The model of the rotational diffusion of the azo-dye molecules under the action of polarized uv light was used to explain the formation of the photoinduced order in azo-dye layers. We consider both the approximations of negligible and strong molecular interaction during the process of the reorientation under the field of a polarized light. We constructed an experimental setup, based on a photoelastic modulator, that allows accurate in situ measurements of the phase retardation delta of thin film as a function of the exposure time t(exp) and exposure power W (W/ cm(2) ). A good agreement with experiment was observed. Fitting the experimental curves delta ( t(exp) ) for different power values W, we can estimate the coefficient of rotational diffusion D, azo-dye order parameter S ( t(exp) ), and other parameters of the rotational diffusion model.