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21 September 2011 Freeform lens for an efficient wall washer
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LEDs are a promising alternative to existing illuminants for a wide range of lighting applications. Besides efficiency and high durability, the small light source dimensions compared to conventional light sources open up new possibilities in optical design. In many lighting setups, it is desired to realize a prescribed intensity distribution, for example homogeneous irradiance on a given area on a wall or floor. This can be realized using LEDs in combination with specially designed freeform lenses and/or mirrors. For high efficiency, it is necessary to collect at least 70 - 80 degrees half-angle (measured against the z axis) of the light that the LED emits into a 90 degree half-angle. This results in a lens that resembles a hemisphere. The numerical design problem thus requires a mathematical description that can handle such strongly curved surfaces with strongly varying surface slopes. Surface parametrizations with a rectangular topography, like e.g. Cartesian tensor product B-splines, have severe drawbacks when handling such surfaces. We report on the use of an alternative surface approximation scheme that uses a triangular mesh. We describe an algorithm that optimizes the two surfaces of a lens for a wall washer that generates homogeneous irradiance on a wall area of 2.8 × 2.8 m2 while mounted to the ceiling. The homogeneity is better than 80% and the optical efficiency including Fresnel losses is about 85%.
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Adrien Bruneton, Axel Bäuerle, Peter Loosen, and Rolf Wester "Freeform lens for an efficient wall washer", Proc. SPIE 8167, Optical Design and Engineering IV, 816707 (21 September 2011);

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