Logo of the Physikalisch-Technische Bundesanstalt

International interlaboratory comparison for the high-accuracy flatness measurement of plane surfaces with an aperture of 300 mm in diameter

An international interlaboratory comparison coordinated by PTB started in 2020. This round robin test on optical flatness measurement was performed using an optical flat of 330 mm in diameter made of Zerodur. The participants have been measuring the flatness of the inner part of the surface with an aperture of 300 mm using optical measurement procedures. This prevents possible scratches at the surface, which can never be completely precluded with tactile procedures. The interlaboratory comparison is designed as a high-accuracy measurement comparison. The participants are therefore expected to achieve an expanded measurement uncertainty of less than 15 nm (k =2).

This so-called “EURAMET supplementary comparison on a 300 mm Zerodur optical flat” [EUR-L-S28] is based on the experiences and findings obtained during the previous EURAMET project #672 on the “Determination of form/topography of high-quality flats”. The procedures for realizing and evaluating such an interlaboratory comparison were developed and tested within the scope of this earlier EURAMET project. For this purpose, a high-quality 205 mm Zerodur plane plate was used as an SUT, and the flatness measurements on apertures of 150 mm and 100 mm in diameter were assessed. This previous comparison was also organized by PTB, and the results were published in 2017 [Qua17].

The EURAMET comparison that has now started will enhance the earlier EURAMET project by using an optical flat of 330 mm in diameter. Furthermore, measurements will be possible both horizontally and vertically. This international interlaboratory comparison is also listed in the BIPM’s Key Comparison Database [BIPM-KCDB].

The figure (left) shows a photo of the Zerodur plate (diameter: 330 mm; thickness: 71 mm). The figure (right) shows the SUT in its holder as well as the transport case.

Figure: Photo of the 330 mm Zerodur plate (left); SUT in holder and transport case (right).

Selected publications

[EUR-L-S28] EURAMET.L-S28 300 mm flat (#1490), www.euramet.org/technical-committees/tc-projects/details/project/high-precision-flatness-over-300-mm/

[Qua17] S. Quabis, M. Schulz, G. Ehret, M. Asar, P. Balling, P. Křen, R. H. Bergmans, A. Küng, A. Lassila, D. Putland, D. Williams, H. Pirée, E. Prieto, M. Pérez, L. Svedova, Z. Ramotowski, M. Vannoni, F. Hungwe and Y. Kang: Intercomparison of flatness measurements of an optical flat at apertures of up to 150 mm in diameter, Metrologia 54 (1), pp. 85-93 (2017), doi.org/10.1088/1681-7575/aa535c

[BIPM-KCDB] Supplementary Comparison „Flatness of surface plates” in the Key Comparison Database (KCDB) at BIPM, https://www.bipm.org/kcdb/comparison?id=1643


Dr.-Ing. Gerd Ehret

Phone: +49 531 592-4220
E-Mail: gerd.ehret(at)ptb.de