Scanning deflectometric form measurement avoiding path-dependent angle measurement errors

For the highly accurate topography measurement of nearly flat optical surfaces, scanning deflectometric methods are capable of achieving nanometer accuracy. In these systems, an autocollimator is typically used as the deflectometric sensor and a pentaprism is applied for the scanning process. When ultimate accuracy is desired, a drawback of these systems is that the autocollimator output signal often depends slightly on the optical path length, resulting in topography errors during scanning. Here, we present a new deflectometric method which separates the angle measurement from the scanning process and, thereby, avoids possible errors due to different optical path lengths. In contrast to conventional deflectometry, the new technique achieves an almost exact autocollimation by appropriately tilting the specimen during scanning. The tilt angle necessary to achieve autocollimation complies with the deflectometric angle determined in conventional deflectometry. The tilt angle is measured with an additional autocollimator at a fixed distance without errors due to different optical path lengths. The separation of angle measurement and the scanning process enable both tasks to be optimized independently. This opens up new possibilities of reducing lateral resolution by facilitating smaller apertures and of assessing topographies with larger curvatures. The concept was tested successfully by a demonstrator setup. The first measurements on a test specimen agree with results obtained with the established Extended Shear Angle Difference (ESAD) technique at the one nanometer level.