ESAD Shearing Deflectometry: A Primary Flatness Standard with Sub-Nanometer Uncertainty
To overcome the limitations of conventional interferometry, a technique has been developed which allows the absolute topography measurement of near-plane and slightly curved optical surfaces of arbitrary size with low measurement uncertainty. The Extended Shear Angle Difference (ESAD) method combines deflectometric and shearing techniques in a unique way to minimize measurement errors and to optimize measurand traceability. A device for the topography measurement of optical surfaces up to 500 mm in diameter, achieving sub-nanometer repeatability, reproducibility and uncertainty, was built at the Physikalisch-Technische Bundesanstalt (PTB). The ESAD method is optimally suited for creating a primary standard for straightness and flatness with highest accuracy by which the three-flat test or liquid mirrors can be replaced as starting points of the traceability chain in flatness measurement. In the following, the improved ESAD device which uses optimized opto-mechanical components is presented. Central aspects of the proper design and use of deflectometric systems are highlighted, including the optimal use of pentaprisms.
Wei Gao, Yasuhiro Takaya, Yongsheng Gao and Michael Krystek
R. D. Geckeler "ESAD Shearing Deflectometry: A Primary Flatness Standard with Sub-Nanometer Uncertainty", Key Engineering Materials, Vols. 381-382, pp. 543-546, 2008