Lateral-scanning through-focus microscopy for high-speed three-dimensional localization of subsurface defects

Accurate three-dimensional (3D) localization and detection of subsurface defects in transparent samples are crucial in precision manufacturing, process engineering, consumer electronics, semiconductor inspection, and related fields. However, conventional optical measurement techniques often struggle to achieve both a large field of view and high localization accuracy simultaneously. To this end, we propose a lateral-scanning through-focus microscopy. By tilting the optical axis at a specific angle relative to the sample surface, the system captures focused images of defects at varying depths within transparent samples across a single image. Combined with horizontal scanning, this approach enables 3D localization of subsurface defects over an extended field of view. Experiments conducted on SiO₂ samples under varying illumination conditions demonstrate that ring light illumination achieves sub-pixel accuracy in 3D defect localization, with a relative standard deviation of 0.77% for depth measurements. Furthermore, the system's wide-field 3D localization capability was further validated using microsphere samples, with the measurable range constrained only by the displacement stage's travel range.