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.

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For the highly accurate topography measurement of nearly flat optical surfaces, scanning deflectometric methods are capable of achieving nanometer accuracy. In these systems, an autocollimator is typically used as the deflectometric sensor and a pentaprism is applied for the scanning process. When ultimate accuracy is desired, a drawback of these systems is that the autocollimator output signal often depends slightly on the optical path length, resulting in topography errors during scanning. Here, we present a new deflectometric method which separates the angle measurement from the scanning process and, thereby, avoids possible errors due to different optical path lengths. In contrast to conventional deflectometry, the new technique achieves an almost exact autocollimation by appropriately tilting the specimen during scanning. The tilt angle necessary to achieve autocollimation complies with the deflectometric angle determined in conventional deflectometry. The tilt angle is measured with an additional autocollimator at a fixed distance without errors due to different optical path lengths. The separation of angle measurement and the scanning process enable both tasks to be optimized independently. This opens up new possibilities of reducing lateral resolution by facilitating smaller apertures and of assessing topographies with larger curvatures. The concept was tested successfully by a demonstrator setup. The first measurements on a test specimen agree with results obtained with the established Extended Shear Angle Difference (ESAD) technique at the one nanometer level.

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