Abstract: An apparatus for determining the position of a surface of a specimen relative to a reference point. The apparatus has a beam source which produces a narrow beam and a beam splitter splits the narrow beam into a primary beam and the secondary beam. The primary and secondary beams are directed by the beam splitter or by a reflector onto the surface of the specimen. The impact points of the primary and secondary beams on the surface of the specimen form images which are monitored by a photo-sensor array. A processor uses the images of the impact points to calculate the distance between the surface of the specimen and the reference point. The use of two such apparatus allows for the thickness of the specimen to be measured.