Due to interference effects which occur during the growth of surface thin films, the resulting spectral emissivity varies continuously. It is shown in this paper, that these temporal variations of emissivity allow the determination of the complex index of refraction (i.e. optical constants n, k) of a thin absorbing film, without explicit knowledge of the optical constants of the substrate layer. In particular, this work presents results obtained by the analysis of the thermal radiation of a thin thermally grown iron-oxide film on a heated steel specimen (at approx. 400°C) measured by a standard CCD camera with a near infrared band-pass filter (~1μm).