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Proceedings Paper

Correction of contrast in projection systems by means of phase-controlled prism coatings and band-shifted twist compensators
Author(s): Alan E. Rosenbluth; Minhua Lu; Kei Hsiung Yang; Kenneth Ho; Rama Nand Singh; Teruhiro Nakasogi
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Paper 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 tri-prism geometry to correct this effect, reducing cross-polarized reflectivity to approximately 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 <EQ 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.

Paper Details

Date Published: 25 April 2000
PDF: 28 pages
Proc. SPIE 3954, Projection Displays 2000: Sixth in a Series, (25 April 2000); doi: 10.1117/12.383385
Show Author Affiliations
Alan E. Rosenbluth, IBM Thomas J. Watson Research Ctr. (United States)
Minhua Lu, IBM Thomas J. Watson Research Ctr. (United States)
Kei Hsiung Yang, IBM Thomas J. Watson Research Ctr. (United States)
Kenneth Ho, IBM Thomas J. Watson Research Ctr. (United States)
Rama Nand Singh, IBM Thomas J. Watson Research Ctr. (United States)
Teruhiro Nakasogi, IBM Japan Ltd. (Japan)

Published in SPIE Proceedings Vol. 3954:
Projection Displays 2000: Sixth in a Series
Ming Hsien Wu, Editor(s)

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