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Top, two compact uorescent lamps. Bottom, two uorescenttube lamps. A matchstick, left, is shown for scale.
Compact uorescent lamp with electronic ballast
A uorescent lamp or a uorescent tube is a lowpressure mercury-vapor gas-discharge lamp that usesuorescence to produce visible light. An electric currentin the gas excites mercury vapor which produces short-wave ultraviolet light that then causes a phosphor coatingon the inside of the bulb to glow. A uorescent lamp
Typical F71T12 100 W bi-pin lamp used in tanning beds. The(Hg) symbol indicates that this lamp contains mercury. In the US,this symbol is now required on all uorescent lamps that containmercury.
One style of lamp holder for T12 and T8 bi pin uorescent lamps
converts electrical energy into useful light much more ef-ciently than incandescent lamps. The luminous ecacyof a uorescent light bulb can exceed 100 lumens perwatt, several times the ecacy of an incandescent bulbwith comparable light output.
2 1 HISTORY
Inside the lamp end of a preheat bi-pin lamp. In this lamp thelament is surrounded by an oblong metal cathode shield, whichhelps reduce lamp end darkening.
Fluorescent lamp xtures are more costly than incandes-cent lamps because they require a ballast to regulate thecurrent through the lamp, but the lower energy cost typ-ically osets the higher initial cost. Compact uorescentlamps are now available in the same popular sizes as in-candescents and are used as an energy-saving alternativein homes.Because they contain mercury, many uorescent lampsare classied as hazardous waste. The United States En-vironmental Protection Agency recommends that uores-cent lamps be segregated from general waste for recyclingor safe disposal.
1.1 Physical discoveries
Fluorescence of certain rocks and other substances hadbeen observed for hundreds of years before its naturewas understood. By the middle of the 19th century, ex-perimenters had observed a radiant glow emanating frompartially evacuated glass vessels through which an electriccurrent passed. One of the rst to explain it was the Irishscientist Sir George Stokes from the University of Cam-bridge, who named the phenomenon uorescence afteruorite, a mineral many of whose samples glow stronglydue to impurities. The explanation relied on the natureof electricity and light phenomena as developed by theBritish scientists Michael Faraday in the 1840s and JamesClerk Maxwell in the 1860s.
Little more was done with this phenomenon until 1856when a German glassblower named Heinrich Geisslercreated a mercury vacuum pump that evacuated a glasstube to an extent not previously possible. When an elec-
trical current passed through a Geissler tube, a stronggreen glow on the walls of the tube at the cathode endcould be observed. Because it produced some beautifullight eects, the Geissler tube was a popular source ofamusement. More important, however, was its contribu-tion to scientic research. One of the rst scientists to ex-periment with a Geissler tube was Julius Plcker who sys-tematically described in 1858 the luminescent eects thatoccurred in a Geissler tube. He also made the importantobservation that the glow in the tube shifted position whenin proximity to an electromagnetic eld. Alexandre Ed-mond Becquerel observed in 1859 that certain substancesgave o light when they were placed in a Geissler tube.He went on to apply thin coatings of luminescent materi-als to the surfaces of these tubes. Fluorescence occurred,but the tubes were very inecient and had a short oper-ating life.
Inquiries that began with the Geissler tube continued aseven better vacuums were produced. The most famouswas the evacuated tube used for scientic research byWilliam Crookes. That tube was evacuated by the highlyeective mercury vacuum pump created by HermannSprengel. Research conducted by Crookes and others ul-timately led to the discovery of the electron in 1897 byJ. J. Thomson and X-rays in 1895 by Wilhelm Roentgen.But the Crookes tube, as it came to be known, producedlittle light because the vacuum in it was too good and thuslacked the trace amounts of gas that are needed for elec-trically stimulated luminescence.
1.2 Early discharge lamps
While Becquerel was interested primarily in conduct-ing scientic research into uorescence, Thomas Edisonbriey pursued uorescent lighting for its commercial po-tential. He invented a uorescent lamp in 1896 that used acoating of calcium tungstate as the uorescing substance,excited by X-rays, but although it received a patent in1907, it was not put into production. As with a fewother attempts to use Geissler tubes for illumination, ithad a short operating life, and given the success of theincandescent light, Edison had little reason to pursue analternative means of electrical illumination. Nikola Teslamade similar experiments in the 1890s, devising high-frequency powered uorescent bulbs that gave a brightgreenish light, but as with Edisons devices, no commer-cial success was achieved.Although Edison lost interest in uorescent lighting, oneof his former employees was able to create a gas-basedlamp that achieved a measure of commercial success. In1895 Daniel McFarlan Moore demonstrated lamps 2 to3 meters (6.6 to 9.8 ft) in length that used carbon diox-ide or nitrogen to emit white or pink light, respectively.As with future uorescent lamps, they were considerablymore complicated than an incandescent bulb.
