Fig. 1. Electromagnetic spectrum

Fig. 2. Experimental setup for terahertz time-domain spectroscopy

Fig. 3. Key reconstruction images：（a）unwrapped，（b）in a vinyl box，（c）in a foam rubber cardboard box^［20］

Fig. 4. Cross section of the time domain imaging profile

Fig. 5. Terahertz spectra of azurite，malachite and verdigris^［2］

Fig. 6. Terahertz spectra of different pigments^［23］

Fig. 7. Electromagnetic wave penetrates the thickness of paintings

Fig. 8. Terahertz time domain imaging of leaves^［41］

Fig. 9. （a）Gold coating detected by terahertz time domain imaging，（b）gypsum/canvas/gypsum structure，（c）wood floor and canvas textures reconstructed at different time delays and spatial resolutions^［44］

Fig. 10. Image of a butterfly extracted between the gypsum layers（gypsum/graphite/gypsum）^［34］

Fig. 11. Test panel with strokes of（a）umber covered with lead white，（b）cross-section THz map of the umber/lead white interface，（c）paint layer thickness^［45］

Fig. 12. （a）Visible image of scanning region，（b）X-ray imaging of scanning region，（c）peak-peak time parameter terahertz image，（d）terahertz frequency integrated image，（e）terahertz image of 0.55-0.62THz，（f）false color imaging^［47］

Fig. 13. （a）Terahertz transmission image of crack area，（b）terahertz time domain waveform of different selected points^［51］

Fig. 14. Cross-section imaging of oil painting，（a）under terahertz B-SCAN，（b）OCT imaging at the same position，（c）terahertz imaging after algorithm combination^［68］

Fig. 15. （a）LF Raman of red lead，（b）LF Raman of cinnabar，（c）LF Raman of proportional mixture of red lead and cinnabar^［67］