Deflectometric profilometers provide the metrology for the accurate measurement of optical surfaces which are not
easily accessible to classical interferometers, e.g., due to the dynamics of their topography, their size, or the need of
independence from calibrated reference surfaces of matched topography. Applications include the measurement of
optics used in synchrotron beamlines and the definition of flatness standards. One application of deflectometric
scanning is the Extended Shear Angle Difference (ESAD) method, which has been developed to provide highly accurate
measurements of the slope and topography of near-plane and slightly curved optical surfaces. ESAD combines
deflectometric and shearing techniques in a unique way to enable error minimization and traceability. A facility
achieving sub-nm repeatability, reproducibility, and uncertainty of topography measurement was built at the
Physikalisch-Technische Bundesanstalt (PTB). ESAD utilizes the straight propagation of light as well as traceable
measurands to provide a flatness standard with highest accuracy which is to serve as the starting point of the traceability
chain in flatness measurement. It will be reported on the ESAD device and on measurement results. Measurement
comparisons involving ESAD deflectometry and an independent deflectometric scanning system will be presented.
Information on the adjustment of the pentaprism, the specimen under test, and the axes of the autocollimator will be
given. It will be reported on the reduction of the systematic errors in the angle difference measurement down to the
milliarcsec (few nrad) level.