An iterative calibration method for improving absolute measurement accuracy of stereo deflectometry

Stereo deflectometry is a measurement technique for specular surfaces with the advantages of full-field, non-contact and fast measurement. In this paper, a calibration method is investigated to enhance absolute measurement accuracy of stereo deflectometry. Traditional calibration method applies a flat mirror to obtain geometrical parameters of stereo deflectomety by placing the mirror at several space positions. Calibration errors introduced in this process result in inaccurate absolute measurement of stereo deflectometry. In order to improve calibration accuracy, a novel iterative algorithm based on a specular step gauge is explored. Calibration results obtained by traditional calibration method are used as initial input. A step gauge is measured based on the input. Calibration parameters are optimized through an iterative process by obtaining the most accurate measurement data of the step. In order to test the developed method, Two samples including a specular step gauge and a specular circle step are measured with a stereo deflectometry system. Comparison studies are conducted by measuring the samples based on CMM (Coordinate Measuring Machine) and stereo deflectometry with optimized parameters and unoptimized parameters. Experimental results show the developed method can improve the absolute measurement accuracy of stereo deflectometry from millimetre level to tens of micrometers.