Lighting The Led Luminous Efficiency Of The Thermal Characteristics And Test And Evaluation Methods

LED(light emitting diode) is very hot in the field of lighting recently,it attractsthe world because of its potentially high efficacy,long life,pure color,solid state andeco-friendly.America,Europe,Japan,South Korea and China had all establishednational plans to develop LED technology,which really make the technology progressquickly in last ten years.Efficacy,as one of the most important parameters of lightsources,has approached 1001m/W for LED commercial products and much higherthan the data for LED samples made by laboratories,which extends LED'sapplication from display,sign and signal to general lighting.However,LED has somecharacteristics quite different from conventional light sources,of which efficacyrelationship to temperature is the most important.LED's efficacy decreasesconsiderably with increasing of the temperature of p-n junction (T_j),the emitter.Currently LED transfer efficiency from electric power to light power is about 30%and 50% is expected in future,this means that more than half of the power heats theLED itself and makes T_j increase,thus decrease its efficacy.Three topics result fromthe problem:1).How to decrease T_j in lumiaire? 2).How to measure T_j in lumiaire? 3).How to evaluate the LED's efficacy in case of its dependent on T_j which increasesdefinitely when LED works? The dissertation,Research on Thermal Characteristics ofEfficacy and Methodology of the Efficacy of LED for Lighting Application,investigates the three questions.At beginning,the dissertation analyzes systematically the reasons why T_jincreases:heat produced by LED itself and its small calorific capacity.Second,a theoretical mode was set to analyze heat sink of LEDs in a luminaire.The mode was proved correct by measuring T_j of a protocol luminaire with LEDslighting on inside.The analysis also shows that thermal resistance from LED solderpoint to air,which is depended on luminaire design only,is much higher than thethermal resistance of LED itself,this means the design luminaire is important toLED thermal management.Several well-designed luminaires were tested their T_js tocheck the design then we indicates some designs are better than others.Third,to measure the T_j when LEDs work in a luminaire,two methods basing onthe relationship of LED's T_j and its forward voltage were investigated.One is tocalibrate the relationship and test forward voltage by driving LED with a very short and small pulsed current,another is by big stable current same as that of LEDnormal work.Some modification is needed for circuits of the luminaire when usesmall current,this makes the measurement quite inconvenient.Several luminaireswere tested by both methods and results are similar.However,uncertainty analysisshows that method with big current has higher accuracy,thus better than methodwith small current.Fourth,a Compound Parabolic Concentrator (CPC) is introduced to collect thelights from a LED in conjunction with a detector which in turn measures the total flux,here integrate sphere is not necessary.We analyzed the problems of methodologies ofLED's efficacy by pulsed current and in certain T_j,and suggested a new definitioncalled efficacy at standards heat sink,test of a real luminaire shows that definition ismore reasonable for LED lighting application.In the end of the dissertation,we suggest more experiments are needed for thenew definition of LED efficacy in later work,and instrument will be designed toprove the CPC measurement method of LED.Finishing of the two works meanspossibly new methodology and evaluation of LED efficacy come out.