Study On Calibration Technology Of Phase Measuring Deflectometry Based On Standard Plane Mirror

Special free-form surface has been widely used in many fields,such as camera lens and primary mirror of large astronomical telescope,due to its advantages of reducing optical elements and improving imaging quality.When using interferometer to measure free-form surface,it is usually necessary to add compensating mirror to the optical path,but the design of the compensating mirror will prolong the measurement period,and the price is expensiveis.Because of the characteristics of full field of view,fast measurement and simple device,phase measuring deflectometry is an effective tool for measuring special surface.But its disadvantage is that the system calibration is complicated,and the precision of calibration is closely related to the final measurement precision.Therefore,one of the emphases of phase measuring deflectometry is how to calibrate the geometric parameters of the system simply and accurately.Stereoscopic deflectometry system uses binocular vision to realize the positioning of the measuring mirror.It does not need the nominal surface shape of the surface to be measured or the screen moving device,and can realize the measurement of free surface,so it is widely used.The geometric parameters of the system need to be calibrated before measurement.The parameters of stereoscopic deflectometry system are usually solved by three attitude method of plane mirror,or the plane mirror with markers.In the three-attitude solution method of the plane mirror,when the difference between the three positions of the plane mirror is small,the deviation between the attitude parameters of the plane mirror solved and the real value is large,which easily leads to the calculation error of the geometric parameters of the system.The surface shape error of the plane mirror with markers is usually on the order of micron,which will result in the inaccurate correspondence between pixel feature points and virtual image feature points on the screen,thus affecting the final calibration accuracy.In this paper,a high precision standard plane mirror is used as a reflector to obtain accurate corresponding feature points,and the deviation of feature points caused by the low precision of the plane mirror with markers is reduced.In order to improve the reliability and stability of the optimized initial value,paper target was used to assist the positioning of the plane mirror attitude parameters.Finally,the method of alternating direction optimization is used to obtain the high precision calibration results.Considering the influence of the screen shape factor in the measurement,this paper adopts the method of minimizing the measurement error based on the standard plane mirror to iteratively obtain the screen shape,thus reducing the measurement error caused by the deviation between actual plane shape of the screen and the ideal plane.