Thephotons of ultraviolet have greater energy than those of visible light, from about 3.1 to 12 electron volts, around the minimum energy required toionizeatoms. Although long-wavelength ultraviolet is not considered anionizing radiation because itsphotons lack sufficient energy, it can inducechemical reactions and cause many substances to glow orfluoresce. Many practical applications, including chemical and biological effects, are derived from the way that UV radiation can interact with organic molecules. These interactions can involveabsorption or adjusting energy states in molecules, but do not necessarily involve heating.[citation needed] Short-wave ultraviolet light isionizing radiation. Consequently, short-wave UV damagesDNA and sterilizes surfaces with which it comes into contact.
For humans,suntan andsunburn are familiar effects of exposure of the skin to UV, along with an increased risk ofskin cancer. The amount of UV radiation produced by the Sun means that the Earth would not be able to sustain life on dry land if most of that light were not filtered out by theatmosphere.[1] More energetic, shorter-wavelength "extreme" UV below 121 nm ionizes air so strongly that it is absorbed before it reaches the ground.[2] However, UV (specifically, UVB) is also responsible for the formation ofvitamin D in most landvertebrates, including humans.[3] The UV spectrum, thus, has effects both beneficial and detrimental to life.
The lower wavelength limit of thevisible spectrum is conventionally taken as 400 nm. Although ultraviolet rays are not generallyvisible to humans, 400 nm is not a sharp cutoff, with shorter and shorter wavelengths becoming less and less visible in this range.[4] Insects, birds, and some mammals can see near-UV (NUV), i.e., somewhat shorter wavelengths than what humans can see.[5]
Ultraviolet rays are usually invisible to most humans. Thelens of the human eye blocks most radiation in the wavelength range of 300–400 nm; shorter wavelengths are blocked by thecornea.[6] Humans also lackcolor receptor adaptations for ultraviolet rays. Nevertheless, thephotoreceptors of theretina are sensitive to near-UV, and people lacking a lens (a condition known asaphakia) perceive near-UV as whitish-blue or whitish-violet.[4] Under some conditions, children and young adults can see ultraviolet down to wavelengths around 310 nm.[7][8] Near-UV radiation is visible to insects, some mammals, and somebirds. Birds have a fourth color receptor for ultraviolet rays; this, coupled with eye structures that transmit more UV gives smaller birds "true" UV vision.[9][10]
"Ultraviolet" means "beyond violet" (fromLatinultra, "beyond"), violet being the color of the highest frequencies ofvisible light. Ultraviolet has a higher frequency (thus a shorter wavelength) than violet light.
UV radiation was discovered in February 1801 when the German physicistJohann Wilhelm Ritter observed that invisible rays just beyond the violet end of the visible spectrum darkenedsilver chloride-soaked paper more quickly than violet light itself. He announced the discovery in a very brief letter to theAnnalen der Physik[11][12] and later called them "(de-)oxidizing rays" (German:de-oxidierende Strahlen) to emphasizechemical reactivity and to distinguish them from "heat rays", discovered the previous year at the other end of the visible spectrum. The simpler term "chemical rays" was adopted soon afterwards, and remained popular throughout the 19th century, although some said that this radiation was entirely different from light (notablyJohn William Draper, who named them "tithonic rays"[13][14]). The terms "chemical rays" and "heat rays" were eventually dropped in favor of ultraviolet andinfraredradiation, respectively.[15][16] In 1878, the sterilizing effect of short-wavelength light by killing bacteria was discovered. By 1903, the most effective wavelengths were known to be around 250 nm. In 1960, the effect of ultraviolet radiation on DNA was established.[17]
The discovery of the ultraviolet radiation with wavelengths below 200 nm, named "vacuum ultraviolet" because it is strongly absorbed by the oxygen in air, was made in 1893 by German physicistVictor Schumann.[18]
Theelectromagnetic spectrum of ultraviolet radiation (UVR), defined most broadly as 10–400 nanometers, can be subdivided into a number of ranges recommended by theISO standard ISO 21348:[19]
Germicidal but strongly absorbed by outer skin layers, so does not reach living tissue.
200–235
Vacuum ultraviolet
6.20–124 0.993–19.867
Strongly absorbed by atmospheric oxygen, though 150–200 nm wavelengths can propagate through nitrogen.
VUV
10–200
Several solid-state and vacuum devices have been explored for use in different parts of the UV spectrum. Many approaches seek to adapt visible light-sensing devices, but these can suffer from unwanted response to visible light and various instabilities. Ultraviolet can be detected by suitablephotodiodes andphotocathodes, which can be tailored to be sensitive to different parts of the UV spectrum. Sensitive UVphotomultipliers are available.Spectrometers andradiometers are made for measurement of UV radiation. Silicon detectors are used across the spectrum.[20]
Vacuum UV, or VUV, wavelengths (shorter than 200 nm) are strongly absorbed by molecularoxygen in the air, though the longer wavelengths around 150–200 nm can propagate throughnitrogen. Scientific instruments can, therefore, use this spectral range by operating in an oxygen-free atmosphere (pure nitrogen, orargon for shorter wavelengths), without the need for costly vacuum chambers. Significant examples include 193-nmphotolithography equipment (forsemiconductor manufacturing) andcircular dichroism spectrometers.[21]
Technology for VUV instrumentation was largely driven by solar astronomy for many decades. While optics can be used to remove unwanted visible light that contaminates the VUV, in general, detectors can be limited by their response to non-VUV radiation, and the development ofsolar-blind devices has been an important area of research. Wide-gap solid-state devices or vacuum devices with high-cutoff photocathodes can be attractive compared to silicon diodes.
