Fig. 1. Schematic diagram of terahertz wave propagation process

Fig. 2. Glucose tablets of different roughness

Fig. 3. Schematic diagram of the reflection mode measurement method. Where, A1, A2 and A3 stands for the focusing lens, BS stands for beam splitter, M1, M2, M3, M4, M5 and M6 stands for the plane mirror, PM1, PM2, PM3 and PM4 stands for the off-axis parabolic mirror, GaAs stands for the gallium arsenide photoconductive antenna, WP stands for the Wollaston prism, Lock stands for Lock-in amplifier

Fig. 4. Reflection spectrum of terahertz waves incident vertically on the surface of smooth, 60 and 360 mesh sandpaper-pressed samples

Fig. 5. Reconstruction of a smooth-surface spectrum from a rough-surface of 60

Fig. 6. Reconstruction of a smooth-surface spectrum from a rough-surface of 360