Abstract: An optical surface measurement device is disclosed. A beam of light is focused through an optical system into a spot on a sample and the optical system collects light reflected and scattered by the sample forming an image of the illuminated part of the sample on a focus detector. A control system responds to the focus detector moving the optical system to keep the spot in focus on the sample as the sample is tracked past the apparatus. The optical system may be moved as a whole or one of the lenses therein may be moved with respect to the others. Alternatively the movable lens may be set to oscillate and its position is recorded each time focus is achieved. The focus detector has two outputs one of which is larger than the other when the surface of the sample is above the plane focussed by the optical system and the other being larger when it is below. The control system uses both sum and difference signals calculated from those outputs to discriminate the point of focus.