Concealed explosive detection on personnel using a wideband holographic millimeter - wave imaging system

A novel wideband millimeter-wave imaging system is presently being developed at Pacific Northwest National Laboratory (PNNL) that will allow rapid inspection of personnel for concealed explosives, handguns, or other threats. Millimeter-wavelength electromagnetic waves are effective for this application since they readily penetrate common clothing materials, while being partially reflected from the person under surveillance as well as any concealed items. To form an image rapidly, a linear array of 128 antennas is used to electronically scan over a horizontal aperture of 0.75 meters, while the linear array is mechanically swept over a vertical aperture of 2 meters. At each point over this 2-D aperture, coherent wideband data reflected from the target is gathered using wide-beamwidth antennas. The data is recorded coherently, and reconstructed (focused) using an efficient image reconstruction algorithm developed at PNNL. This algorithm works in the near-field of both the target and the scanned aperture and preserves the diffraction limited resolution of less than one-wavelength. The wide frequency bandwidth is used to provide depth resolution, which allows the image to be fully focused over a wide range of depths, resulting in a full 3-D image. This is not possible in a normal optical (or quasi-optical) imaging system. This system has been extensively tested using concealed metal and plastic weapons, and has recently been tested using real plastic explosives (C-4 and RDX) and simulated liquid explosives concealed on personnel. Millimeter-waves do not penetrate the human body, so it is necessary to view the subject from several angles in order to fully inspect for concealed weapons. Full animations containing 36-72 frames recorded from subjects rotated by 5-10°, have been found to be extremely useful for rapid, effective inspection of personnel.