Extreme UV (EUV or sometimes XUV) is characterized by a transition in the physics of interaction with matter. Wavelengths longer than about 30 nm interact mainly with the outervalence electrons of atoms, while wavelengths shorter than that interact mainly with inner-shell electrons and nuclei. The long end of the EUV spectrum is set by a prominent He+ spectral line at 30.4 nm. EUV is strongly absorbed by most known materials, but synthesizingmultilayer optics that reflect up to about 50% of EUV radiation atnormal incidence is possible. This technology was pioneered by theNIXT andMSSTA sounding rockets in the 1990s, and it has been used to make telescopes for solar imaging. See also theExtreme Ultraviolet Explorersatellite.[citation needed]
Some sources use the distinction of "hard UV" and "soft UV". For instance, in the case ofastrophysics, the boundary may be at theLyman limit (wavelength 91.2 nm, the energy needed to ionise a hydrogen atom from its ground state), with "hard UV" being more energetic;[22] the same terms may also be used in other fields, such ascosmetology,optoelectronic, etc. The numerical values of the boundary between hard/soft, even within similar scientific fields, do not necessarily coincide; for example, one applied-physics publication used a boundary of 190 nm between hard and soft UV regions.[23]
Levels of ozone at various altitudes (DU/km) and blocking of different bands of ultraviolet radiation: In essence, all UVC is blocked by diatomic oxygen (100–200 nm) or by ozone (triatomic oxygen) (200–280 nm) in the atmosphere. The ozone layer then blocks most UVB. Meanwhile, UVA is hardly affected by ozone, and most of it reaches the ground. UVA makes up almost all UV light that penetrates the Earth's atmosphere.
Very hot objects emit UV radiation (seeblack-body radiation). TheSun emits ultraviolet radiation at all wavelengths, including the extreme ultraviolet where it crosses into X-rays at 10 nm. Extremely hotstars (such as O- and B-type) emit proportionally more UV radiation than the Sun.Sunlight in space at the top of Earth's atmosphere (seesolar constant) is composed of about 50% infrared light, 40% visible light, and 10% ultraviolet light, for a total intensity of about 1400 W/m2 in vacuum.[24]
The atmosphere blocks about 77% of the Sun's UV, when the Sun is highest in the sky (at zenith), with absorption increasing at shorter UV wavelengths. At ground level with the sun at zenith, sunlight is 44% visible light, 3% ultraviolet, and the remainder infrared.[25][26] Of the ultraviolet radiation that reaches the Earth's surface, more than 95% is the longer wavelengths of UVA, with the small remainder UVB. Almost no UVC reaches the Earth's surface.[27] The fraction of UVA and UVB which remains in UV radiation after passing through the atmosphere is heavily dependent on cloud cover and atmospheric conditions. On "partly cloudy" days, patches of blue sky showing between clouds are also sources of (scattered) UVA and UVB, which are produced byRayleigh scattering in the same way as the visible blue light from those parts of the sky. UVB also plays a major role in plant development, as it affects most of the plant hormones.[28] During total overcast, the amount of absorption due to clouds is heavily dependent on the thickness of the clouds and latitude, with no clear measurements correlating specific thickness and absorption of UVA and UVB.[29]
The shorter bands of UVC, as well as even more-energetic UV radiation produced by the Sun, are absorbed by oxygen and generate the ozone in theozone layer when single oxygen atoms produced by UVphotolysis of dioxygen react with more dioxygen. The ozone layer is especially important in blocking most UVB and the remaining part of UVC not already blocked by ordinary oxygen in air.[citation needed]
Ultraviolet absorbers are molecules used in organic materials (polymers,paints, etc.) to absorb UV radiation to reduce theUV degradation (photo-oxidation) of a material. The absorbers can themselves degrade over time, so monitoring of absorber levels in weathered materials is necessary.[citation needed]
Suspendednanoparticles in stained-glass prevent UV rays from causing chemical reactions that change image colors.[citation needed] A set of stained-glass color-reference chips is planned to be used to calibrate the color cameras for the 2019ESA Mars rover mission, since they will remain unfaded by the high level of UV present at the surface of Mars.[citation needed]
Commonsoda–lime glass, such as window glass, is partiallytransparent to UVA, but isopaque to shorter wavelengths, passing about 90% of the light above 350 nm, but blocking over 90% of the light below 300 nm.[32][33][34] A study found that car windows allow 3–4% of ambient UV to pass through, especially if the UV was greater than 380 nm.[35] Other types of car windows can reduce transmission of UV that is greater than 335 nm.[35]Fused quartz, depending on quality, can be transparent even tovacuum UV wavelengths. Crystallinequartz and some crystals such as CaF2 and MgF2 transmit well down to 150 nm or 160 nm wavelengths.[36]
Wood's glass is a deep violet-blue barium-sodium silicate glass with about 9%nickel(II) oxide developed duringWorld War I to block visible light for covert communications. It allows both infrared daylight and ultraviolet night-time communications by being transparent between 320 nm and 400 nm and also the longer infrared and just-barely-visible red wavelengths. Its maximum UV transmission is at 365 nm, one of the wavelengths ofmercury lamps.[citation needed]
Two black light fluorescent tubes, showing use. The longer tube is a F15T8/BLB 18 inch, 15 watt tube, shown in the bottom image in a standard plug-in fluorescent fixture. The shorter is an F8T5/BLB 12 inch, 8 watt tube, used in a portable battery-powered black light sold as a pet urine detector.
