Ceramic Phosphors for LED Lighting
LED light sources are beginning to displace traditional light sources at a rapid rate. This is because of their many advantages, including high efficiency, color tuning capability, compactness, low operating temperatures, high luminance, and flexibility. The ability of LEDs to achieve high luminance (high luminous flux from small source areas) is particularly intriguing and has led to the use of LEDs in applications such as projection and automotive headlamps. These applications require high luminance sources to generate collimated or carefully shaped illumination patterns because of étendue limits that any optical system must obey. Generating either high luminance white light or RGB light from direct emission from different colored LEDs is currently not feasible. The primary reason is that current direct green emitting LEDs have much lower efficiency than red or blue LEDs. This “green gap” is especially unfortunate because these wavelengths occur near the maximum of our eye sensitivity. In the case of high luminance white light sources, one must also combine RGB sources without increasing étendue. This involves complex and expensive color mixing optics. As a consequence, nearly all high luminance, high-efficiency solid-state lighting lighting (SSL) applications use phosphor conversion of highly efficient blue InGaN (and sometimes red InGaAlP) LEDs. (In fact, most SSL applications in general use phosphor conversion for similar reasons.) However, ordinary LED phosphor conversion approaches often fail in high luminance applications because of the very high thermal loads placed on the conversion material. I will discuss some of our work at OSRAM SYLVANIA on high thermal conductivity ceramic phosphor materials that have provided outstanding conversion solutions for these applications, leading to the first high performance all LED automotive headlamps and state-of-the-art green phosphor converted LED projection light sources. In addition to the their excellent thermal and material properties, I will also discuss some of the interesting optical design considerations of ceramic phosphor converted light sources, such as the influence of optical scattering in ceramics, light extraction, and color uniformity.
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