The photoinduced birefringence in DR1-PMMA thin films has been investigated by a polariscopic imaging technique, taking into account the bell-shape of the pump beam irradiance. The technique provides a powerful tool to measure the irradiance dependence of the photoinduced effects during the time evolution of the system. We observe a time behaviour which may be partially ascribed to a continuous distribution of mechanisms, resulting in a sigmoidal contribution in excitation and in a stretched exponential contribution in relaxation. These results are hardly described by current available models. Moreover, we report a simple exponential dependence on irradiance, both in excitation and relaxation. (C) 2008 Elsevier B.V. All rights reserved.