High-precision in-situ measurement with deflectometry
The in-situ measurement of complex optical surfaces is a challenging task in the intelligent optical manufacturing. The phase measuring deflectometry is a powerful measuring method for complex optics, and it has higher measuring efficiency, stability and dynamic range compared to interferometry. Deflectometry is essentially a calibration problem, and the measuring accuracy is directly determined by the quality of geometrical calibration. A self-calibration method is developed to specify the relative positions of the camera and screen. The accuracy of the geometrical positions can be improved by an order of magnitude by minimizing the deviations of the traced points with respect to the true correspondences. According to the statistical properties of the deviations in reverse ray tracing, the form errors and the position errors can be separated, and the positioning error of the workpiece can be corrected accordingly. In order to correct the phase errors caused by the defocus and aberrations of the off-axis catadioptric imaging system, the space-variant point spread functions are modelled, and the phase bias is estimated by forward convolution between the captured images and the point spread function models. The measurement accuracy can be improved from 80 nm RMS to 20 nm RMS.
Xiangchao Zhang is currently the vice director of the Shanghai Engineering Centre of Ultra-Precision Optical Manufacturing, Fudan University, China. He is a senior member of SPIE and a member of OSA, ASPE, and IEEE. He graduated from University of Science and Technology of China in 2005 and received his PhD degree at University of Huddersfield, UK in 2009. Prof Zhang’s research interests include precision optical measurement, surface metrology and intelligent manufacturing. He has achieved a series of important results on freeform surface characterisation, deflectometric measurement, digital holographic microscopy etc and published more than 100 papers. He has leaded several research grants including National Natural Science Foundation of China, National Key R&D Program of China etc. He won a Second Prize of Science and Technology Development, Ministry of Education of China in 2016. He is a peer reviewer of more than 30 international journals and a project reviewer of NSFC of China and EPSRC of UK.