Calibration of a stereo microscope based on non-coplanar feature points with iteratively weighted radial alignment constraint

Abstract

A commercial stereo light microscope, such as the OLYMPUS SZX7, has nonnegligible lens distortion for three-dimensional microscopic image correlation. To tackle the problem, an effective calibration method of using a stereo-vision system is proposed. It is implemented using non-coplanar feature points with weighted radial constraints, where the weights are incorporated into the objective function to estimate the internal and external parameters of the vision system iteratively. In the calculation of the distortion coefficients, a non-parametric distortion model expressed by a bicubic spline surface is used. The iterative convergence of the objective function and the influence of the attitude number of the calibration target on the calibration results are analysed, and verified with a digital stereo light microscope established with OLYMPUS SZX7. Both shape and displacement measurements have been performed to demonstrate the proposed method. The experimental results show that the proposed method can achieve higher accuracy compared with Zhang's classical method and the unweighted two-step method.