Dielectric multilayer angular filters for coupling LEDs to thin light guides

Abstract

A new approach is described to couple light from high-power blue LEDs (1x1mm2) into a thin large area light guide using a dielectric interference multilayer as an angular filter. The goal is to achieve large area luminaires such as backlight systems. The method overcomes the drawback of structuring holes or recessions in light guides when using a system with side-emitting LEDs. Several new LED-masking filters have been designed to improve the polarization dependency and coupling efficiency from a previous design that used a stack of inorganic bi-layers. Firstly, a polymeric multilayer manages to increase the coupling efficiency from 52% to 69%. Secondly, a birefringent multilayer filter enables to fully suppress the Brewster’s angle effect and to realize identical behaviour for both s- and p- polarized light at large angles. Thirdly, by adding layers of a third inorganic material to the original bi-layer stack with refractive index in between the other two, the transmittance equality is improved and the coupling efficiency can reach up to 63%. A novel angular filter is proposed for a large area planar collimating luminaire. A planar collimator of 2x18o cone angle with scalable and simplified structure is demonstrated. It has a thickness of only 5mm. Compact collimators and planar collimators have been analyzed with the ray tracing software LightTools. Measurements on this collimator filter and optical designs match well with the predicted performance. A new approach to thin film filter design, the so called saddle-point method of global optimization was explored and showed promising initial results. A 40-layer collimator filter with a 30% lower merit function was obtained and is discussed.