IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 9, MAY 1, 2008 739 Tailoring of the Color Conversion Elements in Phosphor-Converted High-Power LEDs by Optical Simulations Christian Sommer, Franz-Peter Wenzl, Paul Hartmann, Peter Pachler, Marko Schweighart, Stefan Tasch, and Günther Leising Abstract—Ray-tracing simulations are used to identify the de- mands for angular homogeneity of the white light emitted from phosphor-converted white light-emitting diodes having color con- version elements (CCEs) of constant thickness. The simulations re- veal that a constant thickness of the CCE by itself is not sufficient for a homogeneous white light emission. Rather the height and the broadness of the CCE as well as the phosphor concentration have to be precisely adjusted in order to assure a homogeneous white light emission. Index Terms—Light-emitting diodes (LEDs), lighting, simula- tion. I. INTRODUCTION A S a result of the rapid progress within the last few years, solid state lighting is on the way to supersede today’s in- candescent and fluorescent lamps for lighting and automotive applications [1]–[4]. Besides efficiency, the major challenge for further improvements is the quality of the white light in terms of the angular homogeneity, especially for phosphor-converted white light-emitting diodes (LEDs). Recent progress in coating technology has shown that either a conformal coating (a layer that replicates the LED chip surface) or a color conversion element (CCE) with constant thickness improves the angular homogeneity of the white light in compar- ison with other CCE geometries [3]. However, in this study, we use ray-tracing simulations to highlight that a CCE of constant thickness by itself does not suffice angular homogeneity. While the emission of the conversion particles is isotropic [3], that of the bare LEDs is more directional. As a consequence, even in case of a CCE with constant thickness (in this study represented by a square shaped geometry of equal thickness) only specific parameter triples for the height, the broadness and the phosphor concentration are able to balance this and guarantee angular ho- mogeneity. Manuscript received January 7, 2008. C. Sommer and F.-P. Wenzl are with the Institute of Nanostructured Materials and Photonics, Joanneum Research Forschungsges.mbH, A-8160 Weiz, Austria (e-mail: Christian.Sommer@joanneum.at; Franz-Peter.Wenzl@joanneum.at). P. Hartmann, P. Pachler, M. Schweighart, and S. Tasch are with Tri- donicAtco Optoelectronics GmbH, A-8380 Jennersdorf, Austria (e-mail: Paul.Hartmann@tridonicatco.com; Peter.Pachler@tridonicatco.com; Marko.Schweighart@tridonicacto.com; Stefan.Tasch@tridonicatco.com). G. Leising is with the Institute of Solid State Physics, Graz University of Technology, A-8010 Graz, Austria (e-mail: G.Leising@tugraz.at). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2008.921063 Fig. 1. Sketch of the simulation model: The LED model consists of a light- emitting area on top of a substrate, which is placed on a submount. The CCE has a square-shaped geometry of equal height and consists of yellow phosphor particles embedded in a silicone matrix. A hemispherical detector of 1-cm radius is centrically placed above the LED chip in order to cover the upper hemisphere completely. For the simulations, the height ( ), the broadness ( ), and the phos- phor concentration in the silicone matrix ( ) were varied. All the simulations have been performed with the commer- cial software package ASAP, which benefits from its flexibility in the generation of geometrical models of the device and the as- signment of optical properties (refractive index; extinction, re- flection, and transmission coefficient; scattering properties; etc.) to the individual components that build up the device. The first step of the simulation procedure is the setup of an appropriate simulation model for the blue emitting LED, which in the case of the present study relies on an LED die in thin-GaN configuration. The model considers the overall geometry of the LED chip (see Fig. 1), the geometries of the individual compo- nents the chip is composed of, as well as their respective optical properties. The light emitting area on top of the substrate, which is con- sidered to be placed on a submount that is mounted on an alu- minium substrate by chip-on-board technology, has dimensions of 940 940 m and gold pads for wire bonding in the two cor- ners of one side. The local radiant emittance was experimentally derived from an LED under operation and the respective Bitmap file is used to define the ray-tracing source for the LED model. The angular distribution is assumed to be of Lambertian type, which has been verified by comparing the simulated radiation patterns with experimentally determined data recorded with a Goniospectroradiometer. An important part of the simulation process is the implemen- tation of a multitude of individual CCEs into the LED model 1041-1135/$25.00 © 2008 IEEE