Deflectometric systems for absolute flatness measurements at PTB

Abstract

Recently, two new scanning deflectometric flatness reference (DFR) measurement systems were installed at the Physikalisch-Technische Bundesanstalt. These instruments are aimed at measurements of the absolute flatness of optical surfaces with sub-nanometre uncertainties. System 1 is mainly designed for horizontal specimens with sizes up to 1 m and weights up to 120 kg. The other setup, i.e. system 2, is designed for vertical specimens. The two DFR systems use three different deflectometric procedures, which are based on scanning a pentaprism or the so-called double mirror unit (DMU) across the specimen. These 90° beam deflectors eliminate—to a great extent—residual guidance errors of the scanning stages, which is required to attain topography measurements with sub-nanometre uncertainty. The setups of the two new systems, the principles of the three different measurement modes, the alignment procedures, simulation results and first measurements are presented.