Illumination of LCOS micro displays using planar lightguides

Liquid crystal on silicon (LCOS) microdisplay is typically less than frac12 inch in diameter and usually a field sequential illumination system is used for colour creation. In most high power microdisplay based projectors the colour sequences are produced with colour wheels and the microdisplay is a MEMS device, but in lower power projectors of near to eye displays (NED) the displays are LCD panels and LEDs are commonly used in their illumination. In the light path of a reflective LCOS panel, the Polarizing beam splitter (PBS) that separates the incoming and outgoing beams is in significant role. There are many varieties how the PBS is done but, for example, in compact viewfinders, it can be a plate with diffractive wiregrid or with multiple layers utilizing the interference of light. The PBS is placed between the LCOS panel, and the image forming optics is typically set at 45 degree angle. In this work a curved film type PBS is considered, which shortens the back focal length and makes the system more compact. The proper distribution of light is created with a lightguide where the light is coupled in using butt coupling, with each red, green and blue LEDs in row. For more efficient light mixing the light guide is lengthened and an RGBGR row of LEDpsilas is used. The microstructures on the light guide are designed so that the light distribution matches with the curved PBS and the f-number of the image forming optics. The system performance together with experimental results is discussed.