Ablack light lamp emits long-wave UVA radiation and little visible light. Fluorescent black light lamps work similarly to otherfluorescent lamps, but use aphosphor on the inner tube surface which emits UVA radiation instead of visible light. Some lamps use a deep-bluish-purpleWood's glass optical filter that blocks almost all visible light with wavelengths longer than 400 nanometers.[37] The purple glow given off by these tubes is not the ultraviolet itself, but visible purple light from mercury's 404 nm spectral line which escapes being filtered out by the coating. Other black lights use plain glass instead of the more expensive Wood's glass, so they appear light-blue to the eye when operating.[citation needed]
Incandescent black lights are also produced, using a filter coating on the envelope of an incandescent bulb that absorbs visible light (see section below). These are cheaper but very inefficient, emitting only a small fraction of a percent of their power as UV.Mercury-vapor black lights in ratings up to 1 kW with UV-emitting phosphor and an envelope ofWood's glass are used for theatrical and concert displays.[citation needed]
Black lights are used in applications in which extraneous visible light must be minimized; mainly to observefluorescence, the colored glow that many substances give off when exposed to UV light. UVA /UVB emitting bulbs are also sold for other special purposes, such astanning lamps and reptile-husbandry.[citation needed]
9 watt germicidal UV bulb, in compact fluorescent (CF) form factor
Commercial germicidal lamp in butcher shop
Shortwave UV lamps are made using afluorescent lamp tube with no phosphor coating, composed offused quartz orvycor, since ordinary glass absorbs UVC. These lamps emit ultraviolet light with two peaks in the UVC band at 253.7 nm and 185 nm due to themercury within the lamp, as well as some visible light. From 85% to 90% of the UV produced by these lamps is at 253.7 nm, whereas only 5–10% is at 185 nm.[38] The fused quartz tube passes the 253.7 nm radiation but blocks the 185 nm wavelength. Such tubes have two or three times the UVC power of a regular fluorescent lamp tube. These low-pressure lamps have a typical efficiency of approximately 30–40%, meaning that for every 100 watts of electricity consumed by the lamp, they will produce approximately 30–40 watts of total UV output. They also emit bluish-white visible light, due to mercury's other spectral lines. These "germicidal" lamps are used extensively for disinfection of surfaces in laboratories and food-processing industries.[39]
'Black light'incandescent lamps are also made from an incandescent light bulb with a filter coating which absorbs most visible light.Halogen lamps withfused quartz envelopes are used as inexpensive UV light sources in the near UV range, from 400 to 300 nm, in some scientific instruments. Due to itsblack-body spectrum a filament light bulb is a very inefficient ultraviolet source, emitting only a fraction of a percent of its energy as UV, as explained by theblack body spectrum.
Specialized UVgas-discharge lamps containing different gases produce UV radiation at particular spectral lines for scientific purposes.Argon anddeuterium arc lamps are often used as stable sources, either windowless or with various windows such asmagnesium fluoride.[40] These are often the emitting sources in UV spectroscopy equipment for chemical analysis.[citation needed]
Theexcimer lamp, a UV source developed in the early 2000s, is seeing increasing use in scientific fields. It has the advantages of high-intensity, high efficiency, and operation at a variety of wavelength bands into the vacuum ultraviolet.[citation needed]
A 380 nanometer UV LED makes some common household items fluoresce.
Light-emitting diodes (LEDs) can be manufactured to emit radiation in the ultraviolet range. In 2019, following significant advances over the preceding five years, UVA LEDs of 365 nm and longer wavelength were available, with efficiencies of 50% at 1.0 W output. Currently, the most common types of UV LEDs are in 395 nm and 365 nm wavelengths, both of which are in the UVA spectrum. The rated wavelength is the peak wavelength that the LEDs put out, but light at both higher and lower wavelengths are present.[41]
The cheaper and more common 395 nm UV LEDs are much closer to the visible spectrum, and give off a purple color. Other UV LEDs deeper into the spectrum do not emit as much visible light.[42] LEDs are used for applications such asUV curing applications, charging glow-in-the-dark objects such as paintings or toys, and lights for detecting counterfeit money and bodily fluids. UV LEDs are also used in digital print applications and inert UV curing environments. As technological advances beginning in the early 2000s have improved their output and efficiency, they have become increasingly viable alternatives to more traditional UV lamps for use in UV curing applications, and the development of new UV LED curing systems for higher-intensity applications is a major subject of research in the field of UV curing technology.[43]
UVC LEDs are developing rapidly, but may require testing to verify effective disinfection. Citations for large-area disinfection are for non-LED UV sources[44] known asgermicidal lamps.[45] Also, they are used as line sources to replacedeuterium lamps inliquid chromatography instruments.[46]
Gas lasers,laser diodes, andsolid-state lasers can be manufactured to emit ultraviolet rays, and lasers are available that cover the entire UV range. Thenitrogen gas laser uses electronic excitation of nitrogen molecules to emit a beam that is mostly UV. The strongest ultraviolet lines are at 337.1 nm and 357.6 nm in wavelength. Another type of high-power gas lasers areexcimer lasers. They are widely used lasers emitting in ultraviolet and vacuum ultraviolet wavelength ranges. Presently, UVargon-fluoride excimer lasers operating at 193 nm are routinely used inintegrated circuit production byphotolithography. The current[timeframe?] wavelength limit of production of coherent UV is about 126 nm, characteristic of the Ar2* excimer laser.[citation needed]
