Fig. 1. Imaging principle of laser scanning confocal microscope. (a) Device principle; (b) pinhole structure; (c) horizontal and vertical scanning

Fig. 2. Two modes of silica detected by laser scanning confocal microscope. (a) Scattering mode; (b) fluorescence mode

Fig. 3. Typical subsurface defects collected by confocal microscope. (a) Pitting defect; (b) pit defect

Fig. 4. Processing effects of Fig. 3(b) by using defect enhancement algorithm. (a) Enhanced image; (b) aggregated image

Fig. 5. Principle of double-threshold aggregation algorithm. (a) Original image; (b) processed image

Fig. 6. Principle of improved MC algorithm based on octree algorithm. (a) Establishment of volume data; (b) octree segmentation

Fig. 7. Flow chart of three-dimensional reconstruction algorithm

Fig. 8. Reconstruction of pit defect in simulation. (a) Simulated defect; (b) reconstructed defect in simulation; (c) residual of reconstruction

Fig. 9. Tomography image obtained from simulation. (a) Cross-section of defect from simulation; (b) confocal tomography image obtained from simulation

Fig. 10. Restoration ratio of point cloud after reconstruction by three different algorithms

Fig. 11. Detection results of subsurface defects. (a) Scratch defect; (b) microcrack defect; (c) pit defect

Fig. 12. Subaperture scanning images

Fig. 13. Reconstruction results of subsurface defects. (a)(b) Reconstruction results of scratch defects; (c)(d) reconstruction results of microcrack defects; (e)(f) reconstruction results of pit defects

Fig. 14. Destructive test results of subsurface defects of fused silica. (a) Etching test results of pit defects^[17]; (b) polishing-residual subsurface defects^[18]; (c) scratch defects^[19]; (d) pit defects^[20]