300 GHz THz walk-through body scanner
A 3-D terahertz imaging system for security gate applications by terahertz wave in 275−305 GHz has been developed and experimentally demonstrated to detect concealed objects carried by pedestrians. This system performs 3D measurement by combining depth detection using FM-CW radar, vertical scanning of the detection spot using a 1D high-speed mechanical beam scanner, and horizontal movement of the irradiated area and the detection spot as the pedestrian walks. The high-speed beam scanner consists of an F-Theta telecentric lens and a polygon mirror. It achieves a vertical line scan rate of 142 lines/sec. The spatial resolution was about 10 mm, consistent with the design value, and the depth resolution was about 10 mm, twice the theoretical value estimated from the FM-CW radar principle. The depth-dependent lateral distortion in 3D images, so call telecentricity, measuremed by the body scanner imaging system was also evaluated, as it is important for oblique 3D image of pedestrians. As a result, images with the same magnification were obtained over a range of more than 500 mm of depth of focus. Finally, the detection of concealed objects carried by pedestrians was demonstrated, showing that the system can cope with a pedestrian walking at a 7 km/sec.