Study on material-classification of objects detected by the THz passive body scanner for security screening

The terahertz (THz) passive body scanner is used for monitoring persons carrying concealed weapons and contrabands at such places as airports and stations. It takes images of radiation from objects and gives the information on positions, sizes and shapes of the object. However, it has not been enough for inspecting various illegal goods such as explosives, guns, knives, drugs and narcotics. We propose a method to analyze materials of the objects from the signal observed by the passive body scanner on the basis of knowledge on their emittances in order to improve the ability for screening. Using a passive THz body scanner, we performed measurements of various materials attached to a blackbody plate with a temperature of human body, concealed behind clothes. Experimental results show the detected objects can be certainly classified to metal or nonmetal. Moreover, we made a reflection measurement for the same objects by using a THz source. We discuss on the comparison between passive and active measurement by using the THz body scanner.