POLARIZATION DISCRIMINATION TECHNIQUES FOR OPTICAL PROCESSING

Abstract

The object of this study was to determine the utility of polarization -discrimination techniques for active optical processing. A baseline of static performance must be established before these techniques can be applied to real -time processing. The theoretical foundation for an alternative to the Vander Lugt technique of re- cording complex spatial filters had been laid by Marathay in 1969. On the basis of his theory, a photosensitive Vectograph TM technique was investigated, in which the image is produced as a polarization pattern rather than as the silver grain image of conventional photography. (1) Photographic characteristics of sensitized Vectographs were determined. The resolution ( >700 1p /mm) and the gray scale achieved indicate that the Vectograph material can also be used for recording Vander Lugt filters. (2) Theoretical analysis showed that the technique can be used to image and process objects in the same system without removal of the filter. It can also be used to generate halfwave plates for the pupil functions described by Toraldo di Francia for superresolution. (3) The Vectograph can perform addition and subtraction of functions recorded on it; it also is suitable for recording real -bipolar filter functions. (4) Variable- contrast images can be recorded. A variable-contrast Vectograph tar- get (VCVT), developed for optical testing, can not only vary the contrast of the recorded image but also reverse its contrast. In a similar fashion, a spatially variable birefringent filter (SVBF) was developed that permits spatial control of the wave- length of the transmitted light. Static and dynamic electro-optical properties of liquid crystal mixtures of cholesteryl-chloride, cholesteryl-nonanoate, and cholesteryl-oleyl-carbonate were studied to determine their feasibility as the modulator in a proposed photoconductor-liquid crystal sandwich, which would be used as a reversible recording medium. (1) Previous research had indicated that cholesteric liquid crystals are circularly dichroic in a narrow wavelength band. The present work showed that light transmitted within this band is actually elliptically polarized. The degree of ellipticity depends on the relationship between the probing wavelength and the wavelength at which the sample becomes circularly dichroic. Outside this narrow wavelength band, the crystals exhibited pure optical activity. (2) The dynamic electro-optical properties of the cholesteric trimixture were measured. An alternating electric field applied parallel to the helical axis of the liquid crystals resulted in a hysteresis in the electro-optical rotatory power of the crystals. The magnitude of the hysteresis would limit cycling of these liquid crystals to a maximum frequency of ^0.10 Hz. (3) A bias voltage applied to the crystals in an attempt to improve the frequency response resulted in a field- induced memory. When the bias voltage was maintained, the optical rotatory power failed to stabilize. Prolonged exposure to the bias voltage severely diminished the electro-optical rotatory power of the crystals.