Multilayers of silicon-based amorphous thin films were characterized to test the performance of Fourier transform phase-modulated ellipsometry (FTPME) in absolute measurements (thickness and refractive index). The multilayers consist of a stack of silicon dioxide (SiO2), hydrogenated amorphous silicon (a-Si:H) and NiCr thin films on c-Si substrates. Results provided by FTPME are compared with those from UV-visible phase-modulated ellipsometry and transmission electron microscopy. The thicknesses of the layers determined by the three techniques agrees within 3%. Similar volume void fractions of approximately 8% for the silicon dioxide layers are found by UV-visible and IR ellipsometry.