A study of the accuracy of lens measures

The accuracy of 26 aged and three new lens measures was investigated. Instruments were applied to surfaces of known curvature employing the conventional, hand-held method as well as while rigged on an optical bench. In addition, using the latter method, a controlled amount of surface tilt was introduced. A large inter-instrument variation in accuracy was encountered. In general, hand-held lens measures were found to provide equally accurate measurements on convex and concave surfaces, with the exception of the -20 D surface, and inclinations from 0 degree to 5 degrees did not result in erroneous measurements. Results for inclinations of 15 degrees became unreliable. The different usages of the instrument are discussed.