nucleic acids are most damaged at 265 nm . At around 245 nm, it is primarily the nucleic acids that absorb, while proteins here show a relative absorption minimum between the absorption maximum around 280 nm by aromatic amino acids ( tryptophan , tyrosine and phenylalanine ) and the absorption by the peptide bond between the individual amino acids (maximum at around 220 nm) . Therefore, at 253.7 nm (primary radiation of the low-pressure mercury vapor discharge ), it is also possible to irradiate protein solutions (such as animal sera for cell culture) to inactivate viruses and bacteria contained therein.
UV radiation with wavelengths below 100 nm occurs only with very low intensity in sunlight. The damage depends not only on the energy of the UV radiation, but also on the depth of penetration and the time the tissue was irradiated. For example, UV-C radiation at 253.7 nm is practically completely absorbed on the surface by keratinized skin and is therefore less effective in damaging deeper cell layers than UV-B radiation, which is less absorbed and penetrates into these. Sunburn accidentally caused by a UV-C lamp will therefore subside completely within one day, while the cornea of the eye will be clouded over the long term.
The human body is adapted to natural radiation exposure ( skin type ) or can react to radiation exposure to a limited extent through protective mechanisms (tanning, thickening) that are primarily triggered by UV-B radiation. Due to the reaction time of the repair and protection mechanisms, a slow increase in radiation intensity and dose is crucial for the balance between benefit and hazard. Specifically, depending on the time of day, season and location (latitude, sea level) and the environment (reflective surfaces, sand, snow) in the range of 10–60 minutes per day, exposure of adults to natural sunlight is classified as beneficial to health, but above that as harmful. However, there are strong deviations in adolescents, sick people and different skin types.
Particular care should be taken when exposing the eyes. Ultraviolet causes conjunctivitis and clouding of the cornea. In manual arc welding, a “welding mask” is required because of the short-wave UV radiation. Arcs and spark gaps create a broad spectrum of intense UV radiation that, if used unprotected (exposed body parts), causes a burn of the skin similar to a sunburn after just a few minutes . The skin feels “dry” and begins to “tighten”. First degree burns (reddening) to second degree (blistering) occur.
Long-term damage such as skin aging, skin cancer or cataracts can also occur if the erythema threshold is not exceeded but the radiation is carried out frequently. The skin and eyes register all UV radiation and not just those that are above the erythema threshold.
DNA damage is caused by UV radiation when two neighboring thymine bases covalently bond with one another, so that they form a thymine dimer . These hinder replication or lead to mutations . With the help of the enzyme photolyase and light, these dimers can be split again and the DNA repaired. In all placentates, including humans, the function of photolyase was taken over by the nucleotide excision repair system (NER) in the course of evolution. Children with xeroderma pigmentosum have a defect in the repair enzymes of the NER. This results in an absolute intolerance to natural solar radiation ("moonlight children"). When exposed to natural UV radiation, patients develop malignant skin tumors much faster than people without comparable enzyme defects.
The UV index is an internationally established measure. It describes the solar irradiance effective for sunburn. In the forecast and warning, the UV index is specified as the maximum UV index to be expected (max. UVI). It varies depending on geographic location, geographic altitude, as well as the season and weather conditions.
Further possible damage to organic material through UV radiation are:
- Denaturation of cell protein
- High levels of UV radiation can reactivate herpes labialis .
- Destruction of vegetation : Plants have almost no protection in the UV-C range. Leaves are severely damaged or killed when exposed to radiation in this area. The latter can also lead to the death of the entire plant. UV-A and UV-B are tolerated differently by plants, high intensities lead to death, and land plants can "get used" to UV-A.
- Ultraviolet radiation generates ozone from precursor substances (preferably exhaust gases) when exposed to high levels of solar radiation, even near the ground, which in the smog has a damaging effect on lungs and plants.
