Compensation of birefringence in lead-free polarizing beam splitters for LCOS projectors

Most optical designs for delivering light to LCOS imagers and then from the imagers to the projection lens use polarizing-beam-splitter (PBS) technology. Most of the PBSs used in commercial LCOS projectors contain glass with a significant amount of lead (Pb). Such glasses have inherently low stress birefringence, and therefore maintain the polarization state of light passing through them. However, Pb-bearing glass is an expensive, difficult to process, and hazardous material with special disposal requirements and is therefore not desirable in consumer-electronic products. On the other hand, Pb-free wire-grid plate PBSs require a longer back focal length than would be optimal. Data and modeling results show that uniform high-contrast dark states may be obtained from lead-free-glass Cartesian PBS prisms when a quarter-wave compensator is used between the imager and the PBS.