Reducing of the microscope error for the control of through holes by optimizing the relative threshold offset coefficient with adaptive image binarization

Video measuring microscopes (VMM) are widely used in scientific research and in production to control the geometry of objects of complex shape, however, the measurement results on them strongly depend on lighting conditions and image processing parameters. In this paper, the influence of the backlight level on the choice of the optimal value of the relative threshold offset coefficient relative to the local background for adaptive binarization of images when measuring through holes is investigated. This parameter has been optimized according to the criterion of minimizing the deviation of the measured diameter from the reference data obtained on the coordinate measuring machine (CMM, Carl Zeiss). After processing the images obtained from the microscope and calculating the parameters of the obtained circle profiles, it was shown that the optimal value of the relative threshold offset coefficient demonstrates a close to linear dependence on the level of the lower illumination (30–85 in conventional VMM units). Based on the data obtained, a method was proposed for automating the selection of the relative threshold offset coefficient, which reduces the operator's influence on the measurement results. The materials of the article can be used to increase reproducibility and reduce measurement error in industrial systems of automated optical geometry control of finished products.

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