Optimal use of pentaprisms in highly accurate deflectometric scanning

Abstract

Pentaprisms are key optical elements in devices for the highly accurate measurement of optical surfaces by means of scanning deflectometry. They are used to deflect the beam of an angle measuring device by 90°, as the deflection angle is subject to only second-order error influences when angular errors in the orientation of the prism are present, e.g., due to the mechanical shifting of the prism. This error-reducing property can only be fully made use of if all components of the deflectometric set-up, i.e., the axes of the angle measuring device, the pentaprism and the surface under test, are optimally aligned. In this case, minimal deviation from parallelism of the measuring beam probing the surface is achieved while shifting the prism. The problem of optimal adjustment is investigated in detail by means of ray tracing simulations, and easy-to-implement strategies for the adjustment of all optical components are developed. It is experimentally demonstrated that, in combination with accurate mechanical stages, the angle deviations in deflectometric scanning due to changes in the angular orientation of the pentaprism can be reduced below the 0.001 arcs (5 nrad) level. Therefore, a reduction of the influence of angular errors of the stage shifting the pentaprism on the angle measurement by a factor of at least 1000 is achievable by optimal adjustment.