1. Apparatus for determining the thickness of a transparent workpiece comprising a source of radiation, means for directing a beam of radiation from said source to impinge on said workpiece to produce a first reflected beam from a front surface of the workpiece and a second reflected beam from the back surface of the workpiece at a spacing therebetween, radiation responsive means in the path of said two beams and providing respective pulse wave outputs representative of the distribution of radiation falling on said radiation responsive means, and means for determining the center of gravity of an area under each said pulse, the relationship between said centers of gravity providing a measure of the thickness of the workpiece.