A new technique, referred to as the “n&k Method”, is used to characterize the thin films comprising Silicon-on-Insulator (SOI). With the “n&k Method”, a non-destructive robust measurement of the thickness of both the crystalline silicon top-layer and the buried oxide under-layer, the spectra of refractive index (n), and extinction coefficient (k), and the smoothness of the interfaces is established. The “n&k Method” determines these quantities simultaneously and without multiple solutions for thickness. The non-destructive measurement of interface roughness between the buried oxide under-layer and the silicon substrate is associated with the presence of silicon islands. The native oxide that forms on SOI is also detected and measured. No initial user's input for thickness and optical constants are required in order to obtain these results. The spectra of optical constants are measured accurately and reliably.