Research On Nonlinear Pre-Correction And Calibration Method Of Structured Light Imaging System

Imaging systems based on structured light fringe projection are widely used in computational imaging fields such as 3D measurement due to their non-contact,high efficiency,and high precision.However,the nonlinear distortion and optical distortion of the structured light system reduce the accuracy of the three-dimensional measurement of structured light.In this study,based on the nonlinear model of the structured light imaging system,the influence of each system parameter on the unwrapped phase estimation accuracy was analyzed.A pre-correction for the spatial geometric distortion and nonlinear light intensity distortion of the structured light 3D measurement system was proposed.A fast method for composite calibration of structured light system is designed.Aiming at geometric distortion and light intensity distortion,the mathematical model of each subsystem of structured light imaging system is established,and the key factors affecting the accuracy of 3D measurement are analyzed.The error formulas of the phase principal value,modulated light intensity and background light intensity of the standard phase shift method are deduced,and it is demonstrated that the modulated light intensity,the number of phase shift steps and the random noise of the system are the three most important factors affecting the measurement accuracy of the system.Based on the seventh-order polynomial,the projector’s gray value response model is established,and combined with the optical distortion model,the projection pattern is pre-corrected,and the optical distortion and distortion of the projection pattern are corrected at the same time.The pre-correction method intercepts the gentle gray value interval as the value range of the modulated light intensity,which can effectively avoid the quantization error caused by the platform effect.Through the comparison experiment with the post-compensation method,the experimental data show that the pre-correction method is more effective in suppressing the nonlinear error of the projection system,and its unwrapped phase variance is more than 40% lower than that of the post-compensation method;the phase error of the pre-correction method is only0.016 rad.Especially in the out-of-focus state,the pre-correction method still has a good inhibitory effect on nonlinear errors,and can avoid complex distortion compensation operations.A circular array calibration plate is introduced,and a composite calibration method of structured light system is designed.In order to avoid crosstalk between different feature points,the projection plane is divided into several projection regions,and a homography matrix is established separately for each region.The sub-pixel coordinates of the projection feature points are determined by the invariance of the cross ratio in the projective transformation,which improves the calibration accuracy.While ensuring the calibration efficiency,this calibration method reduces the maximum reprojection error in both directions by an order of magnitude,and the standard deviation of the reprojection error is also reduced to about a quarter of the traditional calibration method.