Abstract: SPR sensor, in particular for detection of a layer of material, essentially consisting of a source for coherent monochromatic electromagnetic waves, a medium with a tunable index of refraction, and an imaging detection system, wherein the medium is designed as an optical resonator that has a first and a second opposing end face, upon each of which is applied at least one coating that is suitable for producing surface plasmon resonances, and wherein the imaging detection system has a polarization device and is designed such that at least one ellipsometric quantity produced by the SPR sensor in the reflected part of the incident coherent monochromatic electromagnetic wave can be detected.