Inside the lamp end of a bi-pin lamp. In this lampthe filament is surrounded by an oblong metalcathode shield, which helps reduce lamp enddarkening.
A fluorescent lamp or fluorescent tube is a gas-discharge lamp that useselectricity to excite mercury vapor. The excited mercury atoms produceshort-wave ultraviolet light that then causes a phosphor tofluoresce, producing visible light. A fluorescent lamp converts electricalpower into useful light more efficiently than an incandescent lamp.Lower energy cost typically offsets the higher initial cost of the lamp.The lamp fixture is more costly because it requires a ballast to regulatethe current through the lamp.While larger fluorescent lamps have been mostly used in commercial orinstitutional buildings, the compact fluorescent lamp is now available inthe same popular sizes as incandescent and is used as an energy-savingalternative in homes.The United States Environmental Protection Agency classifiesfluorescent lamps as hazardous waste, and recommends that they besegregated from general waste for recycling or safe disposal
HISTORY Physical discoveriesFluorescence of certain rocks and other substances had beenobserved for hundreds of years before its nature was understood. Bythe middle of the 19th century, experimenters had observed a radiantglow emanating from partially evacuated glass vessels throughwhich an electric current passed. One of the first to explain it wasthe Irish scientist Sir George Stokes from the University ofCambridge, who named the phenomenon "fluorescence" afterfluorite, a mineral many of whose samples fluoresce strongly due toimpurities. The explanation relied on the nature of electricity andlight phenomena as developed by the British scientists MichaelFaraday and James Clerk Maxwell in the 1840s.
Alexander Edmond Becquerel observed in 1859 that certain substances gaveoff light when they were placed in a Geissler tube. He went on to apply thincoatings of luminescent materials to the surfaces of these tubes. Fluorescenceoccurred, but the tubes were very inefficient and had a short operating life.Inquiries that began with the Geissler tube continued as even better vacuumswere produced. The most famous was the evacuated tube used for scientificresearch by William Crookes. That tube was evacuated by the highly effectivemercury vacuum pump created by Hermann Sprengel. Research conducted byCrookes and others ultimately led to the discovery of the electron in 1897 by J. J.Thomson and X-rays in 1895 by Wilhelm Roentgen. But the Crookes tube, as itcame to be known, produced little light because the vacuum in it was too goodand thus lacked the trace amounts of gas that are needed for electricallystimulated luminescence.
Early discharge lampsWhile Becquerel was primarily interested in conducting scientific researchinto fluorescence, Thomas Edison briefly pursued fluorescent lighting for itscommercial potential. He invented a fluorescent lamp in 1896 that used acoating of calcium tungstate as the fluorescing substance, excited by X-rays, but although it received a patent in 1907,it was not put into production.As with a few other attempts to use Geissler tubes for illumination, it had ashort operating life, and given the success of the incandescent light, Edisonhad little reason to pursue an alternative means of electrical illumination.Nikola Tesla made similar experiments in the 1890s, devising highfrequency powered fluorescent bulbs that gave a bright greenish light, but aswith Edisons devices, no commercial success was achieved.
Although Edison lost interest in fluorescent lighting, one of his formeremployees was able to create a gas-based lamp that achieved a measure ofcommercial success. In 1895 Daniel McFarlan Moore demonstrated lamps 2 to 3meters (6.6 to 9.8 ft) in length that used carbon dioxide or nitrogen to emit whiteor pink light, respectively. As with future fluorescent lamps, they wereconsiderably more complicated than an incandescent bulb.Mercury vapor lamps continued to be developed at a slow pace, especially inEurope, and by the early 1930s they received limited use for large-scaleillumination. Some of them employed fluorescent coatings, but these wereprimarily used for color correction and not for enhanced light output. Mercuryvapor lamps also anticipated the fluorescent lamp in their incorporation of aballast to maintain a constant current.
Neon lamps or Neon lighteningThe next step in gas-based lighting took advantage of the luminescent qualities ofneon, an inert gas that had been discovered in 1898 by isolation from theatmosphere. Neon glowed a brilliant red when used in Geissler tubes. By1910, Georges Claude, a Frenchman who had developed a technology and asuccessful business for air liquefaction, was obtaining enough neon as a byproductto support a neon lighting industry. While neon lighting was used around 1930 inFrance for general illumination, it was no more energy-efficient than conventionalincandescent lighting. Neon tube lighting, which also includes the use of argonand mercury vapor as alternate gases, came to be used primarily for eye-catchingsigns and advertisements. Neon lighting was relevant to the development offluorescent lighting, however, as Claude’s improved electrode (patented in 1915)overcame "sputtering", a major source of electrode degradation. Sputteringoccurred when ionized particles struck an electrode and tore off bits of metal.Although Claude’s invention required electrodes with a lot of surface area, itshowed that a major impediment to gas-based lighting could be overcome.
