Ultraviolet

• 1. Discovery And History

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  In February 1801, the German physicist Johann Wilhelm Ritter observed that invisible rays just beyond the violet end of the visible spectrum darkened silver chloride-soaked paper more quickly than violet light itself. He announced this discovery in a brief letter to the Annalen der Physik and later called them "(de-)oxidizing rays" to emphasize chemical reactivity. The simpler term "chemical rays" was adopted soon afterwards and remained popular throughout the 19th century. Some scientists like John William Draper named them "tithonic rays," claiming they were entirely different from light. By 1878, researchers discovered the sterilizing effect of short-wavelength light by killing bacteria. In 1893, Victor Schumann made the discovery of ultraviolet radiation with wavelengths below 200 nm, which he named "vacuum ultraviolet" because it is strongly absorbed by oxygen in air. A committee of the Second International Congress on Light decided unanimously on the 17th of August 1932, at the Castle of Christiansborg in Copenhagen to divide UV into UVA, UVB, and UVC.

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• 2. Spectral Subdivisions

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  The electromagnetic spectrum of ultraviolet radiation spans 10 to 400 nanometers and can be subdivided into ranges recommended by ISO standard ISO 21348. Ultraviolet A covers 315 to 400 nanometers and is known as long-wave or soft UV that is not absorbed by the ozone layer. Ultraviolet B spans 280 to 315 nanometers and is called medium-wave or intermediate UV that is mostly absorbed by the ozone layer. Ultraviolet C extends from 100 to 280 nanometers and is termed short-wave or hard UV that is completely absorbed by the ozone layer and atmosphere. Near ultraviolet ranges from 300 to 400 nanometers and is visible to birds, insects, and fish. Middle ultraviolet covers 200 to 300 nanometers while far ultraviolet spans 122 to 200 nanometers. Extreme ultraviolet occupies 10 to 121 nanometers and is entirely ionizing radiation by some definitions. Vacuum ultraviolet includes wavelengths from 10 to 200 nanometers that are strongly absorbed by atmospheric oxygen.

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• 3. Solar Interaction And Ozone

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  Sunlight in space at the top of Earth's atmosphere contains about 10% ultraviolet light for a total intensity of approximately 1400 W/m² in vacuum. The atmosphere blocks about 77% of the Sun's UV when the Sun is highest in the sky at zenith. At ground level with the sun at zenith, sunlight consists of 44% visible light, 3% ultraviolet, and the remainder infrared. Of the ultraviolet radiation reaching Earth's surface, more than 95% is the longer wavelengths of UVA with the small remainder being UVB. Almost no UVC reaches the Earth's surface because it is blocked by diatomic oxygen between 100 and 200 nm or by ozone between 200 and 280 nm. Shorter bands of UVC as well as even more energetic solar UV radiation generate the ozone layer when single oxygen atoms produced by UV photolysis 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.

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• 4. Human Health Impacts

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  UV specifically UVB causes the body to produce vitamin D which is essential for life according to the World Health Organization. Five to fifteen minutes of casual sun exposure of hands face and arms two to three times a week during summer months keeps vitamin D levels high. Excessive exposure to UV radiation can result in acute and chronic harmful effects on the eye's dioptric system and retina. The skin circadian system and immune system can also be affected by overexposure. Overexposure to UVB radiation can cause sunburn and some forms of skin cancer including melanoma. All bands of UV radiation damage collagen fibers and accelerate aging of the skin. Both UVA and UVB destroy vitamin A in skin which may cause further damage. UVB photons can cause direct DNA damage while UVA contributes to skin cancer via indirect DNA damage through free radicals such as reactive oxygen species. The WHO-standard ultraviolet index measures total strength of UV wavelengths that cause sunburn on human skin.

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• 5. Artificial Sources And Technology

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  Mercury-vapor lamps consisting of fused quartz tubes filled with mercury and argon emit ultraviolet light with peaks at 253.7 nm and 185 nm. From 85% to 90% of the UV produced by these lamps is at 253.7 nm which is very effective as a germicide. Black light lamps use phosphors on inner tube surfaces that emit UVA radiation instead of visible light. Some lamps use deep-bluish-purple Wood's glass optical filters that block almost all visible light longer than 400 nanometers. Light-emitting diodes or LEDs can be manufactured to emit radiation in the ultraviolet range. In 2019 following significant advances over preceding five years UVA LEDs of 365 nm and longer wavelength were available with efficiencies of 50% at 1.0 W output. Gas lasers laser diodes and solid-state lasers can be manufactured to emit ultraviolet rays covering the entire UV range. Excimer lasers operating at 193 nm are routinely used in integrated circuit production by photolithography.

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• 6. Industrial Applications

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  Ultraviolet radiation is used for very fine resolution photolithography where a chemical called photoresist is exposed to UV radiation passing through a mask. Photolithography processes used to fabricate electronic integrated circuits presently use 193 nm UV and experimentally use 13.5 nm UV for extreme ultraviolet lithography. Certain inks coatings and adhesives are formulated with photoinitiators and resins that harden or cure within seconds when exposed to UV light. Electronic components requiring clear transparency for light to exit or enter can be potted using acrylic resins cured using UV energy. Fast processes such as flexo or offset printing require high-intensity light focused via reflectors onto moving substrates so high-pressure mercury or iron-based bulbs are used. Polymers exposed to UV will oxidize raising surface energy making bonds between adhesive and polymer stronger. UV sources for curing applications include UV lamps UV LEDs and excimer flash lamps.

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• 7. Forensic And Analytical Uses

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  UV serves as an investigative tool at crime scenes helpful in locating and identifying bodily fluids such as semen blood and saliva. UV-vis microspectroscopy analyzes trace evidence like textile fibers paint chips and questioned documents. Multi-spectral imaging allows reading of illegible papyri such as the burned papyri of Villa of Papyri or Oxyrhynchus by taking pictures using different filters tuned to capture certain wavelengths. Simple near-UV sources highlight faded iron-based ink on vellum. Colorless fluorescent dyes emitting blue light under UV are added as optical brighteners to paper and fabrics counteracting yellow tints. To prevent counterfeiting currency driver's licenses and passports may include UV watermarks or fluorescent multicolor fibers visible under ultraviolet light. Ultraviolet helps detect organic material deposits remaining on surfaces where periodic cleaning failed in hotel industry manufacturing and other industries. UV/Vis spectroscopy is widely used to analyze chemical structure especially conjugated systems while quantifying nucleic acids or proteins in biological research.

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