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Interferometric flatness metrology

A Fizeau interferometer (see Figure 1) is used for the areal measurement of the flatness of optical flats (with a maximum diameter of 300 mm). The expanded measurement uncertainty (k = 2) currently amounts to 11 nm. Since a Fizeau interferometer compares the surface of an SUT with a reference surface (Fizeau surface), it is necessary to know the absolute topography of the reference surface.

The reference surface is calibrated by means of a so-called three-plate test similar to the procedure described in [Gri06]. Both the mechanical stability of the setup and its assessment are currently being optimized. The objective is to reduce the uncertainty of the measurement of flat surfaces. In addition, traceability is also ensured via the deflectometric reference system by comparing individual sections.

Figure 1: Interferometer setup for flatness measurements

One property of glass that is still not well understood today is its so-called delayed elasticity. This is the delayed stress relief of the glass (e.g. after being displaced). The consequence is that it can take a long time until the flat surfaces have resumed their original topography. Figure 2 shows examples of measurements on a Zerodur plate of 330 mm in diameter. Additional investigations are necessary to better understand this phenomenon.

Figure 2: Delayed elasticity of a Zerodur flat surface of 330 mm in diameter. The flat surface was first stored horizontally in the transport case and was then set up vertically in a V profile. Finally, the topography was measured over 20 hours. The graph shows the change in the peak-to-valley (PV) values and in the RMS values of the topography over the course of these 20 hours. During that time, the temperature in the room was nearly constantly at 20.4 °C [Ehr19].

Selected publications

[Gri06] U. Griesmann: „Three-flat test solutions based on simple mirror symmetry,“ Appl. Opt.  45, 5856-5865 (2006). doi.org/10.1364/AO.45.005856

[Ehr19] G. Ehret, H. Reinsch, M. Schulz: „Interferometric and deflectometric flatness metrology with nanometre measurement uncertainties for optics up to 1 metre at PTB.“, SPIE Proceedings (11189), Optical Metrology and Inspection for Industrial Applications VI; 1118905 (2019). doi.org/10.1117/12.2538872


Dr.-Ing. Gerd Ehret

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