Multistability and bistable oscillation in the ferroelectric material PMN was found to be very sensitive to laser power, specimen temperature, ambient pressure and flow rate of air past the specimen (J. F. Scott and T. Chen, Integrated Ferroelectrics, 1, 71(1992)). PMN has the potential to make new photonic sensors due to its figure of merit (better, except for speed, than semiconductor materials such as ZnSe). In this paper, an explanation of experimental results is given. The initial temperature dependence of the time behaviour of the transmitted light intensity and period between the switching events versus switching event number have been obtained and are in qualitatively good agreement with the experimental results.