Abstract

Testing an aspheric surface in a conventional phase-shifting interferometer requires use of a null lens to match the test surface. A null lens may consist of refractive, reflective, holographic (CGH) and/or binary optical elements, and it serves to keep the wavefront normal to the test surface.¹ Such a null configuration allows the test wavefront to retrace its path through the interferometer, as illustrated by the diverger lens of a Twyman-Green interferometer (Figure 1). Since the diverger lens is corrected for infinite conjugates, no additional aberrations are introduced into the test beam. The time and expense of using a null lens can be eliminated by testing in a non-null configuration, but the test wavefront will no longer retrace itself (Figure 2). Only one zone of the asphere may retrace itself. In Figure 2, the marginal rays are normal to the asphere, but the rest of the reflected wavefront becomes aberrated. These aberrations will change as the test beam propagates through the interferometer optics. The interferometer must then be calibrated if useful measurements of aspheres are to be made. To study the feasibility of calibrating an interferometer for non-null aspheric testing, a Twyman-Green interferometer has been modeled using lens design code.

© 1994 Optical Society of America