A method of optimising the sensitivity of surface plasmon ellipsometry (SPE) apparatus used to analyse a surface comprising a conducting film is disclosed. The method includes calculating a sensitivity map of plasmon ellipsometry for the film. The sensitivity map comprises data defining variations in sensitivity of the plasmon ellipsometry apparatus with angle of incidence and polarization angle of polarized light incident on the conducting film for analysis by the apparatus. The method further comprises using the sensitivity map to configure the plasmon ellipsometry apparatus with a combination of the angle of incidence and polarization angle located in a region of substantially maximum sensitivity in the sensitivity map.