Chapter46-LightingTYPESOFLAMPSANDLIGHTINGRichardForsterAlampisanenergyconverter.Althoughitmaycarryoutsecondaryfunctions,itsprimepurposeisthetransformationofelectricalenergyintovisibleelectromagneticradiation.Therearemanywaystocreatelight.Thestandardmethodforcreatinggenerallightingistheconversionofelectricalenergyintolight.TypesofLightIncandescenceWhensolidsandliquidsareheated,theyemitvisibleradiationattemperaturesabove1,000K;thisisknownasincandescence.Suchheatingisthebasisoflightgenerationinfilamentlamps:anelectricalcurrentpassesthroughathintungstenwire,whosetemperaturerisestoaround2,500to3,200K,dependinguponthetypeoflampanditsapplication.Thereisalimittothismethod,whichisdescribedbyPlanck’sLawfortheperformanceofablackbodyradiator,accordingtowhichthespectraldistributionofenergyradiatedincreaseswithtemperature.Atabout3,600Kandabove,thereisamarkedgaininemissionofvisibleradiation,andthewavelengthofmaximumpowershiftsintothevisibleband.Thistemperatureisclosetothemeltingpointoftungsten,whichisusedforthefilament,sothepracticaltemperaturelimitisaround2,700K,abovewhichfilamentevaporationbecomesexcessive.Oneresultofthesespectralshiftsisthatalargepartoftheradiationemittedisnotgivenoffaslightbutasheatintheinfraredregion.Filamentlampscanthusbeeffectiveheatingdevicesandareusedinlampsdesignedforprintdrying,foodpreparationandanimalrearing.ElectricdischargeElectricaldischargeisatechniqueusedinmodernlightsourcesforcommerceandindustrybecauseofthemoreefficientproductionoflight.Somelamptypescombinetheelectricaldischargewithphotoluminescence.Anelectriccurrentpassedthroughagaswillexcitetheatomsandmoleculestoemitradiationofaspectrumwhichischaracteristicoftheelementspresent.Twometalsarecommonlyused,sodiumandmercury,becausetheircharacteristicsgiveusefulradiationswithinthevisiblespectrum.Neithermetalemitsacontinuousspectrum,anddischargelampshaveselectivespectra.Theircolourrenderingwillneverbeidenticaltocontinuousspectra.Dischargelampsareoftenclassedashighpressureorlowpressure,althoughthesetermsareonlyrelative,andahigh-pressuresodiumlampoperatesatbelowoneatmosphere.TypesofLuminescencePhotoluminescenceoccurswhenradiationisabsorbedbyasolidandisthenre-emittedatadifferentwavelength.Whenthere-emittedradiationiswithinthevisiblespectrumtheprocessiscalledfluorescenceorphosphorescence.Electroluminescenceoccurswhenlightisgeneratedbyanelectriccurrentpassedthroughcertainsolids,suchasphosphormaterials.Itisusedforself-illuminatedsignsandinstrumentpanelsbuthasnotprovedtobeapracticallightsourceforthelightingofbuildingsorexteriors.EvolutionofElectricLampsAlthoughtechnologicalprogresshasenableddifferentlampstobeproduced,themainfactorsinfluencingtheirdevelopmenthavebeenexternalmarketforces.Forexample,theproductionoffilamentlampsinuseatthestartofthiscenturywaspossibleonlyaftertheavailabilityofgoodvacuumpumpsandthedrawingoftungstenwire.However,itwasthelarge-scalegenerationanddistributionofelectricitytomeetthedemandforelectriclightingthatdeterminedmarketgrowth.Electriclightingofferedmanyadvantagesovergas-oroil-generatedlight,suchassteadylightthatrequiresinfrequentmaintenanceaswellastheincreasedsafetyofhavingnoexposedflame,andnolocalby-productsofcombustion.DuringtheperiodofrecoveryaftertheSecondWorldWar,theemphasiswasonproductivity.Thefluorescenttubularlampbecamethedominantlightsourcebecauseitmadepossibletheshadow-freea...
优质VIP
