Research on digital calibration method for optical surface defect dimension

摘要

A digital calibration method for defect dimension of the optical surface is put forward to get the correspondence between the actual scale of defect on optical surface and the number of pixels of the defect image captured by CCD. Standard scratches, with their width ranging from 0.5μm to 40μm, are fabricated by electron beam exposure and reactive ion beam etching on two kinds of standard calibration board, quartz calibration board with and without chromium film. Calibration experiments are accomplished in five different microscope magnifications. Threshold segmentation, morphological operation and feature extraction are carried out in the images of calibration board to obtain the width of standard scratches in pixels. Interpret the theoretic trend of the calibration function as well as the linear range of it, and fit the calibration function based on the experimental results. According to the analysis and comparing of the calibration results in different microscope magnifications, error source and the factors limiting the resolving accuracy of the calibration system are analyzed. Ultimately, a standardization process including fabrication of the standard scratch, establishment of the standard calibration library for different microscope magnifications and the rapid calibration of actual detect is established. The calibration of the defects on the optical element in the size of 450mm× 450mm is successfully realized.