Fig. 1. Sample of sunflower seeds
(a): Normal seed; (b): Damaged kernel; (c): Worm-eaten kernel; (d): Empty seed

Fig. 2. Experimental apparatus
(a): Terahertz pulse spectrometer; (b): Terahertz transmission imaging attachment

Fig. 3. Terahertz transmission images of four categories of sunflower seeds
(a): Normal seed; (b): Damaged kernel; (c): Worm-eaten kernel; (d): Empty seed

Fig. 4. Time-domain spectra corresponding to different positions of sunflower seed

Fig. 5. Terahertz image filtering results of insect-eroded sunflower seeds
(a): 5×5 template mean filtering;(b): 5×5 template median filtering; (c): NML filtering

Fig. 6. Results of morphological filtering of insect-eroded sunflower seeds terahertz image
(a): Original image; (b): One dilatation result;(c): External gradient; (d): Morphological gradient

Fig. 7. Morphological filtering results of sunflower seed images with different states
(a): Normal seed; (b): Damaged kernel;(c): Worm-eaten kernel; (d): Empty seed

Fig. 8. K-means image segmentation results of sunflower seed images with different states
(a): Normal seed; (b): Damaged kernel;(c): Worm-eaten kernel; (d): Empty seed