The vacuum ultraviolet (V‑UV) band (100–200 nm) can be generated bynon-linear 4 wave mixing in gases by sum or difference frequency mixing of 2 or more longer wavelength lasers. The generation is generally done in gasses (e.g. krypton, hydrogen which are two-photon resonant near 193 nm)[49] or metal vapors (e.g. magnesium). By making one of the lasers tunable, the V‑UV can be tuned. If one of the lasers is resonant with a transition in the gas or vapor then the V‑UV production is intensified. However, resonances also generate wavelength dispersion, and thus the phase matching can limit the tunable range of the 4 wave mixing. Difference frequency mixing (i.e.,f1 +f2 −f3) has an advantage over sum frequency mixing because the phase matching can provide greater tuning.[49]
In particular, difference frequency mixing two photons of anArF (193 nm) excimer laser with a tunable visible or near IR laser in hydrogen or krypton provides resonantly enhanced tunable V‑UV covering from 100 nm to 200 nm.[49] Practically, the lack of suitable gas / vapor cell window materials above thelithium fluoride cut-off wavelength limit the tuning range to longer than about 110 nm. Tunable V‑UV wavelengths down to 75 nm was achieved using window-free configurations.[50]
Lasers have been used to indirectly generate non-coherent extreme UV (E‑UV) radiation at 13.5 nm forextreme ultraviolet lithography. The E‑UV is not emitted by the laser, but rather by electron transitions in an extremely hot tin or xenon plasma, which is excited by an excimer laser.[51] This technique does not require a synchrotron, yet can produce UV at the edge of the X‑ray spectrum.Synchrotron light sources can also produce all wavelengths of UV, including those at the boundary of the UV and X‑ray spectra at 10 nm.[citation needed]
The impact of ultraviolet radiation onhuman health has implications for the risks and benefits of sun exposure and is also implicated in issues such asfluorescent lamps and health. Getting too much sun exposure can be harmful, but in moderation, sun exposure is beneficial.[52]
UV (specifically, UVB) causes the body to producevitamin D,[53] which is essential for life. Humans need some UV radiation to maintain adequate vitamin D levels. According to the World Health Organization:[54]
There is no doubt that a little sunlight is good for you! But 5–15 minutes of casual sun exposure of hands, face and arms two to three times a week during the summer months is sufficient to keep your vitamin D levels high.
Vitamin D can also be obtained from food and supplementation.[55] Excess sun exposure produces harmful effects, however.[54]
Vitamin D promotes the creation ofserotonin. The production of serotonin is in direct proportion to the degree of bright sunlight the body receives.[56] Serotonin is thought to provide sensations of happiness, well-being and serenity to human beings.[57]
UV rays also treat certain skin conditions. Modern phototherapy has been used to successfully treatpsoriasis,eczema,jaundice,vitiligo,atopic dermatitis, and localizedscleroderma.[58][59] In addition, UV radiation, in particular UVB radiation, has been shown to inducecell cycle arrest inkeratinocytes, the most common type of skin cell.[60] As such, sunlight therapy can be a candidate for treatment of conditions such as psoriasis andexfoliative cheilitis, conditions in which skin cells divide more rapidly than usual or necessary.[61]
Sunburn effect (as measured by theUV index) is the product of the sunlight spectrum (radiation intensity) and the erythemal action spectrum (skin sensitivity) across the range of UV wavelengths. Sunburn production per milliwatt of radiation intensity is increased by nearly a factor of 100 between the near UVB wavelengths of 315–295 nm.
In humans, excessive exposure to UV radiation can result in acute and chronic harmful effects on the eye's dioptric system andretina. The risk is elevated at highaltitudes and people living in highlatitude areas where snow covers the ground right into early summer and sun positions even atzenith are low, are particularly at risk.[62] Skin, thecircadian system, and theimmune system can also be affected.[63]
The differential effects of various wavelengths of light on the human cornea and skin are sometimes called the "erythemal action spectrum".[64] The action spectrum shows that UVA does not cause immediate reaction, but rather UV begins to causephotokeratitis and skin redness (with lighter skinned individuals being more sensitive) at wavelengths starting near the beginning of the UVB band at 315 nm, and rapidly increasing to 300 nm. The skin and eyes are most sensitive to damage by UV at 265–275 nm, which is in the lower UVC band. At still shorter wavelengths of UV, damage continues to happen, but the overt effects are not as great with so little penetrating the atmosphere. TheWHO-standardultraviolet index is a widely publicized measurement of total strength of UV wavelengths that cause sunburn on human skin, by weighting UV exposure for action spectrum effects at a given time and location. This standard shows that most sunburn happens due to UV at wavelengths near the boundary of the UVA and UVB bands.[citation needed]
Ultraviolet photons harm theDNA molecules of living organisms in different ways. In one common damage event, adjacentthymine bases bond with each other, instead of across the "ladder". This "thymine dimer" makes a bulge, and the distorted DNA molecule does not function properly.
Overexposure to UVB radiation not only can causesunburn but also some forms ofskin cancer. However, the degree of redness and eye irritation (which are largely not caused by UVA) do not predict the long-term effects of UV, although they do mirror the direct damage of DNA by ultraviolet.[65]
All bands of UV radiation damagecollagen fibers and accelerate aging of the skin. Both UVA and UVB destroy vitamin A in skin, which may cause further damage.[66]
UVB radiation can cause direct DNA damage.[67] This cancer connection is one reason for concern aboutozone depletion and the ozone hole.
