The refractive index and the extinction coefficient of thin films deposited on absorbing substrates, as well as their thicknesses, are obtained from measurements of direct transmittance. The inversion approach is based on application of a spectral projected gradient method (SPGM). First, simulated transmittance spectra are used to compare the performance of the method with published results considering films with different degrees of absorption and thickness, and then inversions from real measurements taken over the solar spectral range are carried out. It is shown that the use of the SPGM, to minimize the difference between measured and computed transmittance spectra, becomes a powerful tool to solve efficiently the problem of obtaining the optical constants of thin films from spectroscopic measurements.