Research On Defect Detection Method For Curved Optical Surface

With the development of optical manufacturing and measurement methods,precision optical elements are used extensively in various fields.During the processing and use of optical components,surface defects will inevitably occur.As the beam scattering and energy loss caused by the surface defects of optical elements will reduce the lifetime of optical elements and the performance of optical systems,it is important to detect and evaluate the surface defects.At present,the detection technology for planar optical components is relatively mature.However,several technical challenges remain in the surface defect detection of curved optical elements.In this paper,a curved defect detection experimental setup based on the dark-field imaging principle is proposed to solve the problems in curved optical elements defect detection.Its main contents are as follows:(1)This paper analyzes the classification and detection standards of optical element surface defects,and then describes the main detection methods of optical element surface defects and their advantages and disadvantages.Finally,the necessity of defects detection on large-diameter curved optical surfaces has been proposed.(2)We study various scattering sources on the surface of the optical element and analyzes their related scattering theory.And then we established a dark-field scattering model and analyzed its feasibility in optical elements' defect detection,which provide a theoretical basis for the defect detection experimental setup.(3)We simulated the scattered light of surface defects with the mothed of FDTD,which provides a theoretical basis for the design of the lighting system.(4)Based on the analysis of the principle of dark-field scattering imaging,we constructed a complete surface defect detection experimental setup for curved optical elements.The system is mainly composed of three parts: a position adjustment module for the optical elements,an illumination module,and an imaging module.We carried out relevant analysis on the parameters of the imaging module and completed the selection of lenses and CCD camera.At the same time,we have completed the path planning for the detection of large-diameter optical elements.(5)We tested different types of components with different calibers,and the experimental results verified the reliability of our experimental setup.In addition,we explored the influence of lighting module on defect detection through the comparative experiment of ring LED light source and high-brightness LED point light source.Finally,the narrowest width of defects can be detectable by our experimental setup is 2.8?m,and the shallowest depth is 1.5nm.(6)In the end,we perform image processing and recognition on the defect images.We compared different filtering methods and edge detection methods,and then we used the maximum entropy threshold segmentation method to binarize the image,and used the minimum circumscribed rectangle method to complete the extraction of the defection image.In this paper,we have realized the surface defects detection of large-diameter curved optical elements with the mothed of dark-field imaging principle,completed the defect detection from theory to experiments.This experimental setup can automatically detect the surface defects of ultra-precision planar optical elements with a maximum aperture of 200 mm and curved optical elements with a maximum aperture of 150 mm.