The most deadly form ofskin cancer, malignantmelanoma, is mostly caused by DNA damage independent from UVA radiation. This can be seen from the absence of a direct UV signature mutation in 92% of all melanoma.[68] Occasional overexposure and sunburn are probably greater risk factors for melanoma than long-term moderate exposure.[69] UVC is the highest-energy, most-dangerous type of ultraviolet radiation, and causes adverse effects that can variously be mutagenic or carcinogenic.[70]
In the past, UVA was considered not harmful or less harmful than UVB, but today it is known to contribute to skin cancer viaindirect DNA damage (free radicals such as reactive oxygen species).[71] UVA can generate highly reactive chemical intermediates, such as hydroxyl and oxygen radicals, which in turn can damage DNA. The DNA damage caused indirectly to skin by UVA consists mostly of single-strand breaks in DNA, while the damage caused by UVB includes direct formation ofthymine dimers orcytosine dimers and double-strand DNA breakage.[72] UVA is immunosuppressive for the entire body (accounting for a large part of the immunosuppressive effects of sunlight exposure), and is mutagenic for basal cell keratinocytes in skin.[73]
UVB photons can cause direct DNA damage. UVB radiationexcites DNA molecules in skin cells, causing aberrantcovalent bonds to form between adjacentpyrimidine bases, producing adimer. Most UV-induced pyrimidine dimers in DNA are removed by the process known asnucleotide excision repair that employs about 30 different proteins.[67] Those pyrimidine dimers that escape this repair process can induce a form of programmed cell death (apoptosis) or can cause DNA replication errors leading tomutation.[citation needed]
UVB damagesmRNA[74] This triggers a fast pathway that leads to inflamination of the skin and sunburn. mRNA damage initially triggers a response inribosomes though a protein known asZAK-alpha in a ribotoxic stress response. This response acts as a cell surveillance system. Following this detection of RNA damage leads to inflammatory signaling and recruitment of immune cells. This, not DNA damage (which is slower to detect) results in UVB skin inflammation and acute sunburn.[75]
As a defense against UV radiation, the amount of the brown pigmentmelanin in the skin increases when exposed to moderate (depending onskin type) levels of radiation; this is commonly known as asun tan. The purpose of melanin is to absorb UV radiation and dissipate the energy as harmless heat, protecting the skin against bothdirect andindirect DNA damage from the UV. UVA gives a quick tan that lasts for days by oxidizing melanin that was already present and triggers the release of themelanin frommelanocytes. UVB yields a tan that takes roughly 2 days to develop because it stimulates the body to produce more melanin.[citation needed]
Demonstration of the effect of sunscreen. The left image is a regular photograph of his face; the right image is of reflected UV light. The man's face has sunscreen on his right side only. It appears darker because the sunscreen absorbs the UV light.
Medical organizations recommend that patients protect themselves from UV radiation by usingsunscreen. Five sunscreen ingredients have been shown to protect mice against skin tumors. However,some sunscreen chemicals produce potentially harmful substances if they are illuminated while in contact with living cells.[76][77] The amount of sunscreen that penetrates into the lower layers of the skin may be large enough to cause damage.[78]
Sunscreen reduces the direct DNA damage that causes sunburn, by blocking UVB, and the usualSPF rating indicates how effectively this radiation is blocked. SPF is, therefore, also called UVB-PF, for "UVB protection factor".[79] This rating, however, offers no data about important protection against UVA,[80] which does not primarily cause sunburn but is still harmful, since it causes indirect DNA damage and is also considered carcinogenic. Several studies suggest that the absence of UVA filters may be the cause of the higher incidence of melanoma found in sunscreen users compared to non-users.[81][82][83][84][85] Some sunscreen lotions containtitanium dioxide,zinc oxide, andavobenzone, which help protect against UVA rays.
The photochemical properties of melanin make it an excellentphotoprotectant. However, sunscreen chemicals cannot dissipate the energy of the excited state as efficiently as melanin and therefore, if sunscreen ingredients penetrate into the lower layers of the skin, the amount ofreactive oxygen species may be increased.[86][76][77][87] The amount of sunscreen that penetrates through thestratum corneum may or may not be large enough to cause damage.
In an experiment by Hansonet al. that was published in 2006, the amount of harmfulreactive oxygen species (ROS) was measured in untreated and in sunscreen treated skin. In the first 20 minutes, the film of sunscreen had a protective effect and the number of ROS species was smaller. After 60 minutes, however, the amount of absorbed sunscreen was so high that the amount of ROS was higher in the sunscreen-treated skin than in the untreated skin.[86] The study indicates that sunscreen must be reapplied within 2 hours in order to prevent UV light from penetrating to sunscreen-infused live skin cells.[86]
Signs are often used to warn of the hazard of strong UV sources.
The eye is most sensitive to damage by UV in the lower UVC band at 265–275 nm. Radiation of this wavelength is almost absent from sunlight at the surface of the Earth but is emitted by artificial sources such as theelectrical arcs employed inarc welding. Unprotected exposure to these sources can cause "welder's flash" or "arc eye" (photokeratitis) and can lead tocataracts,pterygium andpinguecula formation. To a lesser extent, UVB in sunlight from 310 to 280 nm also causes photokeratitis ("snow blindness"), and thecornea, thelens, and theretina can be damaged.[90]
Protective eyewear is beneficial to those exposed to ultraviolet radiation. Since light can reach the eyes 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.[91][92]Ordinary, untreatedeyeglasses 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 aspolycarbonate, inherently block most UV.[93]
UV damagedpolypropylene rope (left) and new rope (right)
UV degradation is one form ofpolymer degradation that affects plastics exposed tosunlight. The problem appears as discoloration or fading, cracking, loss of strength or disintegration. The effects of attack increase with exposure time and sunlight intensity. The addition of UV absorbers inhibits the effect.
Sensitive polymers includethermoplastics and speciality fibers likearamids. UV absorption leads to chain degradation and loss of strength at sensitive points in the chain structure. Aramid rope must be shielded with a sheath of thermoplastic if it is to retain its strength.[citation needed]
Manypigments anddyes absorb UV and change colour, sopaintings and textiles may need extra protection both from sunlight and fluorescent lamps, two common sources of UV radiation. Window glass absorbs some harmful UV, but valuable artifacts need extra shielding. Many museums place black curtains overwatercolour paintings and ancient textiles, for example. Since watercolours can have very low pigment levels, they need extra protection from UV. Various forms ofpicture framing glass, including acrylics (plexiglass), laminates, and coatings, offer different degrees of UV (and visible light) protection.[citation needed]
Because of its ability to cause chemical reactions and excitefluorescence in materials, ultraviolet radiation has a number of applications. The following table[94] gives some uses of specific wavelength bands in the UV spectrum.
