Calculated and experimental differential reflectograms for silicon oxide/silicon nitride/silicon oxide (ONO) structures as utilized by the electronics industry for dielectric layers are shown. The pertinent equations for the simulations of multilayer structures are presented. Experiments and calculations are in good qualitative agreements. Moreover, the differential reflectance values for both experiment and simulation display a strong dependence on the film thickness. The quantitative agreement could be improved if the optical properties of the thin dielectric layers were better known.