- Damage to plastics, color pigments and paints. Organic colors fade, plastic becomes cloudy and becomes brittle (example: decomposition of polyethylene film under the influence of daylight, embrittlement and discoloration of plastics in luminaires for gas discharge lamps). Protection is possible through resistant pigments or a suitable choice of material.
|designation||wavelength||frequency||Photons - energy||Generation / excitation||Technical commitment|
|UV rays||10 ... 380 nm||> 789 THz||
> 5.2 · 10 −19 J
> 3.3 eV
|Disinfection , spectroscopy|
|200 ... 380 nm||> 789 THz||
> 5.2 · 10 −19 J
> 3.3 eV
Gas discharge , synchrotron ,
|Black light fluorescence , phosphorescence ,
checking banknotes , photolithography
|50 ... 200 nm||> 1.5 PHz||
> 9.9 · 10 −19 J
> 6.2 eV
Gas discharge , synchrotron ,
|XUV||10 ... 50 nm||6… 30 PHz||
2.0 · 10 −17 … 5.0 · 10 −18 J
20… 100 eV
|XUV tube , synchrotron||
EUV lithography , X-ray microscopy ,
The natural UV component of daylight is used in detergents by adding optical brighteners . These make gray or yellowed textiles appear “whiter than white” due to limescale deposits, because they convert the UV light into visible blue light, which, as a mixed color with the yellowing of the textiles, results in white. In addition, more visible light is emitted than with a normally reflective object.
Ultraviolet is the primary emission in fluorescent lamps , efficient white light sources in which the ultraviolet emission of a gas discharge of mercury vapor is used to excite phosphors that fluoresce in the visible spectral range .
Other gas discharge lamps also sometimes contain phosphors in order to improve the color rendering by exciting them with the ultraviolet radiation component of the discharge. So-called daylight lamps and full spectrum tubes (and similar names, depending on the manufacturer) emit a light spectrum that is as similar as possible to sunlight, including UV and infrared , in order to enable natural lighting (especially indoors, see also ergonomics ); the amount of UV emission here is harmless to health.
Light emitting diodes (LED), which emit light that appears white to humans, use a blue light emitting diode inside, made of materials such as indium gallium nitride or gallium nitride . Light-emitting diodes, which emit UV radiation, are made of aluminum nitride or aluminum gallium nitride and are used as a direct UV radiation source without a fluorescent coating. UV LEDs can be implemented up to wavelengths just under 250 nm.
Some dyes, such as DAPI used in the life sciences , are excited by UV radiation and emit longer-wave, mostly visible light. Fluorescent substances are used, among other things, to mark biological molecules (e.g. DNA ) in order to observe their behavior in biological systems.
In forensics , the fluorescence of blood and sperm is used to make traces of victims or perpetrators visible. This method is used in the investigation of criminal cases when biological traces (blood, semen, saliva) on walls or in textiles are to be detected. The fluorescence of organic substances is also used in medicine. For example, pigment disorders in the skin can be made more visible with the help of UV lamps (" Wood lamps "). Certain skin germs ( Corynebacterium minutissimum ) are also visible by means of these diagnostic lights by triggering a reddish fluorescence ( porphyrin formation ).
Another application is the origin analysis of chicken eggs. This takes advantage of the fact that rolling leaves characteristic traces on the chicken egg shell, which can be detected with the help of fluorescence. In this way it can be checked whether the eggs are from barn eggs or from laying batteries.
Black light is the colloquial term for UV-A radiation, which is generated by special lamps with UV-A filters. The usual sources are gas discharge tubes equipped with special phosphors to emit ultraviolet radiation at 350 nm or 370 nm with only a small amount of visible light. Other common black light sources are light-emitting diodes (LEDs) based on the compound semiconductors aluminum nitride or aluminum gallium nitride . The latter is an alloy of aluminum nitride with gallium nitride and makes it possible to set the specific wavelength in the ultraviolet range via the mixing ratio of these two substances. Black light can also be produced, with poor efficiency, by incandescent lamps with a glass bulb with a nickel oxide layer that absorbs visible light .
