Influence of CCD nonlinearity effect on the three-dimensional shape measurement of dual frequency grating

The CCD nonlinearity effect in the measurement system will affect the measurement accuracy of complex optical three-dimensional surface. Therefore, a method to eliminate the CCD nonlinearity effect by using dual frequency grating projection is proposed, which can improve the measurement accuracy. Firstly, the influence of CCD nonlinearity effect on three-dimensional shape measurement is analyzed. The analytical derivation and physical explanation of spectrum aliasing are given. Then, the measurement principle of dual frequency grating under the CCD nonlinearity effect is discussed. The light intensity distribution of deformed fringe and the principle of obtaining aliasing spectrum by Fourier transform are analyzed. Finally, the method to judge the measurement accuracy by the equivalent wavelength is given. The basic formula of measuring the height information of three-dimensional surface by using dual frequency grating projection is derived, and the theoretical analysis is carried out. The object is simulated as the maximum absolute value and average absolute value are 24.3181 mm and 1.0839 mm, respectively. The maximum absolute height error and average absolute height error between the measured value and the actual value are 0.8950 mm and 0.0622 mm, respectively. After increasing the fundamental frequency of the dual frequency grating, the corresponding values are reduced to 0.3710 mm and 0.0232 mm, respectively. When the fundamental frequency of the dual frequency grating is increased by 2.5 times, the separation between the fundamental frequency and the advanced spectrum becomes better, and the measurement accuracy is improved. Therefore, using dual frequency grating projection to eliminate CCD nonlinearity effect has strong practicability and is highly advisable.