A portrait taken using only UV light between the wavelengths of 335 and 365 nanometers
Photographic film responds to ultraviolet radiation but the glass lenses of cameras usually block radiation shorter than 350 nm. Slightly yellow UV-blocking filters are often used for outdoor photography to prevent unwanted bluing and overexposure by UV rays. For photography in the near UV, special filters may be used. Photography with wavelengths shorter than 350 nm requires special quartz lenses which do not absorb the radiation.Digital cameras sensors may have internal filters that block UV to improve color rendition accuracy. Sometimes these internal filters can be removed, or they may be absent, and an external visible-light filter prepares the camera for near-UV photography. A few cameras are designed for use in the UV.[citation needed]
Photography by reflected ultraviolet radiation is useful for medical, scientific, and forensic investigations, in applications as widespread as detecting bruising of skin, alterations of documents, or restoration work on paintings. Photography of the fluorescence produced by ultraviolet illumination uses visible wavelengths of light.[citation needed]
Inultraviolet astronomy, measurements are used to discern the chemical composition of the interstellar medium, and the temperature and composition of stars. Because the ozone layer blocks many UV frequencies from reaching telescopes on the surface of the Earth, most UV observations are made from space.[96]
Corona discharge on electrical apparatus can be detected by its ultraviolet emissions. Corona causes degradation of electrical insulation and emission ofozone andnitrogen oxide.[97]
EPROMs (Erasable Programmable Read-Only Memory) are erased by exposure to UV radiation. These modules have a transparent (quartz) window on the top of the chip that allows the UV radiation in.
Colorlessfluorescent dyes that emit blue light under UV are added asoptical brighteners to paper and fabrics. The blue light emitted by these agents counteracts yellow tints that may be present and causes the colors and whites to appear whiter or more brightly colored.
UV fluorescent dyes that glow in the primary colors are used in paints, papers, and textiles either to enhance color under daylight illumination or to provide special effects when lit with UV lamps.Blacklight paints that contain dyes that glow under UV are used in a number of art and aesthetic applications.[citation needed]
Amusement parks often use UV lighting to fluoresce ride artwork and backdrops. This often has the side effect of causing rider's white clothing to glow light-purple.[citation needed]
A bird appears on many Visa credit cards when they are held under a UV light source.
To help preventcounterfeiting of currency, or forgery of important documents such as driver's licenses andpassports, the paper may include a UVwatermark or fluorescent multicolor fibers that are visible under ultraviolet light. Postage stamps aretagged with a phosphor that glows under UV rays to permit automatic detection of the stamp and facing of the letter.
UV fluorescentdyes are used in many applications (for example,biochemistry andforensics). Some brands ofpepper spray will leave an invisible chemical (UV dye) that is not easily washed off on a pepper-sprayed attacker, which would help police identify the attacker later.
In some types ofnondestructive testing UV stimulates fluorescent dyes to highlight defects in a broad range of materials. These dyes may be carried into surface-breaking defects by capillary action (liquid penetrant inspection) or they may be bound to ferrite particles caught in magnetic leakage fields in ferrous materials (magnetic particle inspection).
UV is an investigative tool at the crime scene helpful in locating and identifying bodily fluids such as semen, blood, and saliva.[98] For example, ejaculated fluids or saliva can be detected by high-power UV sources, irrespective of the structure or colour of the surface the fluid is deposited upon.[99]UV–vis microspectroscopy is also used to analyze trace evidence, such as textile fibers and paint chips, as well as questioned documents.
Other applications include the authentication of various collectibles and art, and detecting counterfeit currency. Even materials not specially marked with UV sensitive dyes may have distinctive fluorescence under UV exposure or may fluoresce differently under short-wave versus long-wave ultraviolet.
Using multi-spectral imaging it is possible to read illegiblepapyrus, such as the burned papyri of theVilla of the Papyri or ofOxyrhynchus, or theArchimedes palimpsest. The technique involves taking pictures of the illegible document 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.
Simple NUV sources can be used to highlight faded iron-basedink onvellum.[100]
After a training exercise involving fakebody fluids, a healthcare worker'spersonal protective equipment is checked with ultraviolet to find invisible drops of fluids. These fluids could contain deadly viruses or other contamination.
Ultraviolet helps detect organic material deposits that remain on surfaces where periodic cleaning and sanitizing may have failed. It is used in the hotel industry, manufacturing, and other industries where levels of cleanliness or contamination areinspected.[101][102][103][104]
Perennial news features for many television news organizations involve an investigative reporter using a similar device to reveal unsanitary conditions in hotels, public toilets, hand rails, and such.[105][106]
In pollution control applications, ultraviolet analyzers are used to detect emissions of nitrogen oxides, sulfur compounds, mercury, and ammonia, for example in the flue gas of fossil-fired power plants.[108] Ultraviolet radiation can detect thin sheens ofspilled oil on water, either by the high reflectivity of oil films at UV wavelengths, fluorescence of compounds in oil, or by absorbing of UV created byRaman scattering in water.[109] UV absorbance can also be used to quantify contaminants in wastewater. Most commonly used 254 nm UV absorbance is generally used as a surrogate parameters to quantify NOM.[107] Another form of light-based detection method uses a wide spectrum of excitation emission matrix (EEM) to detect and identify contaminants based on their flourense properties.[107][110] EEM could be used to discriminate different groups of NOM based on the difference in light emission and excitation of fluorophores. NOMs with certain molecular structures are reported to have fluorescent properties in a wide range of excitation/emission wavelengths.[111][107]
A collection of mineral samples fluorescing brilliantly at various wavelengths as seen while being irradiated by UV
Ultraviolet lamps are also used as part of the analysis of someminerals andgems.
