We report here a method for determining the thickness of semiconductor and dielectric thin films based on the optical transmission turning points. An analytical treatment of the transmittance taking into account the shrinkage of the interference fringes caused by thickness inhomogeneities and the bandwidth of the incoming radiation is used. The computation algorithm is original, simple and straightforward. The accuracy of the method is studied, and the effects of the experimental noise, the uncertainty in the value of the substrate refractive index and of the number of available fringes on the spectra are considered. The precision of the thickness is estimated to be around 0.4% (or less) for films whose optical thicknesses (product of the refractive index and the thickness) are in the range 0.5-10 mu m The method is applied to obtain large-area thickness profiles by computing the thickness from spectra taken over different positions of the film surface. In this case, st special mechanical mounting is needed for the acquisition of the transmission data. This procedure presents the advantage of local analysis of the films, allowing the non-flatness of the substrate to be ignored. This last is the major inconvenience in the use of a surface profile measurement system. Results are given for films up to 7 cm large (there is no difficulty in increasing this value even further) and with different thickness profiles.