several layers of organic materials, typically requires an accurate determination of optical constants (n,k) [1]. Several approaches have been proposed to this aim, mostly using spectrophotometric measurements, such as ellipsometry [2] and reflectance and transmittance [3]. The real challenge for both techniques is the n and k extraction from measurements, especially when n and k are potentially of the same order of magnitude, and/or strongly varying with the wavelength, as in most organic materials used in OPV [4]. Indeed, for these materials, the simplest extraction techniques cannot be used. The ‘envelope’ method [5] for example is limited to low absorbing materials. Moreover, multiwavelength approaches usually require to know a-priori an equation for the dispersion model ([6]) (Cauchy equations, Sellmeier relations or a limited number of Lorentz oscillators). In consequence, a brute-force numerical inversion method has to be use to deduce (n,k) from measurements, which is known to lead to multiple solutions [7], requiring additional information to select the true (n,k) value from all multiple solutions. The aim of this paper is to propose an original and complete methodology to extract n and k from R, T measurements without making any a-priori assumption.