Plasmon energy is produced by exciting a plasmon resonance at least one excitation position on a first surface of a first material, and the plasmon energy is detected at at least one measurement position on the first surface after the plasmon energy has propagated from the at least one excitation position to the at least one measurement position. An attenuation of plasmon energy is determined along a plurality of paths between the at least one excitation position and the at least one measurement position, and relative distances between the first surface and a second surface of a second material are determined at a plurality of points on at least one of the surfaces based on the determined attenuation of plasmon energy along the plurality of paths.