After years of work, Moore was able to extend the op-
1.2 Early discharge lamps 3
One of the rst mercury vapor lamps invented by Peter CooperHewitt, 1903. It was similar to a uorescent lamp without theuorescent coating on the tube, and produced greenish light. Theround device under the lamp is the ballast.
erating life of the lamps by inventing an electromag-netically controlled valve that maintained a constant gaspressure within the tube. Although Moores lamp wascomplicated, was expensive to install, and required veryhigh voltages, it was considerably more ecient than in-candescent lamps, and it produced a closer approxima-tion to natural daylight than contemporary incandescentlamps. From 1904 onwards Moores lighting system wasinstalled in a number of stores and oces. Its successcontributed to General Electrics motivation to improvethe incandescent lamp, especially its lament. GEs ef-forts came to fruition with the invention of a tungsten-based lament. The extended lifespan and improved ef-cacy of incandescent bulbs negated one of the key ad-vantages of Moores lamp, but GE purchased the rele-vant patents in 1912. These patents and the inventive ef-forts that supported themwere to be of considerable valuewhen the rm took up uorescent lighting more than twodecades later.At about the same time that Moore was developing hislighting system, another American was creating a meansof illumination that also can be seen as a precursor to themodern uorescent lamp. This was the mercury-vaporlamp, invented by Peter Cooper Hewitt and patented in
Peter Cooper Hewitt
1901 (US 682692; this patent number is frequently mis-quoted as US 889,692). Hewitts lamp glowed whenan electric current was passed through mercury vapor ata low pressure. Unlike Moores lamps, Hewitts weremanufactured in standardized sizes and operated at lowvoltages. The mercury-vapor lamp was superior to theincandescent lamps of the time in terms of energy ef-ciency, but the blue-green light it produced limited itsapplications. It was, however, used for photography andsome industrial processes.Mercury vapor lamps continued to be developed at a slowpace, especially in Europe, and by the early 1930s they re-ceived limited use for large-scale illumination. Some ofthem employed uorescent coatings, but these were usedprimarily for color correction and not for enhanced lightoutput. Mercury vapor lamps also anticipated the uores-cent lamp in their incorporation of a ballast to maintain aconstant current.Cooper-Hewitt had not been the rst to use mercury va-por for illumination, as earlier eorts had been mountedby Way, Rapie, Arons, and Bastian and Salisbury. Ofparticular importance was the mercury vapor lamp in-vented by Kch in Germany. This lamp used quartz inplace of glass to allow higher operating temperatures, andhence greater eciency. Although its light output relativeto electrical consumption was better than that of othersources of light, the light it produced was similar to thatof the Cooper-Hewitt lamp in that it lacked the red por-tion of the spectrum, making it unsuitable for ordinarylighting.
4 1 HISTORY
1.3 Neon lamps
Main article: Neon lighting
The next step in gas-based lighting took advantage ofthe luminescent qualities of neon, an inert gas that hadbeen discovered in 1898 by isolation from the atmo-sphere. Neon glowed a brilliant red when used in Geisslertubes. By 1910, Georges Claude, a Frenchman whohad developed a technology and a successful business forair liquefaction, was obtaining enough neon as a byprod-uct to support a neon lighting industry. While neonlighting was used around 1930 in France for general il-lumination, it was no more energy-ecient than conven-tional incandescent lighting. Neon tube lighting, whichalso includes the use of argon and mercury vapor as al-ternate gases, came to be used primarily for eye-catchingsigns and advertisements. Neon lighting was relevantto the development of uorescent lighting, however, asClaudes improved electrode (patented in 1915) over-came sputtering, a major source of electrode degrada-tion. Sputtering occurred when ionized particles struckan electrode and tore o bits of metal. Although Claudesinvention required electrodes with a lot of surface area,it showed that a major impediment to gas-based lightingcould be overcom