In photo-birefringent films, the relationship rho(I) between optical retardation rho and pump irradiance I may be obtained with imaging techniques applied within a pump probe setup. However, measurement of rho(I) are reported only at long irradiation times and low time-resolution. In this paper we describe a polariscopic imaging technique with higher sensitivity (by a factor of similar to 30), where the probe beam is focused on the entrance pupil of the camera ("Maxwellian view"). The technique is applied to an azo-dye doped polymeric film, obtaining the continuous rho(I) curve after irradiation times from 10 ms to 240 ms. Exponential functions are fitted against the experimental data both as a function of time and irradiance. (C) 2010 Elsevier B.V. All rights reserved.