Evaluation of the microroughness of large flat or curved optics by replication

Abstract

Surface features of optics that are too large to fit into laboratory microroughness-measuring instruments can be replicated onto small witness samples. The replicas are made in two stages. First, a room temperature vulcanizing (RTV) negative is made of a small part of the surface and, secondly, a rigid epoxy replica on a metal substrate is made from the RTV casting. In the first part of this study, reported earlier, surface detail was precisely copied using small well-characterized flats of glass and metal as masters. In the second phase, reported here, the technique has been extended to larger masters which can be either flat or moderately curved. All replicas were evaluated using Nomarski microscopy, a mechanical stylus profilometer and an optical profilometer. The surface figures of the aluminized epoxy replicas were also checked in a Zygo interferometer. In the Nomarski microscope, exact correspondence of surface detail could be seen, and surface profile roughness of second-stage epoxy replicas made from supersmooth (under 10 Å r.m.s.) masters were under 20 Å r.m.s.