TND328/DRev.1–Sep07LEDLightingDefinitionsandCharacteristicsTND328/DRev.1–Sep072DefinitionsTND328/DRev.1–Sep073LuminousFlux[lm]•Luminousfluxisthequantityoftheenergyofthelightemittedpersecondinalldirections.Theunitofluminousfluxislumen(lm).Onelumenistheluminousfluxoftheuniformpointlightsourcethathasluminousintensityof1candelaandiscontainedinoneunitofspatialangle(or1steradian).Steradianisthespatialanglethatlimitsthesurfaceareaofthesphereequaltothesquareoftheradius.Thisconceptisshowninthefigurefor1mradiusofthesphere.Sincetheareaofsphereis4πr²thentheluminousfluxofthepointlightsourceis4πlumens.Pointsourcewithuniformlightand1candela[cd]ofluminousintensity.1meterradius1lumenofLuminousfluxArea=1squaremeterTND328/DRev.1–Sep074LuminousIntensity[cd]•Theluminousintensity(measuredincandelas)istheamountoflightemittedintoacertaindirection,measuredin1mdistance.•IfthepointlightsourceemitsØlumensintoasmallspatialangleß,theluminousintensityisI=Ø/ß.Theunitofluminousintensityiscandela.Thereisastandardthatdetailsthecandeladefinition.Thisincludesthestandardlightsourceandthephysicalconditionsofthemeasurement.TND328/DRev.1–Sep075•Luminousintensityisnotonlydrivenbytheluminousflux,butalsobytheoptics,whichmakesthelightmoreorlessfocused.TND328/DRev.1–Sep076Illuminance•Thisdefinitiondeterminestheamountoflightthatcoversasurface.IfØistheluminousfluxandSistheareaofthegivensurfacethentheilluminanceEisdeterminedbyE=Ø/S.TheunitofilluminationinSIsystemislux[lx],andinfoot-poundsystemitisfoot-candle.Onelxistheilluminanceof1m²surfaceareauniformlylightedby1lmofluminousflux.Thefollowingdrawingexplainsthisdefinition.Onefoot-candleis10.76lux.1lm1squaremeter1lxTND328/DRev.1–Sep077•Definesacolorasthetemperature[K]thata"blackbody"sourcemustreachinordertoproducethatsamecolor.CorrelatedColorTemperatureCIE(1931)xychromaticitydiagramincludingthePlanckianlocus,withtemperaturesindicated.Wavelengthsofmonochromaticlightareshowninblue.ThelinescrossingthePlanckianlocusarelinesofconstantcorrelatedcolortemperature.NoonsunlightMonochromaticColorWaveLengthSourcehttp://en.wikipedia.org/wiki/Chromaticity_diagramTND328/DRev.1–Sep078Example:ChromaticitycoordinateGroupsforOsram’sLWY87C«MicrosideLED»fordisplaybacklightingapplicationsTypicalcolortemperature:6200°KTND328/DRev.1–Sep079CharacteristicsTND328/DRev.1–Sep0710Current/VoltageCharacteristicsForwardvoltagekeepsdecreasing.SmallLEDsdedicatedtobacklightingnowexhibitaforwardvoltageinthe3.1to3.2Vrange.ThishasastrongimpactonLEDdriverchoice.LWY3SGdatasheetTND328/DRev.1–Sep0711ChromaticityCoordinateShiftOsram:LWY87CTheLED’scolorcharacteristicsaredependentfromtheforwardcurrent.SpecialattentionmustbepaidwhenusingLEDdriverswithRed-Green-BlueLEDs.Lightdimmingshouldnotmodifythecolorrendering.PreferredsolutionisPWMdimmingtoensurethatLEDsaredrivenwithknownforwardcurrent.TND328/DRev.1–Sep0712TemperatureInfluenceForwardvoltage:AllLEDsexhibitforwardvoltagevariationasjunctiontemperaturechanges.Thetemperaturecoefficientdependsfromjunctiontype.InGaAlPLEDs(YellowandAmberRed)haveacoefficientofbetween-3.0mV/Kto-5.2mV/K,andtheInGaNLEDs(Blue,VerdeGreen,andWhite)haveacoefficientofbetween–3.6mV/Kand-5.2mV/K.(GoldenDragonLWW5SG)Intensitychangeovertemperature:Lightintensitydecreasesasthetemperatureincreases.Thisisaresultofchangingefficienciesinthesemiconductor,andnotaresultofthechangeintheforwardvoltageovertemperaturechanges.Thistemperaturechangeisnon-linear.TND328/DRev.1–Sep0713TemperatureInfluencePermissiblecurrent:LEDmanufacturershaverealizedsignificantimprovementstoallowhighcurrentoperation.Nevertheless,highcurrentLEDsusedforcameraphonesexhibitshorterlifetimethansmallLEDsdedicatedtobacklight.Thisisnotastrongissueincaseofflash-lightbecauseofshortpulseduration,butitmustbetakenintoaccountforvideomode.Datasheet:LWW5SG
