We introduce a new technique, extended spectral analysis of multiple beam interferometry (ESA-MBI), which significantly expands the variety of surfaces that can be investigated in the surface forces apparatus (SFA). The underlying principle of the technique is to capture the intensity versus wavelength profile of light transmitted through the surfaces; the distance between them is then determined by matching the measured spectra with that predicted from the theory of wave propagation through a stratified medium. ESA-MBI requires two key ingredients : (1) a camera system capable of measuring the intensity versus wavelength spectra and (2) the implementation of theory to predict it. We describe these two ingredients and show how ESA-MBI can be used in conjunction with the SFA to investigate the interactions between two opposed surfaces where one, or both of the surfaces is metallic. Finally, we describe a procedure wherein ESA-MBI is employed to measure the refractive indices of thin metal films, and we report our results for thermally evaporated silver and gold. This procedure is necessary to maximize the accuracy of surface separation measurements.