A Scanning Gauge for Measuring the Form of Spherical and Aspherical Surfaces

Abstract

A new instrument is described for measuring the form (or figure) of spherical and aspherical surfaces. The technique is basically mechanical and utilizes two high-precision air bearings. The test surface is mounted on one of these bearings, while the other carries an arm with a sensitive measuring probe which scans across the surface. Both contacting and non-contacting probes may be used. The instrument has been developed mainly to satisfy the growing need to test highly aspheric surfaces, particularly in connection with the manufacture of lenses and mirrors for thermal imaging applications. However, the instrument has many advantages over existing techniques when applied to the testing of normal spherical surfaces. In particular, it requires no test plates, it can be used to test steeply curved surfaces which are either concave or convex in form (the latter is always a problem with most other methods), and finally it can be used to test surfaces in either their polished or grey states.