Testing high accuracy optics using the phase shifting point diffraction interferometer

Advanced figuring technology has enabled manufacturing of high accuracy optics for precision applications. The measurement technologies to verify them are largely based on Fizeau interferometry, which is limited in terms of accuracy because of external accessories such as reference flat. Lack of appropriate verification method is adversely affecting the manufacturing and optimization of precision optics. In this paper, we explore a fundamentally different interferometry arrangement, D7 produced by Difrotec. A phase shifting point diffraction interferometer (PSPDI) and present measurement results for concave spheres with an accuracy of λ/1000 PV, and compared this full-shot result with wavefront maps obtained by subaperture stitching (SAS) to verify stitching accuracy. We also describe measurement of asphere cavity using SAS, with higher accuracy, λ/500 RMS, discuss strategies to measure concave/convex spheres and aspheres with R-number ≥ 0.5 with nanometer accuracy, and conclude with perspectives on the future applications of PSPDI D7.