A method of measuring a load on a surface produced by a contact force. An elastic film or coating is located on a supporting surface to provide an indication of normal and tangential or shear forces applied to the surface. A thickness measurement, corresponding to a normal displacement, and a tangential displacement of the film, corresponding to a shear displacement, is obtained and input to a response function to reconstruct the normal and shear load components of the contact force on the surface. A differential thickness measurement of the film is detected by an optical method in which a luminescent output from a luminophore in the film is measured. A luminescent output from a reference luminophore located below the film is also measured to provide a measurement system independent of the illumination source used to produce the luminescent output.