Advances in deflectometric form measurement

Phase measuring deflectometry is an accepted technique for measuring the shape of specular surfaces. While defloctometry is known to provide high sensitivity in the nanometer range, the absolute form measuring accuracy is typically inferior by several orders of magnitude. The comparatively low accuracy of typical implementations of phase measuring deflectometry is determined by several influencing factors. On the one hand, many system models used do not consider all relevant system parameters, such as refraction in the display substrate or its flatness deviation. On the other hand, due to the complex system geometry, many calibration procedures are susceptible to deviations due to low condition numbers of the mathematical problems. To increase the absolute accuracy of phase measuring deflectometry, the authors have analyzed in detail the calibration procedures, the measurement process, and the evaluation algorithms and have made numerous extensions and optimizations. The present contribution gives an overview of the obtained findings and the applied measures. The performance of the approach is evaluated based on measurements of challengingly curved measurement objects. Based on these selected objects, form measurement deviations of better than 1 μm are documented.