Digital calibration method for defects evaluation of large fine optical surfaces

The digital calibration method, which is employed in the Surface Defects Evaluation System (SDES) for the defects evaluation of large fine optical surfaces, is presented in this paper. A criterion board, which comes from special design and careful fabrication, is employed to relate the dimensions of the defects and those of their images. The calibration procedure, including collecting of calibration images, digital image processing and calibration function fitting, is described in detail in this paper. Calibration experiments on scratch width and dig diameter were carried out at three different microscope magnification conditions. Experiment results show that following the proposed digital calibration method, micron-sized defects distributed sparsely on a large-aperture fine optical surface can be evaluated with micron accuracy and high efficiency.