焦距测量中线纹清晰度评价函数的研究  

Sharpness Evaluation Function for Line Patterns in Focal Length Measurement

作　　者：刘济豪 卢荣胜[1] 张紫龙 张艾琳 Liu Jihao;Lu Rongsheng;Zhang Zilong;Zhang Ailin(School of Instrument Science and Opto-Electronics Engineering,Hefei University of Technology,Hefei 230009,Anhui,China)

机构地区：[1]合肥工业大学仪器科学与光电工程学院,安徽合肥230009

出　　处：《光学学报》2023年第9期117-126,共10页Acta Optica Sinica

基　　金：国家自然科学基金(51875164)。

摘　　要：当自动对焦用于玻罗板线纹等方向性强的目标时,线纹方向的不确定性导致一些清晰度函数的鲁棒性降低甚至无法找到对焦位置。针对上述问题,通过Hough直线检测得到离焦线纹的角度,构建一种基于Hough变换的线纹清晰度评价函数,该函数的灰度算子垂直于线纹,对线纹方向变化具有较强的鲁棒性。为了比较线纹方向对清晰度函数性能的影响,对5组不同角度的线纹图像进行评价,得到清晰度曲线,并引入平缓区标准差等参数对曲线进行定量评定。实验结果表明,与传统的评价函数相比,基于Hough变换的评价函数的平缓区均值减小59.85%,标准差减小46.05%,对焦偏差减小92.63%,平均运行时间减少36.37%,镜头焦距的相对误差减小62.12%。Objective When the focal length of a lens is measured by the magnification method,a microscope is used to obtain a line pattern image of the Perot plate.Line pair spacing is then measured to determine the focal length of the lens and the coordinates of its focal point.Consequently,the sharpness of the line pattern image,namely,the focusing accuracy,affects the measurement accuracy of the focal length and focal coordinates of the lens.Due to its poor accuracy,inefficiency,and subjectivity,manual focusing has been gradually replaced by autofocusing.Currently,a grayscale-based evaluation function is frequently employed to determine the sharpness of a microscopic image.Although increasing the calculable directions of the evaluation function in a haphazard manner may improve the robustness of the function to some degree,it considerably prolongs the focusing time.To address the aforementioned issue,this study,starting from a line pattern's grayscale distribution,designs an evaluation function that retains favorable robustness and focusing accuracy without extending the focusing time when the angle of the line pattern varies.Methods The direction of the line pattern is correlated with the grayscale distribution of the line pattern's edges.Grayscale variation is not apparent in the direction along the line pattern.In contrast,the grayscale value varies drastically in the direction perpendicular to the line pattern.A larger amplitude of the grayscale operator in the evaluation function corresponds to a smaller effect of noise on the evaluation function.Consequently,obtaining the angle of the line pattern and introducing the evaluation function are applicable to the autofocusing of line patterns.Specifically,Hough transform-based line detection is performed on the first image acquired(the image with a large defocusing amount).Being highly resistant to interference,Hough transform-based line detection is able to overcome negative effects,such as edge loss due to defocusing and more precisely determine the angle of a blurred

关 键 词：测量 自动对焦 清晰度评价 焦距测量 放大率法 HOUGH变换 

分 类 号：TP391.41[自动化与计算机技术—计算机应用技术]