Correction of contrast in projection systems by means of phase-controlled prism coatings and band-shifted twist compensators

Projectors that use LCOS lightvalves face special contrast requirements. Most configurations for reflective light valves employ tilted beam-dividing coatings that see both bright and dark polarization states. The optics must then be designed to eliminate polarization mixing at these coatings, which ordinarily arises when the S and P planes for different rays are non-parallel. We show how phase-controlled coatings can exploit the double-pass symmetry of the Plumbicon tri-prism geometry to correct this effect, reducing cross-polarized reflectivity to similar to 1E-3 when the light valve is mirror-like in black-state. Though contrast in different rays varies as a function of both ray skew component and coating angle of incidence, we show that for NA less than or equal to 0.2 the computation involved in calculating beam contrast is essentially equivalent to tracing a single ray. Light valves that use a normally-black TN mode exhibit a non-mirror-like phase dispersion in their black-state, complicating contrast control in the optics. Scatter depolarization at the edges of pixel electrodes is enhanced in these light valves, because the inherent twist causes the backplane polarization to be rotated out of alignment with pixel edges. We show that all of these contrast degradation mechanisms can be addressed by incorporating into the light valve a compensating layer having opposite birefringence to the black-state TN active layer. Moreover, when the compensating layer and driven layer are blue-shifted to a shorter LC thickness than would ordinarily be appropriate for the wavelength band of interest, a highly achromatic response is obtained at all gray levels.