Abstract: The position of the boundary of an object is measured by forming an optical image of the part of the object of interest as the object part is moved past the end of a thin optical fibre. A photodetector receives the light from the fibre and applies an output to a comparator where it is compared with a reference derived from the light source illuminating the object. The output of the comparator triggers the recording of coordinate value signals from transducers responsive to the relative positions of the object and the end of the optical fibre. The signal paths connected to the recorder are arranged to have the same delays so that no error arises from different speeds of movement. A computer can control the movement of the object and record the positions of different parts of the boundary so that the size of the object can be calculated. The reference can be derived from an area concentric with but larger than the end of the optical fibre.