Research on Camera Calibration of the Binocular Vision Light Pen Measurement System

To binocular vision light pen measuring system, calibrating the camera fast and accurate, can improve the practicality of the system application. In the establishment of ideal camera imaging model, considering the lens radial distortion and tangential distortion factors, studies the practical imaging model parameters. Plane template calibration algorithm is improved. Using points which are near calibration template center and distortion smaller solve the initial model parameters. Application internal mapping Newton's method of subspace confidence domain method solves global optimization model parameters. Designing plane calibration template and using coded identification method, identify the signs point in the calibration template. The calibration efficiency is improved. The experiments show that the calibration method can be achieved binocular stereo vision camera calibration. Introduction Binocular vision, light pen measuring system is based on using two cameras to imaging, and through a series of coordinate transformation relationship to determine the spatial position and posture of light pen and then get space coordinate of measuring points. The mutual mapping relationship between space three-dimensional coordinates of the object surface some point and its corresponding 2 d coordinates in the image is determined by the camera imaging model. In general, the camera model parameters can be obtained through experiment and optimization calculation, and the process of solving camera model geometry and optical parameters is called camera calibration. At present, there is no universal calibration method that can be used for all kinds of situation, if the specific applications are different and the calibration method is also different. Here is a commonly used calibration method. Direct linear transformation (DLT) method [1], its principle is simple and easy to implement, But the disadvantage is that calibration precision is not high and intermediate parameter has constraint each other. moreover, it is not easy to separate internal parameter values from external parameter values. In order to improve the calibration accuracy further, based on the DLT method, Tsai [2] first proposed two-step method based on radial alignment constraint (RAC), which has higher precision and is suitable for precision measurement, but only considering the radial distortion, it does not apply to simple calibration. In order to improve the calibration of flexibility, Zhang Zhengyou [3] first proposed plane template calibration method, which can obtain internal and external parameter values for the camera model by making the camera collect more than three images of the calibration board from different angles and use information of the known feature point location on the template and correspondence of the corresponding projection points in the image, because the calibration is simple, it is widely used. But because of only considering the radial distortion, it is not applicable with large distortion fisheye lens. In addition, camera self-calibration method [4-6] and the calibration method based on active vision [7].which have low calibration precision and complex calibration process, is not suitable for optical measurement system. 4th International Conference on Computer, Mechatronics, Control and Electronic Engineering (ICCMCEE 2015) © 2015. The authors Published by Atlantis Press 1299 We adopt the improved plane model method for two camera calibration aiming at Light pen for binocular vision measurement system in this paper. For the calibration method in this paper, the camera radial distortion and tangential distortion are considered, calibrating part points of template center near is adopted in the solution process of model parameters’ initial value, which improves the accuracy of the parameter’s initial value. For actual imaging model considering the distortion, all points on the template should be used to do the optimization algorithm for precise. At the same time, in order to improve the efficiency of calibration, the coding landmark to identify all the feature points on the calibration template is used. Modeling of the camera Ideal model of the camera. The camera mode can be obtained through the simple center Perspective projection and rigid body rotation and transform translation. This model is linear, not considering the lens distortion, which is as shown in Fig.1. In the camera model, Ow, Xw, Yw, Zw is world coordinate system. Oc, Xc, Yc, Zc is the camera coordinate system, the Oc is the camera lens optical center, Xc, Yc axis is parallel to the imaging plane, Zc axis is perpendicular to the imaging plane ,the intersection between them is (u0, v0), which is the main point of the camera ,namely OI. OcOI is the focal length f. OI,xy is the image plane coordinate system, where the x axis and y axis are parallel to Xc, Yc shaft axis respectively, OI is the intersection point of the Zc shaft and CCD. O0,u,v is the image plane coordinate system of computer, where the origin of coordinates O0 is selected in the top left corner of the image, the u axis is to the right and parallel to the X axis, while the u Shaft is downward and parallel to the Y axis. Fig.1 Ideal model of the camera Fig.2 The serial number of all marking point in the plane calibration template By camera model between different coordinate system transformation relations, can get the corresponding relations between any Point (Xw, Yw, Zw) of the space and its projection point P (u, v) in the image: