Vision measurement has been widely used in the field of dimensional measurement and surface metrology. However, traditional methods of vision measurement have many limits such as low dynamic range and poor reconfigurability. The optical modulation system before image formation has the advantage of high dynamic range, high accuracy and more flexibility, and the modulation accuracy is the key parameter which determines the accuracy and effectiveness of optical modulation system. In this paper, an optical modulation system with pixel level accuracy is designed and built based on multi-points reflective imaging theory and digital micromirror device (DMD). The system consisted of digital micromirror device, CCD camera and lens. Firstly we achieved accurate pixel-to-pixel correspondence between the DMD mirrors and the CCD pixels by moire fringe and an image processing of sampling and interpolation. Then we built three coordinate systems and calculated the mathematic relationship between the coordinate of digital micro-mirror and CCD pixels using a checkerboard pattern. A verification experiment proves that the correspondence error is less than 0.5 pixel. The results show that the modulation accuracy of system meets the requirements of modulation. Furthermore, the high reflecting edge of a metal circular piece can be detected using the system, which proves the effectiveness of the optical modulation system.
Visual inspection for a highly reflective surface is commonly faced with a serious limitation, which is that useful information on geometric construction and textural defects is covered by a parasitic image due to specular highlights. In order to solve the problem, we propose an effective method for removing the parasitic image. Specifically, a digital micromirror device (DMD) camera for programmable imaging is first described. The strength of the optical system is to process scene ray before image formation. Based on the DMD camera, an iterative algorithm of modulated region selection, precise region mapping, and multimodulation provides removal of the parasitic image and reconstruction of a correction image. Finally, experimental results show the performance of the proposed approach.