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LambdaResearchApplication Note for LED modelingLight emitting diodes(LEDs)have come a long way from the small 5mm bulb-like indicator LEDs of the lastcentury.They have now become powerful enough to be considered a "stand alone"light source incommercial applications.Today they are found everywhere,in many shapes and forms,and cover a widerange of output power and color.Within the next decade,they will begin to seriously compete with other lightsources,such as fluorescent lights and incandescent bulbs for general lighting applications.The need foraccurate design and analysis software is now obvious,as these small and versatile light sources are far moredifficult to model than the other more conventional light sources.TracePro is an illumination design and analysis tool that enables the user to analyze and design LED modelsas a light source.TracePro's flexibility in the area of source design can really make a difference in thesemodels,as LEDs have very different optical characteristics from one model to the next.Depending on thepreferences and needs of the user,there are many ways to model LEDs.This application note will offerexposure to different methods and considerations,which must be taken into account in order to achieverealistic LED models.LED structureAn LED is a semiconductor component that emits light.LED sources are available as dies made from layers ofsemiconductor materials called homo junctions,hetero junctions or double hetero junctions.Depending on theirnature,composition,and assembly,these junctions can emit light from within the material and gets out through eithertheir surfaces and/or their edges.Materials used are typically made out of different proportions of Ga,As,In,P,AL,Zn,S and Si depending on the output wavelength desired.In commercial applications,the die is placed on a substrate.connected to an anode and a cathode,and then covered with a transparent epoxy.The newest high intensityapplications require specially packaged LED models,which are sometimes mounted inside modular form factorassemblies with heat sinksLED design considerationsLEDs,especially the bare dies,are quite small,for effective design of optical systems that employ LEDs,one mustmodel the emission characteristics of the LEDs.The following section will illustrate two high intensity LED designs builtfrom the same manufacturer specifications in TracePro.For multicolor LED design,multiple dies (red-green-blue combinations)can be packaged within a single device.Forwhite LEDs,a single blue or UV emitting diode die is used over which a phosphorous material is deposited.Whenilluminated by the radiation from the die,this phosphor generates what is perceived by the human eye to be white light.For example,phosphors emitting more in the red spectrum will make a'wam white",closer to the light emitted by anincandescent light source.These effects can be simulated in TracePro using its integrated macro language.LED examples:Luxeon Star White side emitting modelThe first example is a detailed Luxeon Star White LED model with a side-emitting lens from Lumileds Lighting,builtentirely in TracePro(see Fig.1a).The datasheet(DS25,see ref.)lists mechanical design data,which enables users tobuild the part within TracePro using the different tools available.Since manufacturer specifications are often incomplete(lacking material data and emission measurements for confidentiality reasons)and manufacturing tolerances of theLED die mass-production process creates characteristic variations,the resulting model may perform differently fromthe datasheet.Contacting the manufacturer could provide more information to help design a more accurate model,butone can use TracePro to bring the optics model into agreement with lab measurements.Copyright 2005 Lambda Research Corporation-All Rights Reserved25 Porter Road-Littleton,MA 01460-1-978-486-0766-www.lambdares.com-sales@lambdares.comEmitter bodySide-emitting lensCupLED die0402atsink assemblyFigure 1a.Completed geometrical LED model:The emitter (CenterFigure 1b.White spectrum:The modeled LED will emit a coldstructure)includes a side-emitting lens,a cup,the emitter body andwhite light spectrum due to its blue-white light appearance seenthe pins.The outer area is the form factor assembly,which includesby the human eye.The spike at 440nm is what is emitted by thesolder pads on an FR4 board,under which is an Aluminum heat sink.die.The part between 500nm and 650nm is generated by theThe die is the central block seen through the lens.phosphor material over the die.The LED shown in Fig.1a is side emitting,meaning that most light will be sent to the side (at an angle around 80degrees off the surface normal).Modifying the lens on top will change the angular light distribution significantly (seeangular distnbution and spectral emissions in Figures 7a and 7b).By adding a phosphor over the die,one obtains anominal spectral distribution as shown in Fig.1b.This spectrum is the one used in our ray traces for these examples.This model was built according to the manufacturer's specifications.The sapphire-based die inside the LED is modeledas an emitting block with general optical properties.A higher level of detail would include the thin film layer structure ofthe die,complete with material properties and the micro-wiring between the pins and die and the exact measurementsthe optical and geometrical properties of each aspect of the LED.The die and reflector inside the emitter were fittedinto the original geometry.For this example,we will consider the die's side surface light emissions to be negligible.Theside-emitting lens was developed to go on top of the emitter geometry.Once the geometry is built,different opticalproperties are applied to complete the model.Here are some pointers to consider when modeling LEDs:An LED die generally can be considered a top surface emitter.Using the side surfaces as emitters to varyingdegrees can improve the accuracy of the angular distribution patterns out of the mode depending on hetechnology used by the manufacturer.Some LEDs emit as much as 50%of their light through the sides(since their layers can serve as light guides),with the remaining light going out their top surface.However,inour case here we only used a 90 degrees Gaussian-like emission pattern on the top surface of the die asside surface losses are negligible.-The die is usually encased in an epoxy which serves both to protect the die and as a lens/light guide.Thespecifications rarely list the lens index of refraction or the actual material the epoxy is made from,which cancause problems for proper coupling into a light guide.Typical epoxies have indices of refraction ranging from1.49 to 1.65.The shape of the epoxy is critical to an accurate model reproduction,but the manufacturer mayprovide its general shape and dimensions.Optical properties of the reflector (or cup)can be varied by changing the reflectivity and scatteringproperties,enabling users to further enhance their model's accuracy when compared to the LED's angularCopyright 2005 Lambda Research Corporation All Rights ReservedPage-2Figure 2.Lighting up the LED:This image shows the rays generated by the LED.The large majorityof the rays leave the lens through its sidesLED examples:Point Source LEDThe second example is much simpler.This LED is built by applying a Surface Source with a Surface Property to a disk-shaped object.The Surface Property is constructed to describe the angular distribution of the LED by weighting theproperty's absorption (see Fig.3).This angular distribution is obtained using the LED's datasheet for guidance.Thisexample effectively creates a point source when used in a far field model.The design ignores the geometry of the partsuch that the part is merely a placeholder to position and orient the LED.This method works for general lightingapplications by using only the average angular distribution of mass-produced LEDs.Figure 3.Surface Property with weighted absorption.To improve the model's accuracy,many angles are used,keeping the LED model true to the manufacturer's specificationsrelative to the angular distribution.Figure 4 illustrates the model's ray output.The property data is applied to one surface of the planar-disk.During the raytrace,TracePro will consider the applied surface property and modify the ray distribution according to the table ofabsorption values entered.For a more thorough and exact analysis,the previous method based upon development ofthe entire LED geometry is recommended.Combining the two methods-i.e.,providing an emission (absorption)pattern on the surfaces of the die solid-is frequently used to control the die's angular distribution along with thecomplete geometrical modelCopyright 2005 Lambda Research Corporation All Rights ReservedPage-3