Abstract
This paper is concerned with auto-focus of microscopes for the surface structure of transparent materials under transmission illumination, where two distinct focus states appear in the focusing process and the focus position is located between the two states with the local minimum of sharpness. Please note that most existing results are derived for one focus state with the global maximum value of sharpness, they cannot provide a feasible solution to this particular problem. In this paper, an auto-focus method is developed for such a specific situation with two focus states. Firstly, a focus state recognition model, which is essentially an image classification model based on a deep convolution neural network, is established to identify the focus states of the microscopy system. Then, an endpoint search algorithm which is an evolutionary algorithm based on differential evolution is designed to obtain the positions of the two endpoints of the region where the real focus position is located, by updating the parameters according to the focus states. At last, a region search algorithm is devised to locate the focus position. The experimental results show that our method can achieve auto-focus rapidly and accurately for such a specific situation with two focus states.
Highlights
In the field of materials science, the laser is widely used to micromachine transparent materials, including glass, crystal, polymer, etc. [1,2,3]
The focus state recognition (FSR) model and the squeeze adjustment algorithm are combined to tune the parameters in the endpoint search algorithm with memory and sorting (ESMS) for the improvement in accuracy and convergence speed
Once the two endpoint positions are found, the region search algorithm acts to get the real focus position. This method is a depth from focus (DFF)-based auto-focus method, in which the Sober operator is used for image definition measurement, and another important part of the DFF-based auto-focus method is the focus search algorithm, which is composed of ESMS, FSR, squash adjustment, and region search
Summary
In the field of materials science, the laser is widely used to micromachine transparent materials, including glass, crystal, polymer, etc. [1,2,3]. The light sources are placed between the object to be observed and the camera system, which is conducive to observing the surface of materials, especially for opaque materials. The light sources are placed behind the object to be observed. Due to the different positions of light sources, compared with reflection illumination, transmission illumination is more suitable to observe the interior of transparent materials. To achieve efficient laser micromachining of transparent materials from surface to interior, it is necessary to avoid the continuous switching of two illumination modes that interrupts micromachining and readjusts light parameters. In many applications, transmission illumination is of certain significance to auto-focus of microscope for surface structure of transparent materials
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