The whole system was constructed by the author; this included the computer automation of the equipment and the writing of the associated software. Light was provided by a 15mW He-Ne laser (=633nm) at an incident angle of 450 to the sample plane. The polarisation of the light beam before reflection was controlled by a Glan Taylor polariser to be either p or s-polarised, which was then focused onto the sample by a lens of focal length 30cm. The sampling area was determined by the size of the laser spot; this was approximately 100m in diameter. The reflected light beam was passed through an analysing Glan Taylor polariser onto a photo-diode, where the reflected intensity was measured.