In general, ultraviolet detectors use either a solid-state device, such as one based onsilicon carbide oraluminium nitride, or a gas-filled tube as the sensing element. UV detectors that are sensitive to UV in any part of the spectrum respond to irradiation bysunlight andartificial light. A burning hydrogen flame, for instance, radiates strongly in the 185- to 260-nanometer range and only very weakly in theIR region, whereas a coal fire emits very weakly in the UV band yet very strongly at IR wavelengths; thus, a fire detector that operates using both UV and IR detectors is more reliable than one with a UV detector alone. Virtually all fires emit someradiation in the UVC band, whereas theSun's radiation at this band is absorbed by theEarth'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, includinghydrocarbons, metals,sulfur,hydrogen,hydrazine, andammonia.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.
Ultraviolet radiation is used for very fine resolutionphotolithography, a procedure wherein a chemical called a photoresist is exposed to UV radiation that has passed through a mask. The exposure causes chemical reactions to occur in the photoresist. After removal of unwanted photoresist, a pattern determined by the mask remains on the sample. Steps may then be taken to "etch" away, deposit on or otherwise modify areas of the sample where no photoresist remains.
Electronic components that require clear transparency for light to exit or enter (photovoltaic panels and sensors) can be potted using acrylic resins that are cured using UV energy. The advantages are low VOC emissions and rapid curing.
Effects of UV on finished surfaces in 0, 20 and 43 hours
Certain inks, coatings, andadhesives are formulated with photoinitiators and resins. When exposed to UV light,polymerization occurs, and so the adhesives harden or cure, usually within a few seconds. Applications include glass and plastic bonding,optical fiber coatings, the coating of flooring,UV coating and paper finishes in offsetprinting, dental fillings, and decorative fingernail "gels".
UV sources for UV curing applications includeUV lamps, UVLEDs, andexcimer flash lamps. Fast processes such as flexo or offset printing require high-intensity light focused via reflectors onto a moving substrate and medium so high-pressureHg (mercury) orFe (iron, doped)-based bulbs are used, energized with electric arcs or microwaves. Lower-power fluorescent lamps and LEDs can be used for static applications. Small high-pressure lamps can have light focused and transmitted to the work area via liquid-filled or fiber-optic light guides.
UV radiation is useful in preparing low-surface-energypolymers for adhesives. Polymers exposed to UV will oxidize, thus raising thesurface energy of the polymer. Once the surface energy of the polymer has been raised, the bond between the adhesive and the polymer is stronger.
Using acatalytic chemical reaction fromtitanium dioxide and UVC exposure,oxidation of organic matter convertspathogens,pollens, andmoldspores into harmless inert byproducts. However, the reaction of titanium dioxide and UVC is not a straight path. Several hundreds of reactions occur prior to the inert byproducts stage and can hinder the resulting reaction creatingformaldehyde, aldehyde, and other VOC's en route to a final stage. Thus, the use of titanium dioxide and UVC requires very specific parameters for a successful outcome. The cleansing mechanism of UV is a photochemical process. Contaminants in the indoor environment are almost entirely organic carbon-based compounds, which break down when exposed to high-intensity UV at 240 to 280 nm. Short-wave ultraviolet radiation can destroy DNA in living microorganisms.[114] UVC's effectiveness is directly related to intensity and exposure time.
UV has also been shown to reduce gaseous contaminants such ascarbon monoxide andVOCs.[115][116][117] UV lamps radiating at 184 and 254 nm can remove low concentrations ofhydrocarbons andcarbon monoxide if the air is recycled between the room and the lamp chamber. This arrangement prevents the introduction of ozone into the treated air. Likewise, air may be treated by passing by a single UV source operating at 184 nm and passed over iron pentaoxide to remove the ozone produced by the UV lamp.
A low-pressure mercury vapor discharge tube floods the inside of ahood with shortwave UV light when not in use,sterilizing microbiological contaminants from irradiated surfaces.
Ultraviolet lamps are used tosterilize workspaces and tools used in biology laboratories and medical facilities. Commercially available low-pressuremercury-vapor lamps emit about 86% of their radiation at 254 nanometers (nm), with 265 nm being the peak germicidal effectiveness curve. UV at these germicidal wavelengths damage a microorganism's DNA/RNA so that it cannot reproduce, making it harmless, (even though the organism may not be killed).[118] Since microorganisms can be shielded from ultraviolet rays in small cracks and other shaded areas, these lamps are used only as a supplement to other sterilization techniques.
UVC LEDs are relatively new to the commercial market and are gaining in popularity.[failed verification][119] Due to their monochromatic nature (±5 nm)[failed verification] these LEDs can target a specific wavelength needed for disinfection. This is especially important knowing that pathogens vary in their sensitivity to specific UV wavelengths. LEDs are mercury free, instant on/off, and have unlimited cycling throughout the day.[120]
Disinfection using UV radiation is commonly used inwastewater treatment applications and is finding an increased usage in municipal drinkingwater treatment. Many bottlers of spring water use UV disinfection equipment to sterilize their water.Solar water disinfection[121] has been researched for cheaply treating contaminated water using naturalsunlight. The UVA irradiation and increased water temperature kill organisms in the water.
Ultraviolet radiation is used in several food processes to kill unwantedmicroorganisms. UV can be used topasteurize fruit juices by flowing the juice over a high-intensity ultraviolet source. The effectiveness of such a process depends on the UVabsorbance of the juice.
