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The schlieren method was the most common method for measuring the far-field focal spot in high power laser facility. The reconstruction of far-field focal spot was used by manual and autocorrelation matching method usually. Although the problem of the automatic reconstruction of focal spot was solved by the autocorrelation matching method, it searched the best matching point in the whole region of the mainlobe cutting image, which would greatly reduce the efficiency of the reconstruction of far-field focal spot. To solve this problem, a fast method for searching the best matching point based on steepest descent method was proposed in this paper. Firstly, the mainlobe image and sidelobe image were cut into the mainlobe cutting image and sidelobe cutting image, which were named as cutzb and cutpb respectively, and the size of cutting images was 300*300.Then a new image that a circle the same size as the schlieren ball was dug from cutzb image was named as cutzb', while cutzb' and cutpb were converted into two column vectors respectively, and the SAM values of the two vectors are calculated. Secondly, calculated the gradients of 8 neighborhoods of current pixel, the next searching position was the neighborhood located in the direction of the largest gradient (the largest decrease) of 8 neighborhoods while using the steepest descent method. After all the SAM values of the two vectors that met requirement were searched. The upper left corner of mainlobe cutting image was the best matching point when the SAM value was minimal. Finally, according to the best matching point, the far-field focal spot was merged by the mainlobe image and sidelobe image, and the weighted average method was applied to fuse the stitching area of the merged image. The experimental results show that the method can not only realize the automatic reconstruction of the measurement of far-field focal spot while using the schlieren method, but also greatly improve the efficiency of the reconstruction of the far-field focal spot, and realize the accurate measurement of the far-field focal spot in the high dynamic range.
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