Phosphors
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Phosphors
luminescing centers will cancel each other out. Activators enhance the light-producing properties by many orders of magnitude and also determine the color of the emitted light. Zinc sulfide, for example, can be activated by copper, silver, or gold, Certain solid materials can absorb enyielding a variety of colors. ergy from x-rays, ultraviolet light, or highenergy particles, then re-emit the energy If prepared with appropriate activator as visible light. Such "phosphors" are used impurities, other materials, such as certain in fluorescent lamp bulbs, electrolumisilicates, borates, phosphates, or metal oxnescent lamps, and television screens. ides, can become phosphors. For instance, Without phosphors, I would not be able to when activated by magnesium ions, zinc see these words as I type them on my comsilicate, zinc and cadmium borate, and zinc puter screen. beryllium silicate become efficient phosphors, emitting in the red to green region. Scientists had known for some time that Other phosphors commonly used in fluocertain minerals were capable of giving off rescent lamps are tungstates, phosphates, visible light when excited by other forms of and halophosphates of zinc, calcium, and radiation. Then, in 1801, Johann Wilhelm magnesium. Ritter, a physicist in Germany, discovered that some materials luminesce brightly Fluorescent lamps operate on a simple when exposed to light from the invisible principle. If the inside of a glass tube is spectral region beyond violet; Ritter was coated with a thin film of phosphor, then thus the first to discover ultraviolet radiafilled with a gas through which an electrition. The term fluorescence, first used in cal discharge is passed, the ultraviolet light 1852, was derived from the mineral from the gas discharge provides energy to fluorospar, which emits a violet light when make the phosphors shine in visible light. exposed to ultraviolet radiation. Though this was known from Becquerel's work in 1859, the first widespread use of In 1859 in France, Alexandre Edmond fluorescent lighting did not occur until alBecquerel described afluorescentlamp he most seventy years later. had produced; more than a century earlier Benjamin Franklin had described the prinFluorescent lighting was first used for ciple on which it was based—that the lumiadvertising in Europe in 1925, though exnescence of lightning is caused by electric periments to apply the principle for practidischarge through the atmosphere. In cal interior lighting did not take place until a decade later in the United States. These 1860, the Royal Society of London observed an electric discharge lamp that exvery successful experiments allowed for commercial introduction of low-voltage cited rarefied carbon dioxide gas into producing a brilliant white light. fluorescent lamps three years later. In 1939 only straight tubular designs of fluorescent Chemists began to search for practical lamps were available, and only in 15-, 20-, solid materials to be used as phosphors. 30-, and 40-watt sizes. Two decades later, The first
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