 in Japan used continuous terahertz solid-state frequency multiplier with frequency of 540 GHz to conduct TS imaging on three letters "T", "H" and "Z" at different depths of post-it notes.The back projection method and wiener filter were used to reconstruct the spatial distribution of three letters.Terahertz fault chromatography can reconstruct the 3D distribution of the refractive index by reflecting the terahertz pulse at different depths in the sample.The depth distribution information of the refractive index can be obtained by analyzing the time delay of the peak value of the reflected pulse.