Study On Microscopic Autofocus Technology Based On Digital Image Processing

In the field of micro-nano inspection,it is necessary to inspect the line width and defects of the micro-nano structure.When inspecting large-size micro-nano devices,fast and stable focus scanning is required.The traditional manual focusing method takes a long time and is low in efficiency,and cannot meet the requirements of high precision and stability.With the gradual development of precision instruments in the direction of automation and intelligence,higher requirements have been placed on digital microscopes,and various autofocus technologies based on digital microscopes have emerged.Among them,the autofocus method based on image processing is the most widely used.This method can be divided into two major categories: Defocus Depth Method and Depth-of-Focus Method.In this paper,the depth-of-focus method is selected for research.Based on the research of autofocus technology at home and abroad,this article takes the microscopic imaging system as the research object,and carries out a series of practical applications in the three important links of the focus evaluation function,focus window,and focus search algorithm in the depth of focus method.the study.Firstly,the theory of microscope imaging principle,optical imaging model,point spread function and optical transfer function are introduced,which lays the theoretical foundation of autofocus technology.Secondly,the focus evaluation function is studied in depth.Aiming at the problem of poor focus stability when the spatial focus evaluation function focuses on micro-nano structures with multiple edge directions,combined with the multi-channel characteristics of HVS,a new focus evaluation function Bre2d＿Rob function is proposed.The experimental results show that the focus evaluation function can It is suitable for microscopic images with multiple edge directions,and the focus stability is better.Again,the selection of the focus window is studied.A variable step size focusing window selection method is proposed.Compared with the center window method,it can adapt to the situation when the imaging subject deviates from the center area.Then,a search strategy for the autofocus system was formulated.The traditional hill-climbing search method is improved,and the entire focus search process is divided into two stages: coarse search and fine search.In the fine search stage,three consecutive images are used to determine the search direction to improve the anti-interference and search efficiency of the search algorithm.Finally,combined with practical applications,an overall autofocus scheme with a PC as the image processing unit was proposed,and an autofocus experiment platform was built.A large number of experimental verifications were carried out to test the focusing effect,accuracy and stability of the autofocus system.The experimental results show that the auto-focus method proposed in this paper is practical,with a high focus success rate,and has certain practical engineering application value.