The mathematics associated with thickness relationship and the optical response is well documented but normally requires at least 2 measurements to be made (in 2 polarisations or 2 wavelengths) as the Refractive index needs to be determined as well, i.e. a thick layer with low density (or RI) will give the same response as a thin layer with high density. With traditional SPR there is only one polarisation which means this tends to be an ambiguity that you have to live with. We use a waveguide method with two polarisations which resolves this and there is also a coupled waveguide SPR version marketed by Peterion(?) which offers some thickness info.