Fig. 1. Principle of nondestructive detection by infrared thermography method

Fig. 2. Structure of mural

Fig. 3. Photograph of east wall

Fig. 4. Surface temperature of mural after square wave excitation. (a) Original thermal image of local mural in east wall; (b) temperature rise after square wave excitation

Fig. 5. Local image 1 of east wall. (a) Visible photograph; (b) original thermal image 338^th frame; (c) phase image of PPT(frequency is 0.01 Hz)

Fig. 6. Local image 2 of east wall. (a) Visible photograph; (b) original thermal image 32^th frame; (c) phase image of PPT(frequency is 0.02 Hz)

Fig. 7. PCT results of local murals in east wall. (a) PC2; (b) PC5; (c) PC7

Fig. 8. PCT results of red figure image on east wall. (a)Visible photograph; (b) original thermal image 386^th frame; (c) PC2; (d) PC3; (e) PC5; (f) PC6

Fig. 9. Thermal tomography results of red figure image on east wall at different depths. (a) Visible image; (b) surface; (c) 2.40 mm; (d) 8.45 mm; (e) 10.78 mm; (f) 13.87 mm

Fig. 10. Thermal tomography results of local murals on east wall at different depths. (a) Visible image; (b) surface; (c) 2.40 mm; (d) 8.45 mm; (e) 10.78 mm; (f) 13.87 mm

Fig. 11. Complete mosaic results of three dimensional thermal tomography at different depths. (a) Endothermic map of complete mural; (b) 1.47 mm; (c) 4.57 mm; (d) 7.68 mm; (e) 10.78 mm