Aspheres are widely used in modern optical systems because of their advantage of promising higher image quality with fewer elements compared with traditional spherical systems. Surface figure metrology becomes more challenging as increasingly higher performance demands aspheres with a larger aperture, higher accuracy, and even more complex surface forms, e.g., larger slope variation or a freeform surface.
Wavefront interferometry is a standard solution for the measurement of optical surface error or wavefront aberration, which is usually at the submicron or even nanometer scale. An interferometer fundamentally outputs a test beam and records the fringes formed by interference of the reference beam and the test beam. The reference beam is reflected by a well-polished reference surface, and the test beam is modulated by the test surface or system. The surface error or wavefront aberration is then obtained by analyzing the fringe pattern. Readers are referred to Refs. 1–3 for details of the basic principle of wavefront interferometry if required.
The standard form of the reference surface, which is the last surface of the transmission flat (TF) or transmission sphere (TS) mounted at the exit pupil of the interferometer, is either flat or spherical. Hence, flat or spherical surfaces can be measured in the so-called null test configuration. The name “null test” comes from the fact that a nominally null fringe pattern is obtained because the test wavefront perfectly matches the test surface. However, for aspheres, non-null fringes are, of course, observed, and the number of fringes depends on the aspheric departure from the best-fit sphere of the surface. On the other hand, the number of resolvable fringes of the interferometer is limited by the Nyquist frequency. The dynamic range of measurement of a commercial interferometer is typically restricted to only tens of fringes.
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