Research On Large-caliber Mirror Surface Shape Detection Based On Stripe Reflection Method

The processing level of large-aperture mirrors has always been limited by the level of optical detection,especially the surface shape detection at the transition stage from grinding to polishing.At present,there is no more mature and effective detection method.Based on the above problems,this paper selects the stripe reflection method to conduct a three-dimensional profile measurement of the optical mirror in the transition stage.This method uses structured light with phase information to project onto the surface of the object to be measured.In the deformed fringe image,the gradient data of each point on the mirror surface is obtained by calculating the phase change of the deformed fringe.Finally,the gradient data is integrated to fit the three-dimensional shape of the mirror objectThis paper studies in detail the key techniques such as gradient acquisition,gradient integration and calibration of the detection system in the fringe reflection method.First,we studied the methods and principles of the digital phase shift technology and phase expansion that need to be used for gradient acquisition.The computer simulation is used to compare the phase expansion of the Goldstein branch cut method and the quality map guidance method in the phase expansion technology under different signal-to-noise ratios.In the study of the gradient integration algorithm,this paper proposes an improved iterative compensation algorithm based on the Southwell integration model.This method uses the attenuation coefficient to control the compensation height,and iteratively accumulates the height residuals to gradually improve the face shape.The simulation results show that the improved iterative algorithm has the advantages of faster convergence rate and higher accuracy of mid-high frequency fitting.In order to reduce the error caused by the high uncertainty in the actual measurement on the gradient data,we jointly improve the iteration method and the SCOTS gradient solution algorithm to propose a double iteration algorithm.The algorithm updates the gradient data once during each iteration and the updated gradient gradually converges to the true gradient value of the actual surface shape.Its superiority is verified in the simulation comparison experiment.The high-precision detection results of the stripe reflection method are based on high-precision system calibration.In this paper,the 2D plane black and white grid calibration target is used to calibrate the CCD camera to obtain the internal and external parameters and distortion coefficient of the camera,and then the camera projection vector is calibrated.The linearity compensation of the gray value of the non-linear problem of the LCD screen is performed to avoid the influence of the streak artifacts on the fitting accuracy of the surface shape.The corrected root mean square error of the brightness is only 1/15 before the correction.In the actual detection experiment,we uses the fringe reflection method to measure a high-order aspheric surface with a diameter of 1540 mm and compares the detection results with the measurement results of the interferometer.The same profile error distribution of the two methods verifies the feasibility of the stripe reflection method for high-precision full-diameter detection of large-diameter mirrors in the transition stage.