Pulsed light (PL) is a technique of killing microorganisms on surfaces using pulses of an intense broad spectrum, rich in UVC between 200 and 280nm. Pulsed light works withxenon flash lamps that can produce flashes several times per second.Disinfection robots use pulsed UV.[122]
The antimicrobial effectiveness of filteredfar-UVC (222 nm) light on a range of pathogens, including bacteria and fungi showed inhibition of pathogen growth, and since it has lesser harmful effects, it provides essential insights for reliable disinfection in healthcare settings, such as hospitals and long-term care homes.[123] UVC has also been shown to be effective at degrading SARS-CoV-2 virus.[124]
Some animals, including birds, reptiles, and insects such as bees, can see near-ultraviolet wavelengths. 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, thus assisting in the control of these arachnids. 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, are much easier to spot with ultraviolet. Urine trails of rodents can be detected by pest control technicians for proper treatment of infested dwellings.
Butterflies use ultraviolet as acommunication system for sex recognition and mating behavior. For example, in theColias eurytheme butterfly, males rely on visual cues to locate and identify females. Instead of using chemical stimuli to find mates, males are attracted to the ultraviolet-reflecting color of female hind wings.[125] InPieris napi butterflies it was shown that females in northern Finland with less UV-radiation present in the environment possessed stronger UV signals to attract their males than those occurring further south. This suggested that it was evolutionarily more difficult to increase the UV-sensitivity of the eyes of the males than to increase the UV-signals emitted by the females.[126]
Many insects use the ultraviolet wavelength emissions from celestial objects as references for flight navigation. A local ultraviolet emitter will normally disrupt the navigation process and will eventually attract the flying insect.
Thegreen fluorescent protein (GFP) is often used ingenetics as a marker. Many substances, such as proteins, have significant light absorption bands in the ultraviolet that are of interest in biochemistry and related fields. UV-capable spectrophotometers are common in such laboratories.
Ultraviolet traps calledbug zappers are used to eliminate various small flying insects. They are attracted to the UV and are killed using an electric shock, or trapped once they come into contact with the device. Different designs of ultraviolet radiation traps are also used byentomologists forcollectingnocturnal insects duringfaunistic survey studies.
Ultraviolet radiation is helpful in the treatment ofskin conditions such aspsoriasis andvitiligo. Exposure to UVA, while the skin is hyper-photosensitive, by takingpsoralens is an effective treatment forpsoriasis. Due to the potential ofpsoralens to cause damage to theliver,PUVA therapy may be used only a limited number of times over a patient's lifetime.
UVB phototherapy does not require additional medications or topical preparations for the therapeutic benefit; only the exposure is needed. However, phototherapy can be effective when used in conjunction with certain topical treatments such as anthralin, coal tar, andvitamin A and D derivatives, or systemic treatments such asmethotrexate andSoriatane.[127]
Reptiles need UVB for biosynthesis of vitamin D, and other metabolic processes.[128] Specificallycholecalciferol (vitamin D3), which is needed for basic cellular / neural functioning as well as the utilization of calcium for bone and egg production.[citation needed] The UVA wavelength is also visible to many reptiles and might play a significant role in their ability survive in the wild as well as in visual communication between individuals.[citation needed] Therefore, in a typical reptile enclosure, a fluorescent UV a/b source (at the proper strength / spectrum for the species), must be available for many[which?] captive species to survive. Simple supplementation withcholecalciferol (Vitamin D3) will not be enough as there is a complete biosynthetic pathway[which?] that is "leapfrogged" (risks of possible overdoses), the intermediate molecules and metabolites[which?] also play important functions in the animals health.[citation needed] Natural sunlight in the right levels is always going to be superior to artificial sources, but this might not be possible for keepers in different parts of the world.[citation needed]
It is a known problem that high levels of output of the UVa part of the spectrum can both cause cellular and DNA damage to sensitive parts of their bodies – especially the eyes where blindness is the result of an improper UVa/b source use and placementphotokeratitis.[citation needed] For many keepers there must also be a provision for an adequate heat source this has resulted in the marketing of heat and light "combination" products.[citation needed] Keepers should be careful of these "combination" light/ heat and UVa/b generators, they typically emit high levels of UVa with lower levels of UVb that are set and difficult to control so that animals can have their needs met.[citation needed] A better strategy is to use individual sources of these elements and so they can be placed and controlled by the keepers for the max benefit of the animals.[129]
The evolution of early reproductiveproteins andenzymes is attributed in modern models ofevolutionary theory to ultraviolet radiation. UVB causesthymine base pairs next to each other in genetic sequences to bond together intothymine dimers, a disruption in the strand that reproductive enzymes cannot copy. This leads toframeshifting during genetic replication andprotein synthesis, usually killing the cell. Before formation of the UV-blocking ozone layer, when earlyprokaryotes approached the surface of the ocean, they almost invariably died out. The few that survived had developed enzymes that monitored the genetic material and removedthymine dimers bynucleotide excision repair enzymes. Many enzymes and proteins involved in modernmitosis andmeiosis are similar to repair enzymes, and are believed to be evolved modifications of the enzymes originally used to overcome DNA damages caused by UV.[130]
Elevated levels of ultraviolet radiation, in particular UV-B, have also been speculated as a cause of mass extinctions in the fossil record.[131]
Photobiology is the scientific study of the beneficial and harmful interactions of non-ionizing radiation in living organisms, conventionally demarcated around 10 eV, the first ionization energy of oxygen. UV ranges roughly from 3 to 30 eV in energy. Hence photobiology entertains some, but not all, of the UV spectrum.
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