In commercial high-brightness phosphor-coated white LED packages the phosphor is put down on the die at the center of the hemispherical encapsulation, representing a quasi point source that provides convenient optical control in luminaire design. However, specific applications may benefit from other package geometries and beam shapes regarding efficiency, color uniformity and thermal management. In order to examine optical arrangements, a solid model of an InGaN LED die and the optical system including the phosphor were simulated using a Monte Carlo forward ray tracing technique. Photoluminescence was implemented as two separate processes: short-wavelength LED emission and phosphor absorption was traced first, followed by reemission of the down converted radiation by the phosphor layer; optical properties of existing phosphors were used. Output parameters of the two ray traces were combined and evaluated for the geometries examined.
KEYWORDS: Light emitting diodes, Monte Carlo methods, Ray tracing, Geometrical optics, Indium gallium nitride, LED lighting, Beam shaping, Solids, Optical properties, Refractive index
In commercial high brightness phosphor coated (PC) LED packages the phosphor is put down on the die in the center of the hemispherical encapsulation, representing a quasi-point source that provides convenient optical control in lighting fixture design. However, specific applications may benefit from other package geometries and beam shapes regarding efficiency, color uniformity and thermal management. In order to examine optical arrangements the solid model of an InGaN LED die and the optical system including the phosphor were simulated using Monte-Carlo forward ray tracing technique.
Photoluminescence was implemented as two separate processes: short wavelength LED emission and phosphor absorbtion was traced first, followed by re-emission of the down-converted radiation by the phosphor layer, optical properties of existing phosphors were used. Output parameters of the two ray traces were combined and evaluated for the geometries examined.
The advantage of enhancing the light extraction of LED dies is twofold: it increases the overall efficiency of the device, reducing internal absorption and subsequently the temperature of the device. We have developed the solid model of a commercially available blue LED in a ray tracing simulation software according to the physical structure of the GaN die grown on SiC substrate. Optical parameters of the various device layers were adjusted within a plausible range to achieve the best match between the measured and the simulated spatial luminous intensity profile. Two approaches for improving the light extraction efficiency were examined using the model: change of refractive index of the encapsulating medium and chip shaping. Simulation results were verified by a series of luminous intensity and total flux measurements in a mini-goniophotometer. We found that the physical experiment confirmed the predicted results.
Besides display application, organic light emitting diodes (OLED) have great potential for the application of thin film light sources. The new device is designed to have a self-healing mechanism against electrical shorts. The entire device can be prepared in a vacuum chamber equipped with plasma treater, monomer evaporators, radiation curing units and inorganic deposition stations. A blend of small electron-donor organic molecules and radiation polymerizable monomers is flash evaporated to provide a molecular-level vapor-phase mixture, which is then condensed and cured on a flexible ITO coated substrate. The procedure is repeated with an electron-acceptor organic substance, which is deposited over the electron-donor layer. A metallic cathode is then deposited over the electron-acceptor layer and the composite OLED product is packaged. The flash evaporation vacuum deposition technique with in-line radiation cross-linking allows the mixing of small OLED molecules with monomers or oligomers at any ratio. Using this technique, a heterogeneous blend can be flash evaporated and molecularly mixed in the vapor phase, re-condensed as a homogeneous film, and then quickly cross-linked before any phase separation occurs. This creates a unique polymer chemistry that is not possible by conventional coating techniques. The electrical characteristics and the thickness of the metallic cathode and the composition of the polymer layers are selected to produce a self-healing mechanism via gasification of elemental carbon generated by dielectric breakdowns and the oxidation of any exposed cathodic surface, thereby providing a self-healing mechanism to prevent propagation of the damage caused by electrical shorts.
Conference Committee Involvement (4)
Sixth International Conference on Solid State Lighting
14 August 2006 | San Diego, California, United States
Fifth International Conference on Solid State Lighting
1 August 2005 | San Diego, California, United States
Fourth International Conference on Solid State Lighting
3 August 2004 | Denver, Colorado, United States
Third International Conference on Solid State Lighting
5 August 2003 | San Diego, California, United States
Course Instructor
SC353: OIDA Roadmap on Solid State Lighting
The business of lighting sources is in a phase of significant flux. The simultaneous goals of addressing both energy and pollution reduction is driving change for the underlying technology behind lighting sources. This presentation by leading industry experts examines some of the trends and their implications to lighting and new applications of optoelectronics.
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