Interferometric inspection of optical surfaces and wavefronts requires permanently increasing accuracy. Therefore interferometric equipment is being improved and improved continuously. Point-diffraction interferometers (PDI) with an “inbuilt” reference wavefront originating from light diffraction by a pinhole aperture are potentially capable to produce the highest possible accuracy of a surface figure or wavefront characterization.
The most mentioned configurations and their versions like Linnik-Smart and Sommargren schemes produce low-contrast spare-striped fringe patterns instead of full-contrast distinct interferograms, like e.g. produced by Fizeau interferometers, with clear phase shifting (PS) data flow.
The concept of a PDI presented in this paper is to provide two perpendicularly outgoing wavefronts – test and reference ones – with mutual intensity regulation and arbitrary and stable phase shifts of one wavefront relative to the other. Such concept is also targeted to provide user-friendly measuring conditions similarly to interferometers which are in common use.
Advantages of such arrangement of the PDI are: high numerical aperture (NA = 0.55), clear fringe patterns of high contrast, high accuracy of surface figure testing with wave-front RMS error 0.125 nm and wave-front RMS repeatability 0.05 nm. Performance of the PDI is illustrated by tables of repeatability and test surface profile plots for different azimuthal angles.
Date Published: 13 May 2013
PDF: 13 pages
Proc. SPIE 8788, Optical Measurement Systems for Industrial Inspection VIII, 878805 (13 May 2013); doi: 10.1117/12.2020618