We examine the photo-orientation of molecules in a linearly polarized field and the ensuing optical anisotropy of a sample. We propose a theoretical model that considers both photoinduced reorientation and rotational diffusion, for the case of linear or axial molecules not interacting among them, as in dilute solutions in viscous media. We perform numerical simulations to highlight the dependence on the parameters of the molecular reorientation processes, on the intensity of the exciting light, and on the use of cross polarized pulses. As a realistic example we simulate the photo-orientation of azobenzene in ethylene glycol.