A method of monitoring the surface of a sample under test is provided and the method comprises the steps of: illuminating the surface with light (20) polarised in a first direction; viewing light reflected from the surface through a polarising filter (27) arranged at 90° to the first direction, wherein the surface of the sample under test is provided with a marked area where diffuse reflection of the incident polarised light will occur in order to improve the contrast between the marked area and the surface of the sample under text.