One of the main question dealing with the ageing of composites or nanocomposites relates to the impact of the fillers on the stability of the polymeric matrix, in other words on how the fillers have an impact on the rate of photodegradation of the materials. Comparing the oxidation kinetics of the neat material with that of the filled one generally brings the answer. However there is an aspect that is never considered, which is the true optical length of the irradiated material. This paper addresses this fundamental aspect, in order to evaluate the influence of light scattering on the photo oxidation kinetics on EVA-calcite composite films, calcite being introduced in the form of micro- or nano-sized powder. Considering the theoretical background that drives the scattering of light, it is shown how the photon resident time varies with the size of the particles and the wavelength of the incident light.