We have built a novel scanning facility based on ESAD (Extended Shear Angle Difference) deflectometry for the ultra-precise and traceable measurement of large near-flat and slightly curved optical surfaces. A primary application of the device is to establish the improved standard for straightness and flatness with sub-nanometer accuracy at PTB. Transfer standards then allow to calibrate optical devices, e.g., interferometers and wafer mappers, with high accuracy. The measurement principle is based on the analysis of differences of reflection angles obtained at surface points with large lateral displacements (shears). The ESAD principle minimizes error influences and is in first order independent of stage errors and wholebody movements of the specimen. ESAD scanning does not rely on external reference surfaces of matched topography and allows an accurate calibration of the angle measuring device. The measurands are directly traceable to the SI units of angle and length.
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