Ambiguities in measuring the physical parameters for twisted-nematic liquid crystal spatial light modulators

There is great interest in using commercially available twisted-nematic liq uid crystal displays as programmable spatial light modulators. Proper use o f such displays, particularly as phase-only filters for optical correlators or as diffractive optical elements, requires knowledge of the physical par ameters of the device including the twist angle, the birefringence, and the orientation of the director axes. A simple experimental technique was publ ished for evaluation of these parameters. When we used this technique, howe ver, we found a number of different sets of values that satisfied the exper imental data. We demonstrate that additional measurements at different wave lengths can resolve these ambiguities and provide valuable information abou t the wavelength dependence of the birefringence. (C) 1999 Society of Photo -Optical Instrumentation Engineers. [S0091-3286(99)00804-1].