Duncan J. Anderson

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

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.

Uniform color illumination for scrolling color LCoS projection

Projection based on the scrolling color single panel reflective liquid crystal on silicon (LCoS) architecture developed within Philips is set to become a competitive technology for high definition rear projection television. The colorimetry of the scrolling color illumination light engine is examined in this paper including the design issues considered in specifying dichroic color filters for uniform color illumination. The scrolling action can be achieved with rotating glass prisms (one for each primary color), which combined with the requirement for compact illumination optics can lead to beamsteering at some of the dichroic filters in the light path. This beamsteering can cause unacceptable color changes of the illuminating stripes as they are scrolled from the top to the bottom of the LCoS panel unless special attention is made to the choice of filter cut-off wavelengths and their sensitivity to angle of incidence variations. One solution involves the design of new dichroic filters that are relatively insensitive to these beamsteering effects. Filters have been designed and fabricated with edge sensitivities < 0.9 nm/deg compared with typical sensitivities of ~ 1.4nm/deg from standard filters; the filter specifications and their system performance will be described. Further system solutions are given that utilize conventional angle-sensitive dichroic filters. The effect of color balancing upon the optical efficiency of the system will also be described.