Detection of Text Information Hidden in an Envelope Using Terahertz Imaging

In order to meet the application requirements of non-contact and non-destructive acquisition of hidden object information, such as cultural relics protection, biomedical and safety inspection, terahertz tomography technology based on linear frequency modulation technology has been developed. The wide-band all-solid-state device, combined with optical collimation system and mechanical scanning positioning device, is used to obtain the depth information of the target. Terahertz imaging results show that pencil handwriting and graphics hidden on A4 paper in express envelope can be clearly distinguished and the information on different pages can be distinguished according to the difference of phase distance.