We present the results of an experimental study of photoanisotropy in polymethylmethacrylate layers containing phenanthrenequinone. We consider the characteristic features of inducing birefringence for different contents of the photosensitive molecules and temperatures of the layer. We propose a mechanism for appearance of photoinduced birefringence; we explain the concentration and temperature dependences of its magnitude based on the dimerization of phenanthrenequinone, established from the IR spectra. We have observed stability of the photoinduced birefringence relative to subsequent exposure to optical radiation, which can be used to improve contrast when converting polarization phase images to amplitude images.