To measure the diameter of polythene on a cable core, as it emerges from the extrusion nozzle and while it is still hot and sticky, an optical method is useful. In the apparatus described, a beam of light is swept across the cable, whose diameter is measured by the time for which the beam is interrupted. The apparatus is not seriously affected by condensation on the lenses, and the design allows for possible horizontal and vertical movement of the cable; one of these possible movements causes loss of focus in the optical system, and the electronic circuits are designed to deal with the resulting imperfect signals. The apparatus is intended for cable of about ⅝ in diameter and works to an accuracy of about ±0.001 in.