A theoretical and experimental study about the ability of the photothermal modulated mirage technique to characterize vertical interfaces (thermal resistances) separating two different opaque and thermally thick media with thermal conductivities larger than the medium surrounding the sample and with any value in their thermal diffusivities has been carried out. For the calculation of the mirage deflection, a theoretical model that incorporates the mirage parameters (sizes of probe and pump beams, probe beam height) and which is valid for any value of the thermal resistance characterizing the interface is presented. The model is validated by experimental measurements on a fabricated sample having a controlled interface width that separates two pieces of copper and steel. Fitting the model calculations and the experimental data allows one to retrieve the value of the thermal resistance. ©1996 American Institute of Physics.