Abstract: A method of assessment of semiconductor wafers by infra-red scanning microscopy illuminates a wafer with infra-red light through its polished face and detects light emanating from the same face by back-scattering from particles within the specimen, the light being back-scattered through an angle of more than 90.degree.. Measures are taken to reduce the effects of specular reflection from the surface, which measures may include the use of stop/mirror arrangements and/or the use of an aperture plate for confocal discrimination.