Abstract: Contact lenses can be made by machining an inner concave surface into a plastics material blank, then mounting the blank in a lathe and machining the outer convex surface thereon. Repeated measurement with a micrometer is needed during the latter machining to ensure that the lens vertex is correctly formed. To avoid this, the sensor described herein comprises a remote laser source from which radiaiton passes the lens vertex in the blank to a detector, the items being arranged so that, when the lens has been correctly machined, a surface diffraction effect occurs at the vertex leading to a detectable signal from the detector. The basis of the technique is the projection of an image of an edge diffraction intensity distribution through a slit to the detector while the non-diffracted component of the laser beam is prevented from reaching the detector.