Distortion correction for images in planar metrology

In consideration of the effect of the distortion error of an image on the accuracy of planar geometrical measurement in the precise visual metrology,a method to correct the image distortion by using standard grid board directly as measurement calibration is proposed.As the position of an under-tested point is unchanged relative to that of grid board when an under-tested object and a grid board lay on the same object plane,the grid board is directly used as the correcting calibration of image distortion instead of the way that converts the external standard into the intrinsic parameter of a camera by modeling.Firstly,the primary position of the under-tested point in the image of grid board is determined;then,its fine distance is decided according to the proportion theorem of line segment divided by parallel line.Finally,planar geometrical measurement is fulfilled by calculating the sum of both distances.Experimental results show that the distortion error by the proposed method has reduced to 20% that of the traditional method and the correctoin accuracy of image has reached 4 μm or higher,while the distance of intersection point of the calibrated grid board is 1 mm and its accuracy is 0.2 μm.The method is more applicable and has high accuracy,for it omits the modeling and its accuracy only depends on the calibrated grid board.