Fig. 1. Coal and gangue laser speckle imaging system

Fig. 2. Coal and gangue laser speckle images and grayscale 3D surfaces. (a) Laser speckle image of coal; (b) laser speckle image of gangue; (c) grayscale 3D surface of coal laser speckle image; (d) grayscale 3D surface of gangue laser speckle image

Fig. 3. Three channel component diagrams in Lab space. (a) a channel; (b) b channel; (c) L channel

Fig. 4. Segmentation process. (a) a channel image; (b) binary image; (c) morphological operation; (d) mask image

Fig. 5. Area of interest

Fig. 6. Texture features of gray-level size zone matrix. (a) Zonal correlation; (b) entropy of neighborhood gray-level; (c) big area emphasis; (d) small area emphasis; (e) gray level non-uniformity; (f) size-zone non-uniformity; (g) speckle areal ratio

Fig. 7. Texture features of gray-level co-occurrence matrix. (a) Angular second moment; (b) entropy; (c)contrast ratio; (d)inverse distance difference

Fig. 8. Gray features of laser speckle images. (a) Mean; (b) standard deviation; (c) skewness; (d) kurtosis

Fig. 9. Recognition accuracy of scattered and natural light images under different illumination levels