Measurement technique for scattering characteristics of surface defects of optical components

Surface defects in optical components represent a major determinant of ultra-smooth optical components quality. The scattering method evaluates the surface quality of optical components by analyzing the scattering effects caused by defects. However, the three-dimensional morphology of surface defects that induce scattering is highly diverse, and their distribution on the image plane cannot be sufficiently characterized using geometric optics alone. In this study, a method is proposed to comprehensively investigate the scattering characteristics of surface defects of precision optical components. Firstly, electromagnetic theoretical models of surface defects with cross-sections shaped like triangles, ellipses, and rectangles were established using the finite element method . By altering the width and depth of these surface defects, this research analyzes the variations in the spatial distribution of the laser scattering field. A sophisticated dark-field microscopy detection system was designed and constructed. We use image stitching, denoising, segmentation, and recognition, enabling the capture of high-resolution scattered light images from standard defect samples. To effectively measure the depth of surface defects, photomultiplier tube (PMT) was used to measure the scattered light intensity. We compare the experimental scattering curves with those derived from the simulation database and determine the reference depth of the defects by evaluating multiple parameters. The results indicate that the relative measurement deviations of defects between experiment and simulation are around 8.01 % (BTDF) and 5.44 % (BRDF). The technique of the study provided a theoretical basis and reference for the scale calibration and nondestructive testing technology for the detection of surface defects of precision optical components.