Iterative method for the optical characterization of biaxially oriented poly(ethyleneterephthalate) films

We present a method to determine the optical parameters (refractive index, optical thickness, and coherence factor) and the geometrical thickness of a weakly absorbing film of poly(ethyleneterephtalate) PET. It is based on the analysis of the interference modulated transmission spectra at normal incidence (in the visible) and of absorbance bands (in the IR). We use successive approximations and interpolations to calculate these properties. To check the results, thickness and refractive index have been determined with an electronic micrometer and an Abbe refractometer, respectively.The method can be applied to other low-roughness films with interference fringes in zones of high transparency. It is non-destructive and allows for a rapid in situ control.