A new technology is described for making transparent thermo-setting plastic replica mouldings of optical quality. The finished mouldings are sufficiently accurate for a wide range of applications in image-forming lenses, and particularly for the manufacture of quantities of identical aspheric lenses. The moulded lenses are made in the form of a thin layer of plastic of suitably chosen properties bonded during the moulding process onto one or both faces of a prepared glass substrate. The substrate is first ground and polished to a convenient flat or spherical form. An accurate optically worked negative glass block is used as a mould tool to form the plastic to the correct shape, and the plastic is then polymerised before the finished lens is removed from the tool. The difference in profile between the mould tool and the glass substrate is taken up by variations in thickness of the plastic layer.