The Research of Calibration Technology on Parts Size Measurement

For parts size measurement system, the traditional method of using fixed focus optical with short focal and of calibrating with standard parts, which has more radial distortion and pixel equivalent error, affecting the measurement accuracy. In order to improve the accuracy of size measurement system, according to the linear imaging model of pinhole camera and the non-linear model of lens, a focusing mathematical model of fixed-focus camera lens is established and cross-ratio invariant calibration method is presented. The impact factors for measurement accuracy is studied by distance changes of objects in small-scale and by geometry projective transformation of lens, the pixel equivalent calibrated is revised with focusing model and lens distortion coefficients is calibrated with cross-ratio invariant, high-precision measurement is achieved with the calibration method in the system. The experimental data show that the proposed calibration technology can achieve higher accuracy.