Ultraviolet (UV) radiation is emitted by both incandescent and fluorescent bulbs, but the levels are low compared to sunbathing bulbs or reptile tanning bulbs. The most powerful source of UV radiation is the Sun. While some scientists have expressed concern about the impact of these lights on human health, the UVA light emitted by these bulbs is too small to have an immediate effect. For scientific purposes, specialized gas-discharge UV lamps containing different gases produce UV radiation in certain spectral lines.
UV radiation was discovered in 1801 by German physicist Johann Wilhelm Ritter, who observed that invisible rays located just beyond the violet end of the visible spectrum darkened paper soaked in silver chloride more rapidly than violet light itself. The UV light emitted by these bulbs is so small that it is impossible for human health to be noticeably affected. Reptile owners, for example, can use artificial UV bulbs to provide them with vitamin D, and small amounts of UV light can help combat seasonal affective disorder. The WHO standard ultraviolet index is a widely disseminated measure of the total intensity of the UV wavelengths that cause sunburn on human skin.
The amount of ultraviolet light produced by the Sun means that Earth couldn't support life on land if the atmosphere didn't filter most of that light. Many pigments and colorants absorb UV rays and change color, so paints and textiles may need additional protection from both sunlight and fluorescent lamps, two common sources of UV radiation. Synchrotron light sources can also produce UV rays of all wavelengths, including those at the limit of the UV and X-ray spectra, at 10 nm. Because the ozone layer prevents many UV frequencies from reaching telescopes located on the Earth's surface, most UV observations are made from space. To prevent counterfeiting currency or falsifying important documents such as driver's licenses and passports, paper can include a UV watermark or multicolored fluorescent fibers that are visible under UV light.
The shorter UVC bands, as well as even more energetic UV radiation produced by the Sun, are absorbed by oxygen and generate ozone in the ozone layer when individual oxygen atoms produced by the UV photolysis of dioxygen react with more dioxygen.
