Kenneth C. Ho

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 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.

Design of phase-controlled coatings to correct skew-ray depolarization in LCOS projectors*

Abstract LCOS projectors usually 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 whenever the S/P planes of different rays are not aligned with one another. (This non-parallelism is a consequence of the compound incidence angles with which a beam of finite NA intercepts a tilted coating.) We show how to correct this effect using phase-controlled coatings that exploit the double-pass symmetry of the Plumbicon tri-prism geometry, reducing cross-polarized leakage to ∼1×10 −3 . In lowest order, any polarization ellipticity that the coatings may introduce in the light valve illumination will cancel in the return pass through the optics (while induced rotation will double). The condition that must be satisfied to correct rotation will only constrain the coatings in a single collective degree of freedom (for each color channel); this condition depends on both the orientations and phase shifts of the coatings. Even beyond this first order result, we show that for NA≲ 0.2 the computation involved in calculating beam contrast can be reduced to the equivalent of tracing a single ray (in double-pass), despite the fact that contrast along different rays in the beam will vary as a function of both the skew component and incidence angles of the rays at coatings. The efficiency of this algorithm makes it practical to refine the design of all coatings in the system simultaneously to take higher order terms into account.

14.4L: Late‐News Paper: Colorimetric Characterization of Wide‐Viewing Angle Technologies

The viewing-angle dependence of TFTLCD color characteristics have been studied for a number of wide-viewing angle technologies. Different color characteristics were obtained for each LC mode. For particular viewing directions relative to the LC director the color shifts can be understood as changes in the relative retardation of different ray components.

Microscope Metrology of Reflective Light Valves

A polarizing microscope is used to measure the electro-optic properties of nematic light valves on a spatial, spectral, and time-resolved basis. Under the quiescent uniform twist approximation(8), the response of the liquid crystal (LC) layer can be broadly characterized by a universal polarization conversion efficiency (PCE) function (essentially the reflectivity between crossed polarizers), whose primary independent variable is the unified parameter Δn d/λ. Spectral resolution capability in the polarizing microscope thus allows measurement of light valve PCE as a function of λ; the PCE response in turn largely determines projector dark state neutrality and color balance across gray scale. Emphasizing the case of a normally black 45° TN reflective (RTN) light valve, we use colorimetric analysis of PCE parameter-space to select an optimal cell gap, based on a criterion of near-achromatic black state. Once black state is achromatized, the non-constant slope of the PCE curve implies a characteristic residual variation in CIE chromaticity as gray level is changed (for non-monochromatic illumination). Spectral resolution allows the microscope to measure spatial variations in Δn d (and hence in PCE response). Variations in Δn d may arise from nonuniformity in either cell gap or pretilt. These may be distinguished by using the microscope to make a spatially resolved measurement of response time.