leybold镀银制程对汽车头灯反射的运用分析

354OpticsApplicationsofHighlyReflectiveSilverCoatingsforCarHead-lampReflectorsTorstenSchmauder,StephanKüper(LeyboldOpticsGmbH,Alzenau,Germany)Keywords:Headlamps,Reflectivity,DeviceEfficiency,…1IntroductionModerncarheadlampsposehighrequirementstotheirdeviceefficiency.Atpre-sent,asingleLEDlightsourcestillfallsshortinluminousfluxcomparedtoaHIDlightandthusseveralopticalelementsaremandatorytoshapethebeam.Asaconsequence,thereflectivityofthe(reflective)opticalelementsbecomesoneofthekeyfactorsfortheoverallperformanceandefficiencyofthedevice.Further,conventionalsystemswithhalogenorHIDlightsourcestestthethermallimitsofthematerialsusedintheirdesign.Here,reflectivityisakeycriticalfactorforthethermaldesignofthecomponents.Finally,withenergyconservationbe-comingmoreandmoreimportant,poordeviceefficiencybecomesunacceptableforenvironmentalreasons.Still,mostoftoday’sheadlampreflectorsarecoatedconventionallywithAlumi-numanddeliverareflectivitybetween82and88%onamanufacturingqualitylevel.Silver,onthecontrary,offersareflectivityupto97%andcanthereforecontributetotheimprovementofthedeviceefficiency.OneofthemajorchallengesforapplyingSilverasthereflectivematerialisitscor-rosionstability.EventhoughSilverisanoblemetalandAluminumischemicallyveryreactive,Aluminumcanbepassivatedandprotectedtoexcellentqualitylevelsbymatureandestablishedmanufacturingprocesses.Finally,thecostofmanufacturingSilvercoatedpartsisimportant,asSilvertar-getscomewiththepricetagofanoblemetal.However,acompletecostperpiececalculationrevealsthatthecostsofSilvercoatedpartsmaybeoverestimatedifviewedinrelationwiththetotalcostofaheadlampandwiththeperformanceimprovementachieved.Thepaperdiscussestheadvantagesandchallengesofreflectorswhicharevacu-umcoatedwithSilverinsteadofAluminum.355ApplicationofHighlyReflectiveSilverCoatingsforCarHeadlampReflectors2OpticalPropertiesofSilverandAluminum2.1ComparisonofSpectraSilverandAluminumexhibitquitedifferentreflectionspectra.Silverrisestoalevelof98%forallwavelengthsabove600nm.Inthevisiblerangefrom400to700nmtheintegralreflectivityis96.9%.Aluminum,ontheotherhandhasa(theoretical)integralreflectivityof92.5%inthevisiblerangeandrisestoapp.97.5%intheinfrared.Indaytodayproduction,however,thereflectivityofAluminummirrorsisfrequentlylower(sometimesbelow82%inthevisiblerange)duetoimpuritiescomingfrominadmissibleresidualgascontentduringvacuummetallization,inadequatelayerthickness/uniformityinducedbycom-plexpartgeometryandrecessedareasaswellasbythenecessaryapplicationofacorrosionprotectivesiloxanetopcoat.ThecalculationsbelowarebasedontheidealreflectivitiesofAluminumplusSiloxanetopcoatandSilver.ThedifferencesbetweenSilverandAluminumcoatedreflectorscalculatedinthispaperthereforerepresentaconservativeestimationoftheachievableimprovements.Figure1:OpticalreflectionspectraofmirrorscoatedwithAluminumwithSiloxanetopcoatandSilverrespectively.SilverhasasignificantlyhigherreflectivityinthevisibleandnearIR.90919293949596979899100200400600800100012001400160018002000220024002600Wavelength[nm]Reflectivity[%]AgAl+SiloxanTC356Optics2.2DeviceEfficiencyThedeviceefficiencyofcarlightsdescribesthepercentageofthelightemittedbythesourcewhichactuallyleavesthedeviceintheintendedwayandisavailableforillumination.Itisdeterminedexclusivelybytheopticaldesignofthedeviceandcanbecalculatedfromtheopticaldataandreflectionspectraoftheindividu-alelements.Lossesareduetoabsorptiononreflectiveopticalelements(mirrors)andduetoreflectiononrefractiveelements(lenses,transparentcovers).Fortheilluminationperformanceweconsiderthevisiblerangefrom400to700nmforthecalculations.Incaseofasinglereflectiveopticalelementtheintegralreflecti-vityisatthesametimethedeviceefficiency.Incaseof2reflectiveelements(likeinFigure2)thedeviceefficiencyistheproductofthetwointegralreflectivitiesofbothreflectiveelements.Inallheadlightsthereisalsoafrontlens,mostlymadeofPolycarbonate,whichfinallyreflectsanother10%ofthelightintothelampbody.Asanexample,inaheadlightdevicelikeinFigure2,Figure2:OpticalLossesinaheadlightutilizingtwoAluminumreflectorsforbeamshaping.Duetothereflectorsalone,14.3%ofthelightislost.AtthePCfrontlensadditional10%arereflectedwithinthedevice.Only77.2%oftheinitiallightfromthelightsourceleavethedeviceintheintendedwayandareavailableforillumination.about14.3%oftheinitiallightarelostonlyduetothereflectors,iftheyarecoa-tedwithAluminum.Afterthefrontlenslittlemorethan77%oftheinitiallightisavailableforroadillumination.IfthemirrorswerecoatedwithSilver,thereflec-tivelossescouldbereducedto6.2%,whichwouldlifttheoverallefficiencyafterthefrontlensto84.4%.Intotal,thelightoutputfromtheSilvercoateddevicewillbe9.1%higherthanfromtheAluminumcoateddevice.Thereflectiononthefrontlenscouldalsobereduceddrasticallybyinterferenceantireflexcoatings,whichhoweverarenottopicofthispaper.357ApplicationofHighlyReflectiveSilverCoatingsforCarHeadlampReflectors2.3HeatManagementAlllightthatislostduetoabsorptionisconvertedtoheatinthesystem.Fortheheatmanagement,therangebetween350and2500nmneedstobeconsideredandcoversmostofthelightthatisemittedbyahalo