Abstract

With higher and higher demands put on the performance of optical systems today, there has been a trend toward using nonspherical surfaces in these systems. Optical metrology is given the responsibility for testing these surfaces. Fortunately, many of these surfaces can be tested in a straightforward manner. Most conic sections fall into this category. By using the geometric properties of these surfaces of revolution, it is possible to test them with conventional interferometers with little extra special equipment. The geometry for these surfaces determines the test configuration and what accessories are required to do the testing. The geometry and actual testing configuration will be covered in detail. It is also to the metrologists' advantage that even if a nonspherical surface is not a conic, the departure from the near conic is small and the asphere can be tested as a conic section. There are mathematical means for finding the near conic. When the departure is on the order of waves, the surface can be tested as the near conic and the results evaluated using standard automatic reduction techniques. With the advent of more sophisticated measuring techniques and data reduction systems, wavefront departures approaching 12 waves can be automatically evaluated. These new techniques also permit the automatic testing and evaluation of surfaces that are nominally a plane or sphere but are so badly aberrated that the interference pattern is complex. As with any optical testing, care must be taken to properly configure and align the test set up. With the proper attention paid to these points, a reasonably noncomplex fringe pattern nearly free of alignment errors can be obtained and evaluated.

© 1981 Optical Society of America