The Research Of Projecting Lens Distortion Calibration And Correction In 3D Imaging System

With the mature of Digital Micromirror Device(DMD) techniques, Digital Light Processing(DLP) projectors based on DMD have been widely utilized in 3D shape measuring systems to project digital structure-light patterns,due to their advantages of high contrast, high intensity, flexible programming, and cost efficiency. The DLP projector can be treated as an inverse camera, so the projector calibration is important for recovering 3D(three-dimensional) shape data of the objects, which can convert the monocular measurement system to the binocular measurement system. In practical applications, most off-the-shelf DLP projectors are not dedicated to precision 3D measurement. Due to the manufacturing and assembling deviations, the nonlinear distortion of lens is inevitable, which cannot be represented by the classical pinhole imaging model. It is necessary to establish the nonlinear model of the projector, especially for measuring large-size objects. Therefore, it is vital to calibrate the geometric parameters of DLP projectors and eliminate the projector lens distortion to improve the precision of 3D shape measuring systems.The nonlinear model of the projector is established and then a novel method is presented to calibrate a DLP projector by using an optical coaxial structure. A beam splitter is used to make imaging axis of the CCD camera and projecting axis of the DLP projector coaxial, so the DLP projector can be treated as a true inverse camera. A plate having discrete markers on the surface is designed and manufactured to calibrate the DLP projector. By projecting vertical and horizontal sinusoidal fringe patterns on the plate surface from the projector, the absolute phase of each marker’s center can be obtained. The corresponding projector pixel coordinate of each marker is determined from the obtained absolute phase. The internal and external parameters of the DLP projector are calibrated by the corresponding point pair between the projector coordinate and the world coordinate of discrete markers. Experimental results show that the proposed method can accurately obtain the parameters of the DLP projector. One advantage of this method is the calibrated internal and external parameters have high accuracy because of independent on a camera, so there is no accumulative error. The other is the optical coaxial structure gives a true inverse camera, so the calibrated parameters are more accurate than that of crossed-optical-axes, especially for the principal points and the radial distortion coefficients of the projector lens. According to the calibrated distortion coefficients, the distortion of the projector lens is graphical displayed and eliminated by using bilinear interpolation. Experimental results demonstrate the effectiveness of the proposed method.