1. A method for measuring the composition of a thin film deposited on a semiconductor wafer, comprising: generating a three-dimensional design of experiment (DOE) for a plurality of semiconductor wafers, a first dimension of the DOE being a relative amount of a first component of the thin film, a second dimension of the DOE being a relative amount of a second component of the thin film, a third dimension of the DOE being a thickness of the thin film; acquiring a spectrum for each of the wafers across a selected spectral range; generating a set of optical dispersion data by extracting a real component (n) of a complex index of refraction and an imaginary component (k) of the complex index of refraction across the selected spectral range for each of the acquired spectrum utilizing a regression process applied to a selected dispersion model; identifying one or more systematic features of the set of optical dispersion data; and generating a multi-component Bruggeman effective medium approximation (BEMA) model utilizing the identified one or more systematic features of the set of optical dispersion data and the generated set of optical dispersion data.