“Black light” is often used for show effects in darkened rooms, such as discos, at magic events or for black light theaters . The radiation stimulates fluorescent substances to shine, and since bright light is avoided, the lighting effects have a particular effect, as is noticeable with textiles, paper, artificial teeth and other materials with optical brighteners.
Applications are also the visualization of security features on documents such as identity papers or tickets, the authenticity check of means of payment and the "neon stamp" on the back of the hand as a "ticket" to a concert or as an owner mark on an art object (against theft).
In connection with traffic speed monitoring , the word black light is also used for identification technology in the invisible spectral range. However, this is not the ultraviolet range, but infrared photography .
UV radiation is used in training courses for the visualization of substances marked with fluorescent dyes:
- Application control of skin protection agents for personal protective equipment (PPE)
- Demonstration of cross-contamination (transmission of germs) within hygiene training courses
- Visualization during hand hygiene training (washing control and application of hand disinfectant)
- Optical spectroscopy : UV / VIS spectroscopy
- Evaluation of the chromatograms in thin layer chromatography
- Gas - analysis ( NO , NO 2 , H 2 S , SO 2 )
- Molecular biology : Making nucleic acids visible with the help of ethidium bromide
- Determining the thickness of the grease: Points at which friction occurs between two objects are often greased. With the help of UV radiation, it is possible to determine whether and how thick a layer of fat is on the objects.
- Determination of the tin side of float glass (photovoltaics, thin-film solar cells)
- Ultraviolet Photoelectron Spectroscopy (UPS)
- Inspection of glass (panes): Fluorescence from defects can be used to detect cracks or defects in glass surfaces.
- Testing of oil hoses: Due to the different spectral characteristics of water and oil in the UV range, oil can be distinguished from water. This can be used, for example, to track down defective oil hoses.
- Detection and classification of coatings, e.g. for the detection of oil in water
- Inspection of overhead line and high voltage systems: Corona discharges occur in the case of defective insulators or torn cables . UV radiation is emitted from the defective high-voltage components. This can be recorded by special cameras.
- Resilience and weathering tests: Testing the resilience of materials that require a particularly long service life. This includes solar cells and materials used in the automotive industry. With the help of modern test systems, it is possible to increase the natural UV radiation so that 63 years of natural UV radiation are simulated within 12 months.
- Exposure of special UV-sensitive films to detect hairline cracks in thin metals
Curing (crosslinking) of polymers
Intensive UV radiation is used in industry for curing special materials:
- Printing industry: for hardening ("drying") special, solvent-free, UV-sensitive printing inks, especially in offset printing .
- Paint shops: for curing UV-curable paints
- Dentistry: light-curing materials
- Curing of radiation- curing adhesives
- Optical industry: for hardening optical products, as in the case of prescription glasses
- Cosmetics industry: light-curing plastics for modeling artificial fingernails
- Window repairs on the laminated glass of automobiles
In electronics , UV radiation is mainly used in the manufacture of microelectronic components and circuits as well as corresponding devices. For example, conductor tracks on circuit boards are produced by exposing a light-sensitive layer on the circuit boards to a mercury vapor lamp . The UV radiation triggers a photochemical reaction in the photoresist . The same principle is also used in the manufacture of integrated circuits ( wafer exposure ), cf. Photolithography (semiconductor technology) . In the past, mercury vapor lamps were also used here - especially with the g-line (434 nm) and the i-line (365 nm). Later KrF and ArF excimer lasers (248 nm and 193 nm). The trend to use ever shorter wavelengths is due to the constant scaling of the transistor structures .
In addition to its use in production, UV radiation is also used for other applications in electronics. One example is the erasure of EPROM memory with a mercury vapor lamp (253.7 nm). Here, the UV radiation causes the release of charge carriers in the floating gate made of polysilicon , the released electrons have enough energy to overcome the potential barrier of the silicon dioxide dielectric and flow away.
