Measurement of radius of curvature with a low-coherence flatness interferometer

Klaus Freischlad

doi:10.1117/12.2690824

29 November 2023 Measurement of radius of curvature with a low-coherence flatness interferometer

Klaus Freischlad

Proceedings Volume 12778, Optifab 2023; 127780N (2023) https://doi.org/10.1117/12.2690824
Event: SPIE Optifab, 2023, Rochester, New York, United States

Abstract

The precision measurement of the radius of curvature of optical surfaces is mainly done using interferometric methods. Most general is the two-position method with a measurement at the cat’s eye and confocal positions, where the radius of curvature is given by the axial displacement of the test piece between the two positions as obtained e.g. from a distance measuring interferometer. This becomes more difficult for surfaces with long radii and large R-numbers, especially so for convex surfaces. Typical examples are laser cavity mirrors, or lenses with very long focal length. For this group of very shallow test surfaces the possibility exists to measure the radius of curvature with a flatness interferometer. It requires just a single measurement without axial part displacement, and represent a very quick and simple way of measuring long radii. However, this method does not operate at or near a fringe-null since a curved surface is compared to a flat reference wavefront, as opposed to the two-position method where the curvatures of the reference and test wavefronts are matched. Hence a more detailed consideration of measurement errors is required to establish the accuracy of the measured radii. This analysis shows that a carefully calibrated low-coherence flatness interferometer can provide radius measurements accurate to better than 1 part in 2000, or 0.05%, with the additional advantages of suppressing unwanted backside fringes. This presentation details the error analysis, and presents measurement results from the OptoFlat low-coherence flatness interferometer.

Conference Presentation

(2023) Published by SPIE. Downloading of the abstract is permitted for personal use only.

Citation Download Citation

Klaus Freischlad "Measurement of radius of curvature with a low-coherence flatness interferometer", Proc. SPIE 12778, Optifab 2023, 127780N (29 November 2023); https://doi.org/10.1117/12.2690824

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