  We present a scheme for active compensation of complex extrinsic polarization
perturbations introduced into an optical system. Imaging polarimeter is used to
measure the polarization state across a beam profile and a liquid crystal
spatial light modulator controls the polarization of the input beam. A sequence
of measurements permits determination of the birefringence properties of a
perturbing specimen. The necessary correction is calculated and fed back to the
polarization modulator to compensate for the polarization perturbation. The
system capabilities are demonstrated on a range of birefringent specimens.
