Fig. 1. Laser ultrasonic visualization nondestructive testing system. (a) Principle diagram; (b) physical map

Fig. 2. Welding test sample with simulated defect and scanning area. (a) Physical map; (b) size

Fig. 3. Laser ultrasonic visualization based on the reciprocity theorem of acoustic wave fields

Fig. 4. Comparison of ultrasonic wave propagation image of 57th frame before and after local statistic filtering. (a) Noisy image; (b) denoised image

Fig. 5. Comparison of (a) entropy and (b) average gradient under different frames before and after local statistic filtering

Fig. 6. Denoised image enhancement by differential operator. (a) Denoised image of 346th frame; (b) denoised image of 346th frame after image enhancement

Fig. 7. (a) Denoised image of 246th frame; denoised images of 246th frame after image enhancement with two order differential operators of (b) Od1, (c) Od2 and (d) Od3

Fig. 8. (a) Denoised image of 346th frame; denoised images of 346th frame after image enhancement with the two order differential operators of (b) Od1, (c) Od2 and (d) Od3

Fig. 9. Denoised images of 346th frame (a) before and (b) after incident wave suppression

Fig. 10. (a) Contour lines of weighting factor ω, regularization coefficient λ and average gradient; (b) contour lines of weighting factor ω, regularization coefficient λ and entropy; (c) contour lines of weighting factor ω, regularization coefficient λ and PSNR; (d) contour lines of weighting factor ω, regularization coefficient λ and SSIM

Fig. 11. Effect of (a) penalty function and (b) adjacent wave subtraction on incident wave suppression on 346th frame images; effect of (c) penalty function and (d) adjacent wave subtraction on incident wave suppression on 376th frame images