Commercialization of fluorescent lampsAll the major features of fluorescent lightingwere in place at the end of the 1920s. Decadesof invention and development had provided thekey components of fluorescent lamps:economically manufactured glass tubing, inertgases for filling the tubes, electricalballasts, long-lasting electrodes, mercury vaporas a source of luminescence, effective means ofproducing a reliable electrical discharge, andfluorescent coatings that could be energized byultraviolet light. At this point, intensivedevelopment was more important than basicresearch.
The patent issue would not be completely resolved for manyyears, General Electric’s strength in manufacturing and marketing thebulb gave it a pre-eminent position in the emerging fluorescent lightmarket. Sales of "fluorescent lumiline lamps" commenced in 1938 whenfour different sizes of tubes were put on the market used in fixturesmanufactured by three leading corporations, Lightly, Art craft FluorescentLighting Corporation, and Globe Lighting, two based in New York City.During the following year GE and Westinghouse publicized the newlights through exhibitions at the New York World’s Fair and the GoldenGate International Exposition in San Francisco. Fluorescent lightingsystems spread rapidly during World War II as wartime manufacturingintensified lighting demand. By 1951 more light was produced in theUnited States by fluorescent lamps than by incandescent lamps.
Components of a Fluorescent LampA fluorescent lamp consists of a phosphor-coated tube, starter, and ballast. The tube isfilled with an inert gas (argon) plus a small amount of mercury vapor. The starterenergizes the two filaments when the lamp is first turned on. The filaments supplyelectrons to ionize the argon, forming a plasma that conducts electricity. The ballastlimits the amount of current that can flow through the tube. The plasma excites themercury atoms, which then emit red, green, blue, and ultraviolet light. The light strikesthe phosphor coating on the inside of the lamp, which converts the ultraviolet light intoother colors. Different phosphors produce warmer or cooler colors.
Incandescent LampIn an incandescent lamp, anelectric current flows through athin tungsten wire called afilament. The current heats thefilament to about 3000 C(5400 F), which causes it to emitboth heat and light. The bulb mustbe filled with an inert gas toprevent the filament from burningout. For many years incandescentlamps were filled with a mixtureof nitrogen and argon. Recentlythe gas krypton has been usedbecause it allows the filament tooperate at a highertemperature, which produces abrighter light.
Advantages LUMINOUS EFFICACYFluorescent lamps convert more of the input power to visible lightthan incandescent lamps. A typical 100 watt tungsten filamentincandescent lamps may convert into only 2% of its power input tovisible white light, whereas typical fluorescent lamp convert about22% of the power input visible white light.Fluorescent lamps efficacy is depend on lamp temperature at thecoldest part of the lamp. It T8 Lamp this is in centre of the tube. InT5 lamp this is at the end of the tube with the text stamped on it. Theideal temperature for a T8 lamp is 25 C (77 F) while the T5 lamp isideally 35 C (95 F).
Lower luminosityCompared with an incandescent lamp, a fluorescent tube is amore diffuse and physically larger light source. In suitablydesigned lamps, light can be more evenly distributed withoutpoint source of glare such as seen from an undiffusedincandescent filament; the lamp is large compared to the typicaldistance between lamp and illuminated surfaces. Lower heatAbout two-thirds to three-quarters less heat is given off byfluorescent lamps compared to an equivalent installation ofincandescent lamps. This greatly reduces the size, cost, andenergy consumption.
LifeTypically a fluorescent lamp will last between 10 to 20times as long as an equivalent incandescent lamp whenoperated several hours at a time.The higher initial cost of a fluorescent lamp is usuallymore than compensated for by lower energyconsumption over its life. The longer life may alsoreduce lamp replacement costs, providing additionalsaving especially where labor is costly. Therefore theyare widely used by businesses and institutions, but notas much by households.
Disadvantages Frequent switchingIf the lamp is installed where it is frequently switched on and off, itwill age rapidly. Under extreme conditions, its lifespan may be muchshorter than a cheap incandescent lamp. Each start cycle slightlyerodes the electron-emitting surface of the cathodes; when all theemission material is gone, the lamp cannot start with the availableballast voltage. Fixtures intended for flashing of lights (such as foradvertising) will use a ballast that maintains cathode temperaturewhen the arc is off, preserving the life of the lamp.The extra energy used to start a fluorescent lamp is equivalent to afew seconds of normal operation; it is more energy-efficient to switchoff lamps when not required for several minutes.
Health and safety issuesIf a fluorescent lamp is broken, a very small amount ofmercury can contaminate the surrounding environment. About99% of the mercury is typically contained in the phosphor,especially on lamps that are near the end of their life. Thebroken glass is usually considered a greater hazard than thesmall amount of spilled mercury. The EPA recommends airingout the location of a fluorescent tube break and using wetpaper towels to help pick up the broken glass and fineparticles. Any glass and used towels should be disposed of in asealed plastic bag. Vacuum cleaners can cause the particles tobecome airborne, and should not be used.