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

Abstract

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 tn-prism geometry to correct this effect, reducing cross-polarized reflectivity 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 NA0.2 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.

Two liquid crystal on silicon panel projector with efficient light-emitting diode illumination engine

“…2͒ will resolve the problem of skew ray depolarization. 8 After the PBS2 the analyzer will again filter out the unwanted ͑s͒ polarization. The combination of the prepolarizer, the analyzer, and the quarter-wave plate will increase the contrast dramatically.…”

Compact LED projector with tapered light pipes for moderate light output applications

“…We have also provided an additional loss of 10% in the simulation to introduce the effect (loss) of the light valve. Between the PBS and the microdisplay, we have added a quarter-wave plate to clean up the depolarization of the skew rays [4] on the splitting layer of the PBS.…”

9.2: Skew Ray Compensated Color Separation Prism for Projection Display Applications

“…A more detailed discussion of the issues outlined above can be found in several seminal papers by Rosenbluth, et. al. [5][6][7].…”