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Optical materials for digital projection

Light sources in digital projection experience a transition from Xenon to laser pumped phosphor and pure RGB laser sources with constantly growing light flux. Today’s state of the art cinema projectors with laser sources are achieving a light flux of up to 75000 lumens to illuminate screens of 30 m diagonal and more. With increasing light flux the thermal load on the optical components increases, limiting the quality of the projection. The optical system of high-end cinema projectors usually consist of a large volume prism assembly and a high-end projection lens system. Especially the prism assembly experiences a high thermal load due to long light paths. The requirements on the performance of the optical glass in terms of maximum transmittance therefore constantly increases. Proper material characterization and selection helps to enable future projection requirements. Laser phosphor projection sources enable a cost effective way to generate high light flux, since they take advantage of the recent cost down of blue laser diodes and do not need to use expensive green and red lasers. The primary colors are e.g. generated by blue laser diode illuminating a phosphor wheel to create yellow light. The yellow light is subsequently split into green and red light by means of e.g. a dichroic filter. With increasing light flux standard phosphor wheels that are based on a phosphor embedded in silicone, degrade at high peak temperatures. Wheels with a ceramic phosphor have a much higher temperature stability and offer a significant improvement. This paper discusses the requirements on optical materials used for digital projection.