  In a planar liquid crystal sample sandwiched between a photosensitive and a
reference plate instabilities occurred, when the cell was illuminated from the
reference side. The instabilities were induced both by polarized white light
source and monochromatic laser beams. Static and dynamic regimes were found;
for laser irradiation dynamic instability was found only in a range of
polarization directions. A model, developed for monochromatic excitation,
predicts that at certain thicknesses dynamic instability is forbidden.
Experiments on a wedge-like cell confirmed this conclusion.
