Reaching accuracies of Lambda/100 with the Three-Flat-Test

Steffen Wittek

doi:10.1117/12.2020058

13 May 2013 Reaching accuracies of Lambda/100 with the Three-Flat-Test

Steffen Wittek

Proceedings Volume 8788, Optical Measurement Systems for Industrial Inspection VIII; 87882L (2013) https://doi.org/10.1117/12.2020058
Event: SPIE Optical Metrology 2013, 2013, Munich, Germany

Abstract

Flat surfaces and the determination of their surface characteristics are of major importance in industry. The absolute measurement of them is a great challenge however. A solution for the Three-Flat-Test was proposed by Kuechel. His solution relies on four normal measurements where the three flats are interferometrically measured in different permutations and one where the test-piece flat is rotated with respect to the reference flat. As the solution is not exact the quality of the result depends on the number of integration steps taken for the fifth measurement. Kuechel has shown that for a 640 x 480 pixel CCD array 20 rotations are sufficient. Using his algorithm a interferometric-measurement accuracy on the order of lambda/100 is likely. In the present work it could be shown that the algorithm works with both high and low frequency errors, here represented by fractal and Zernike surfaces respectively. In a first practical test the algorithm retrieved the wavefront errors of three ZYGO transmission flats. The three flats were given to have accuracies of A = lambda/15, B = lambda/15 and C= lambda/ 20. The application of the algorithm resulted in accuracies of Aa = lambda/ 39, Ba = lambda/ 42 and Ca = lambda/ 40. This implies that all surfaces could be more accurately determined than given by the manufacturer! A rotation stage which can be turned more precisely and transmission flats of known better quality are however expected to lead to the measurement of higher accuracies in the range of lambda/100. Hence this procedure is likely to allow for cheap and fast absolute calibration of transmission flats.

Citation Download Citation

Steffen Wittek "Reaching accuracies of Lambda/100 with the Three-Flat-Test", Proc. SPIE 8788, Optical Measurement Systems for Industrial Inspection VIII, 87882L (13 May 2013); https://doi.org/10.1117/12.2020058

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