Fig. 1. Schematic diagram of the hot-image formation process.

Fig. 2. Locations and peak intensities of hot images. (a) Two hot-image planes are both located at ; (b) the extracted intensity distributions and lateral positions of hot images.

Fig. 3. Evolution of gray value of the diffraction ring and the luminance disk. (a) Diffraction ring image; (b) luminance disk (matching template).

Fig. 4. Gradient direction fields used to indicate the evolution of gray value. (a) Gradient direction field of the diffraction ring; (b) gradient direction field of the luminance disk.

Fig. 5. Flow chart of the GDM method.

Fig. 6. Simulated diffraction rings and the recognition results. (a)–(d) Simulated diffraction rings with different overlap degrees; (e)–(h) similarity distributions and the detected centers.

Fig. 7. Experimental setup for the diffraction optical path that uses SLM to simulate a defect.

Fig. 8. Diffraction ring images and similarity distributions. (a)–(e) Diffraction ring images with different compression ratios; (f)–(j) similarity distributions and the detected diffraction center labeled with a red circle.

Fig. 9. Diffraction ring image and similarity distributions. (a) The collected diffraction ring image at 41 cm; (b) similarity distribution with six initially detected centers; (c) similarity distribution with two centers left after cluster analysis.