A New Stitching Method for Dark-Field Surface Defects Inspection Based on Simplified Target-Tracking and Path Correction.

Chen Chen,Li Li,Sui Sui

doi:10.3390/s20020448

Abstract

A camera-based dark-field imaging system can effectively detect defects of microns on large optics by scanning and stitching sub-apertures with a small field of view. However, conventional stitching methods encounter problems of mismatches and location deviations, since few defects exist on the tested fine surface. In this paper, a highly efficient stitching method is proposed, based on a simplified target-tracking and adaptive scanning path correction. By increasing the number of sub-apertures and switching to camera perspective, the defects can be regarded as moving targets. A target-tracking procedure is firstly performed to obtain the marked targets. Then, the scanning path is corrected by minimizing the sum of deviations. The final stitching results are updated by re-using the target-tracking method. An experiment was carried out on an inspection of our specially designed testing sample. Subsequently, 118 defects were identified out of 120 truly existing defects, without stitching mismatches. The experiment results show that this method can help to reduce mismatches and location deviations of defects, and it was also effective in increasing the detectability for weak defects.

Highlights

Large-aperture precision optics are widely used in various fields, such as the National IgnitionFacility (NIF), inertial confinement fusion system (ICF), ultra-high-power laser systems, shortwave optics, photo-lithography system, etc. [1,2,3,4,5]
The camera-based dark-field imaging system has begun to be used for inspecting defects of large-aperture optical surfaces in recent years [6,7,8,9,10]
Compared with other defect inspection methods, such as atomic force microscopy (AFM) and scanning electron microscopy (SEM), this camera-based inspection system presents a good balance between working performance and time consumed [11,12,13,14]

Summary

Introduction

Large-aperture precision optics are widely used in various fields, such as the National IgnitionFacility (NIF), inertial confinement fusion system (ICF), ultra-high-power laser systems, shortwave optics, photo-lithography system, etc. [1,2,3,4,5]. The camera-based dark-field imaging system has begun to be used for inspecting defects of large-aperture optical surfaces in recent years [6,7,8,9,10]. Through sub-aperture scanning and stitching, the dark-field imaging system is able to inspect micrometer-level defects on large optical surfaces of tens of millimeters or even hundreds of millimeters over a relatively short period of time. Compared with other defect inspection methods, such as atomic force microscopy (AFM) and scanning electron microscopy (SEM), this camera-based inspection system presents a good balance between working performance and time consumed (especially for large optics) [11,12,13,14].

Methods

Findings

Discussion

Conclusion