Fig. 1. Block diagram of defect detection algorithm

Fig. 2. Flow chart of image preprocessing and its effect

Fig. 3. Button defect-free image reconstruction and residual image with defects. (a) Original image; (b) defect-free image; (c) residual image

Fig. 4. Pixel value distribution of original and residual images. (a) Original image; (b) residual image

Fig. 5. (a) Residual image; (b) binary image

Fig. 6. Result diagrams of different defection methods. (a1)-(a3) Original images of three samples; (b1)-(b3) results by background substraction method; (c1)-(c3) results by linear regression method; (d1)-(d3) results by proposed method

Fig. 7. ROC curves by proposed method

Fig. 8. Effect of different parameter C on detection effect. (a) Original image; (b) detection effect at C=1; (c) detection effect at C=3; (d) detection effect at C=6

Fig. 9. Recognition rate of algorithm versus illumination