Ultraviolet radiation is used to treat water, air and surfaces. Due to the speed of the reaction - microbes are inactivated within fractions of a second with a sufficient dose - UV lamps can not only be used to disinfect surfaces, but also to disinfect water, air or even air flows guided in air conditioning ducts. Before the development of laminar flow systems for clean rooms and the massive use of disinfectants that are common today, weak ultraviolet emitters were common in hospitals to keep the number of germs low. The increasing antibiotic resistance of hospital-specific germs could lead to a return of the well-known technology in the near future, since no mutation-related resistances can develop during UV disinfection.
A method that is already quite common today is drinking water treatment with UV radiation. The number of germs in the water is reliably reduced, depending on the dose. In principle, it is not necessary to add chemicals. Chlorine-resistant pathogens, such as cryptosporidia , can be inactivated with UV radiation. Taste, smell or the pH value of the medium are not affected. This is an essential difference to the chemical treatment of drinking or process water. In the home, such devices are also known as " UV filters ".
In general, low-pressure mercury vapor lamps are used for UV disinfection (possibly also medium-pressure lamps), which emit radiation with a wavelength of 254 nm. Shorter wavelengths (less than 200 nm) can break down all organic substances ( TOC ) in water and are used to produce ultrapure water.
With SODIS , UV-A radiation from the sun that has a longer effect is used together with the heat for simple water disinfection at household level in developing countries.
In addition to disinfecting microbes, UV radiation with a wavelength of 254 nm is also used to inactivate viruses . This takes advantage of the fact that the 254 nm radiation has a preferred effect on the virus nucleic acid and less on the proteins. However, radiation with a wavelength of 235 nm is also highly destructive to proteins.
UV radiation is also used for medical and cosmetic purposes. UV-A radiation in particular has an effect on the pigmentation ( melanin formation ) of the human skin, which is used in the wellness area to tan the skin in a solarium . UV-B radiation can be used therapeutically (with a suitable dosage) to stimulate vitamin D production or the central nervous system .
In chemistry, UV radiation is used in the synthesis and decomposition of various substances. An example from photochemistry is that of the synthesis of vitamins D 2 and D 3 . Examples of the decomposition of substances are the chlorine-free bleaching of cellulose and the breakdown of chloramines during water treatment in swimming pools . Here, UV light with a wavelength of 185 nm is used.
Plants use certain parts of the flower ( UV marks ) to attract insects that, like bees and bumblebees , can perceive UV radiation. The UV marks on the flowers are caused by the different reflectivity for ultraviolet light of certain parts of the flower, for example the inside and outside. As a result, bees find the center even with flowers that appear monochrome in the visible area. In the case of more complex flower shapes or flowers that are more difficult to exploit, the path to the food source can be marked by sap marks that absorb UV light .
Street lamps with a high blue and ultraviolet content ( mercury vapor lamps ) attract insects and influence the biological balance. Bats are attracted by flying insects, which can cause them to crash in traffic. The influencing of behavior by UV light is also used in light traps for catching insects, in which UV-rich light sources are used. They are used for pest control and for enumeration / species identification in research.
UV radiation in the workplace
If UV radiation exposure occurs at workplaces , suitable protective measures must be taken to avoid damage to the eyes or skin. Examples of this are windows of vehicles that absorb UV radiation, storage facilities such as sunshades or shifting working hours to earlier or later hours. If exposure cannot be avoided and if it is of interest how high the exposure is during a certain activity, the level of exposure can be recorded using a suitable data logger. The aim is to gain information about the exposure in order to be able to take suitable occupational safety measures and to be able to determine a possible correlation with cancer. In order to create a complete overview of the exposure of the population to UV radiation from the sun and to achieve comprehensive prevention, targeted measurements of the UV exposure during various leisure activities continue to take place.
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