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Flatness metrology is needed in the optical industry as well as for synchrotron mirrors, for precision engineering and for telescope mirrors. Calibrated optical flats are a key element for quality assurance in many fields and applications of science, research, and industry. The National Metrology Institute of Germany (Physikalisch-Technische Bundesanstalt, PTB) offers interferometric flatness calibrations for specimens up to 300 mm in diameter and deflectometric flatness calibrations up to 900 mm for elongated specimens. We are continuously developing our flatness metrology further to meet future requirements, e. g. to achieve uncertainties of a few nanometers as well as to offer measurements of large flats up to 1.5 meters. In this contribution we focus on three current activities. Firstly, we show our new large form measuring system for smooth curved specimen, which can also be used for flatness measurements of specimens with diameters up to 1.5 meters. Secondly, we demonstrate, how sub-millimeter lateral resolutions can be achieved with small angle deflectometry. Thirdly, we present an international round robin comparison of an optical flat. The flatness on an aperture of 300 mm is to be measured and the participants shall reach an expanded uncertainty of less than 15 nm (k=2). This will also ensure a reliable traceability also for subaperture measurements of optical flats with larger diameters.
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