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.