We present a two-step analytical inversion of s-polarized transmittance data of weakly absorbing films on transparent substrates using an important relation between the s-transmittances tau (film alone), tau(1) (substrate alone) and T (film on substrate). This relation allows to calculate tau from the measured quantities tau(1) and T. Instead of a direct determination of the optical constants n, k from the measured s-transmittance, we firstly calculate the real and imaginary parts eta and gamma of the normal component of the wave vector q in the film. Analytical formulae are derived for the calculation of these quantities from the envelopes of the transmittance t of the film at oblique incidence. The optical constants are then easily determined analytically again from. and.. We tested the formulae to investigate the accuracy of the deduced values of the optical constants for different cases of refractive index ratio of film to substrate and show that they are well-suited for implementation in real-time inline monitoring systems for film deposition. (C) 2016 Elsevier B.V. All rights reserved.