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 ach
ieves an arbitrary wavefront shape, that is limited only by the availa
ble modulation depth and resolution. The possibility of using a twiste
d-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 ob
tained both with monochromatic and polychromatic light illustrate the
different sampling, quantization and chromatism effects discussed theo
retically. Useful models for the computation of the point spread funct
ion 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.