Abstract: A method of measuring, for example, a tube, comprises the steps that the tube is retained in a measurement region which is scanned by light rays. The light rays (35, 36) are displaced in one plane at right-angular directions until eclipses caused by the tube occur and cease. At this point, the displacement of the light sources (37,23; 41, 31) of the light rays (35, 36) and the light receivers (22, 40; 32, 44) is measured, and the eclipses are registered. The plane of scanning of the light rays (35, 36) is displaced at right angles to that plane, the displacement being measured and the process being then repeated.An apparatus for carrying out the method comprises a retainer (14) for the tube and a foundation (1-5) along which a frame (10-13) is positively movable. The frame encloses the measurement region and is provided with two light sources (37, 23; 41, 31) and two light receivers (22, 40; 32, 44) for emitting two right-angularly displaceable light rays.