Freeform testability considerations for subaperture stitching interferometry

Paul E. Murphy; Chris Supranowitz

doi:10.1117/12.2536851

15 November 2019 Freeform testability considerations for subaperture stitching interferometry

Paul E. Murphy, Chris Supranowitz

Proceedings Volume 11175, Optifab 2019; 111750Z (2019) https://doi.org/10.1117/12.2536851
Event: SPIE Optifab, 2019, Rochester, New York, United States

Abstract

Fizeau interferometry is a flexible tool for optical surface metrology. Different transmission spheres (TSs) enable testing most spherical surfaces, and selecting a TS to measure form irregularity of a given surface is straightforward. New applications, however, have increased the variety of surfaces to test beyond spheres. Aspheres and freeforms are particularly challenging, as interferometers only resolve small deviations from a sphere without additional corrective optics. Furthermore, the surface irregularity specification may be accompanied by tolerances related to mid-spatial frequencies (MSFs), such as power-spectral density (PSD) or local slope. These MSF specifications may require spatial resolution beyond what a typical full aperture test provides. Subaperture stitching interferometry is particularly well-suited to measuring MSFs, and can also significantly increase non-null aspheric and freeform measurement capability. Selecting the most appropriate TS for a given surface, however, becomes more complicated. We explore how the surface specification interacts with the interferometer’s slope capture limit to determine an “optimal magnification” for that surface. We show how to select the most appropriate TS for the surface, given the optimal magnification and other interferometer constraints (e.g. cavity length, focusing range). We demonstrate this TS selection process for a toroid measurement (with 400 micrometers departure from best-fit sphere). We conclude with guidance for designing aspheres and freeforms that can be measured more easily with stitching interferometry.

Conference Presentation

Citation Download Citation

Paul E. Murphy and Chris Supranowitz "Freeform testability considerations for subaperture stitching interferometry", Proc. SPIE 11175, Optifab 2019, 111750Z (15 November 2019); https://doi.org/10.1117/12.2536851

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