Comparison Study of Three Camera Calibration Methods Considering the Calibration Board Quality and 3D Measurement Accuracy

BackgroundCamera calibration is a key process in optical deformation measurement techniques, like close-range photogrammetry and digital image correlation (DIC). Traditionally, an expensive calibration board with precisely machined feature points is employed in camera calibration. Low-precision calibration boards will lead to unstable camera parameters and worse length measurement accuracy (the relative measurement accuracy high than 0.1%).ObjectiveTo introduce camera calibration methods capable of achieving high measurement accuracy even with low-precision calibration boards, and give practices guide on selecting suitable calibration methods and judging calibration results.MethodsThree calibration methods are introduced and discussed. In the iterative board-points method, all the feature points on the calibration board are flexible. The coordinates of all the feature points and camera parameters are iteratively updated and solved simultaneously. In the fixing 3-points method, three feature points on the calibration boards are selected and the distances between them are measured precisely; the locations of these points are fixed in the optimization process. Both the simulation experiments and the real experiments were carried out to study the performance of the three calibration methods.ResultsExperimental results showed that both the iterative board-points method and the fixing 3-points method can effectively improve the accuracy of calculated camera parameters. Compared with the iterative board-points method, the fixing 3-points method restores the calibration board more precisely, as well as showing improved reproducibility and higher measurement accuracy. The relative accuracy of 3D distance measurement is also increased from 0.1% to 0.01 %, even using a low-precision paper calibration board.ConclusionsHigh measurement accuracy can be obtained using the fixing 3-points calibration method, even with a low-precision calibration board. This will decrease the cost and expand the application of optical measurement, especially in large structure deformation measurement.