An Error Calibration Method Against Aberration In Long Focal-length Measurement

In order to avoid the demanding collimation requirements of parallel light and to increase the measurement accuracy, divergent light and gratings with different periods are used to substitute parallel light and gratings with identical periods. However, Gaussian Equation is used to calculate the focal length of the test lens without consideration of the errors caused by aberration. What’s more, the errors caused by aberration would enlarge in the measurement system based on divergent light and become the key factor affecting the accuracy.Firstly, the imaging light path of the measurement system is simulated, simulation results reveal that the errors caused by aberration would draw deviation into the measurement focal length of the test lens. When the light source is at a fixed distance of 4200mm before the test lens, the errors caused by aberration would draw 5mm deviation into the measurement focal length of 13500mm focal length lens and 40mm deviation into the measurement focal length of 31251mm focal length lens. The deviation is beyond our measurement requirement, the errors caused by aberration must be calibrated. Then, the aberration errors are obtained by numerical method and compared with results simulated by ZEMAX. The modified focal length of the test lens could be calculated after putting the aberration errors into former formula. Thirdly, the measurement software is improved and achieves auto-correction function with the use of VC++. Finally, the measurement data of 13500mm focal length lens and 31251mm focal length lens is calibrated, the modified measurement accuracy is better than 0.08% and 0.028%. Besides, the modified measurement accuracy is compared with the measurement accuracy of interferometer measurement, the relative accuracy is better than 0.06%. These experimental data fully illuminate the proposed calibration method is highly effective and reliable.