  We consider the effect of photoinduced optical anisotropy (POA) in
azopolymers. Using a unified approach to the kinetics of photo-reorientation we
discuss the assumptions underlying the known theoretical models of POA and
formulate a tractable phenomenological model in terms of angular redistribution
probabilities and order parameter correlation functions. The model takes into
account biaxiality effects and long term stability of POA in azopolymers. It
predicts that under certain conditions two different mechanisms,
photoorientation and photoselection, will dominate POA depending on the
wavelength of pumping light. By using available experimental data, we employ
the model to compute dependencies of principal absorption coefficients on the
illumination time. Our calculations clearly indicate the different regimes of
POA and the numerical results are found to be in good agreement with the
experimental data.
