Phase change rewritable optical recording media contain a chalcogenide layer, e.g., GexSbyTez, that undergoes a transition between the amorphous and crystalline phases (by absorption of laser energy). Differences in their optical constants [n,k] result in reflectivity differences in the multilayer optical devices. We have obtained those constants by reflection and transmission [R,T] intensity measurements (200 less than or equal to lambda less than or equal to 2000 nm) at (near) normal incidence for films <50 nm thick. The film's [n,k] are obtained at each wavelength by direct numerical inversion of the [R,T] expression describing the measured [air/film/substrate/air] configuration. In our approach based on a graphical technique combined with two-dimensional Newton-Raphson (N-R) iteration, the existence of [n,k] solutions can always be determined, whereas N-R alone often fails. Investing the nonlinear [R,T] expressions results in multiple [n,k] (one physical and additional mathematically relevant) solutions at every wave length, which combine to provide a powerful indication of the film's thickness (with similar to 1% accuracy). We are thus able to show that when as-deposited amorphous chalcogenides are annealed, their thickness diminishes from 5% to 10% as they crystallize, where the amount depends on the alloy composition.