基于滤波差分的双阈值弱划痕提取算法

高质量光学元件表面缺陷中存在一些深度较浅或者宽度较窄的划痕,在暗场成像检测中,该类划痕产生的散射光灰度值很低,甚至淹没在背景光中,很难被目视或常规机器视觉识别,造成划痕缺陷的漏检。针对该问题,以既有的疵病检测系统为基础,根据划痕灰度的等级特征,提出双阈值法分类处理划痕缺陷。在低阈值的弱划痕处理中,根据弱划痕和背景的频率特征以及空间对比度特征,设计了频域滤波及背景差分算法。通过空间域以及频率域的滤波处理,排除高频噪声以及高亮度噪声,根据几何特征等提取弱划痕图像中的复杂背景。经差分处理后,提取弱划痕并增强对比度,最后与正常灰度级划痕信息一同通过高阈值(正常阈值)进行后续划痕的特征提取,即得到所有的划痕信息,为划痕缺陷总长度计算以及最大长度的分级判定奠定基础。实验结果表明,该算法避免了过低二值化阈值引入的背景等不规则噪声,使得划痕与背景的对比度大大增强。目前该算法已经应用于惯性约束聚变系统中大口径光学表面划痕的定量检测,并且使长度计量的准确度已提升到约80%。

Dual-threshold algorithm study of weak-scratch extraction based on the filter and difference

There are some scratches of shallow depth (about 25 nm) or narrow width (collectively referred to as weak scratches) during defects detection on the super-smooth optical components. In the dark field imaging detection, the scattered light of the weak scratches produces such a low gray value, even masked in the background light, as a result, they are very difficult to identify by visual or conventional machine vision, and could be undetected. To solve this problem, based on the existing surface defects evaluation system (SDES), according to the scratches gray features, the dual-threshold classification method to extract scratches defects is proposed. In low threshold scratches processing, according to the frequency characteristics of the weak scratches, background contrast and spatial characteristics, the algorithm of frequency domain filtering with background difference is adopted. By spatial domain and frequency domain filtering algorithms, interference points and high luminance noise points are eliminated. In addition, background can be extracted since its geometric characteristics are different from weak scratches. Further, weak scratches with enhanced contrast are extracted, and finally, these scratches are used in high threshold information extraction of subsequent scratches feature together with the normal gray level scratches. Thus all the scratches information is obtained for calculating the total length of scratches and grading of defects maximum length. Experimental results show that the algorithm proposed avoids introducing irregular background noise by a low binarization threshold, and also improves the contrast of scratches and background. At present, the algorithm has been applied to inertial confinement fusion systems in large aperture optical surface scratches quantitative detection, and the length detection accuracy has been raised to about 80%.