High-contrast dark-field imaging method for detecting optical surface micro-defects

Abstract

Surface and subsurface defects significantly impact on the performance of optical components, especially on high-power laser optics, of which the damage threshold will be lowered by defects. Therefore, it is required to detect defects in optical manufacturing. In this paper, a novel dark-field microscopic imaging method, Circular-Aperture Microscopy (CAM), is proposed for defect detecting of optical surfaces. In CAM, an illuminating ray transmits through the optical surface and is blocked by a small obscuration attached on the objective. The scattered light by the defect propagates through the circular aperture formed by the obscuration, and forms a high-contrast image on the camera. Because the illuminating ray is blocked, CAM is a dark-field imaging method. Since the illumination light is incident perpendicularly to the surface, there is no shadow effect in CAM. The imaging results of scratches and pitted standard comparison plates, resolution transmission test plates, SiO2 suspensions and other samples show that CAM has the advantages of simple principle, high contrast, high resolution, and high precision, It can provide an effective method for defect detection and control in optical surface manufacturing.