Advances in deflectometricform 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.