Optimized use and calibration of autocollimators in deflectometry

Abstract

Accurate and traceable angle measurement poses a central challenge to deflectometric profilometry. High-resolution electronic autocollimators are capable of providing accurate angle metrology for this purpose. The optimized calibration and the use of autocollimators under well-defined and stable measurement conditions are central to their proper application. To illustrate these issues, the autocollimator in use in the Extended Shear Angle Difference (ESAD) device for the absolute and traceable topography measurement of optical surfaces built at the Physikalisch-Technische Bundesanstalt (PTB) is considered. In contrast to other deflectometric profilers, ESAD combines deflectometric and shearing techniques in a unique way to minimize measurement errors and to optimize measurand traceability. Sub-nanometer repeatability, reproducibility, and uncertainty of the topography measurement are achieved for scans across near-flat surfaces up to 500 mm in diameter. In this paper, the ESAD shearing deflectometer and its measuring capabilities are presented. Information on the optimized use and accurate calibration of autocollimators for deflectometric applications is provided. The calibration of autocollimators by comparison with the highly accurate primary angle standard of PTB is illustrated. Factors influencing their angle response are discussed, such as the position of the aperture stop both along the autocollimator's optical axis and perpendicular to it.