Twisted-nematic liquid-crystal pixelated active lens

Liquid-crystal active lenses are classical imaging systems in which a liquid-crystal spatial light modulator is placed in a pupil plane. The modulator controls the phase of the optical wave in the pupil and achieves an arbitrary wavefront shape, that is limited only by the available modulation depth and resolution. The possibility of using a twisted-nematic liquid-crystal television in the pupil plane is investigated both experimentally and theoretically. The precise evaluation of the modulation characteristics is first discussed. Experimental results obtained both with monochromatic and polychromatic light illustrate the different sampling, quantization and chromatism effects discussed theoretically. Useful models for the computation of the point spread function and the modulation transfer function of a generic pixelated liquid-crystal active imaging system are presented for both in-focus and out-of-focus systems, and their numerical implementation is discussed. (C) 1998 Elsevier Science B.V. All rights reserved.