P-162: Off-Axis Compensation of 90TN0 LCoS Mode

Abstract

In projection applications, as the liquid crystal device size continues to shrink, more demand is placed on the illumination optics to increase lumens per unit area on the device. One method to achieve this is to use smaller f-numbers (larger cone angles); however, as the f-number of the optical system reduces the contrast invariably suffers. In this paper, we examine a twisted nematic liquid on silicon device (90TN0, LCoS) and explore the trade-off between contrast and system f-number. This mode is nominally self-compensating, however additional compensation of the off-axis birefringence is required to achieve higher contrast, especially at lower f-numbers (<2.4). Device birefringence characterization was measured as a function of the applied voltage, wavelength and angle of incidence. Both uncompensated and compensated devices were examined. Contrast as a function of f-number was measured in a conoscope system based upon a wire grid polarization beam splitter (PBS). Substantial improvement in contrast was measured when the c-compensator was added, suggesting that contrast in excess of 1000:1 can be achieved for all primaries at F/1.8. Final measurements in a rear-projection system utilizing a wire-grid PBS, operating at F/2.4, provided an increase in the contrast from 1000 to 1350:1 using the additional compensator.