Absolute deflectometric measurement of topography - influence of systematic deviations

摘要

For industry and research there is an interest in increasing large master surfaces and other unique optics. Even for meter-scale objects, an uncertainty of form measurement in the range of nanometers has to be fulfilled. The deviation in the radius of curvature should reach 1000 kilometers and more. We present a robust optical profiler that allows the measurement of absolute topographic profiles of flat and slightly curved surfaces up to one meter in length. It is based on simultaneous multiple angle measurement, actually carried out with a common electronic autocollimator and a specially designed front aperture. By implementation of the Extended Shear Angle Difference (ESAD) algorithm - developed at the PTB - the complete angular topography can be reconstructed from two data sets of angle difference. At last, integration leads to the height topography. The uncertainty of measurement of the parabolic form contribution is close to the one claimed above and is referenced to a smaller diameter flatness transfer standard. All other form components are measured absolutely. Measurements on an excellent optical flat standard of 60 cm in length show a reproducibility of about 0.2 nm rms. The systematic errors contributing to the uncertainty budget are listed and investigated experimentally. Starting from this, suggestions will be made how advancement can be reached, and it is proposed, that the limit is clearly over 1000 kilometer in radius of curvature.