In many applications it is important to find objects that are carried by people and which can be concealed on their bodies. Detection systems are known which require an individual to pass through a fixed detector passageway (portal) equipped with magnetic sensors. When metallic objects of sufficient mass pass through the passageway, a warning signal is activated because a change in magnetic flux is detected. This type of system either detects or does not detect a metal object and sometimes makes no determination relative to the amount of metal present. Keys, jewelry, watches, and metal-framed eyeglasses may all trigger such a system.
While such magnetic detectors are capable of detecting metal objects passing through the detector passageway, such systems cannot determine whether the detected metal object is a threat object (e.g., a knife or a gun) or an innocuous (non-threat) object (e.g., keys, coins, jewelry, belt buckles). Moreover, these detection systems do not pinpoint the location of the metal objects on the individual's body. Likewise, these systems are useless in detection of modern threats posed by plastic and ceramic items and plastic and liquid explosives.
Moreover, there are situations when a person under surveillance need not be aware that he or she is being monitored. In such a case, the portal systems described above are not appropriate.
Another type of detection system also employs imaging techniques that acquire images of the detection space and then display the image to an operator. Moreover, imaging detection systems can use image recognition methods to convert the image into an indication (such as an audible or visual alarm). In order to recognize a specific threat object, the system has to compare the object with an electronic catalog of images of uniquely-shaped threat objects. In this case, unique orientations of the objects are also important, because an object may have a significantly different appearance if viewed from the sides, the top, etc. The observed uniqueness of a threat object also essentially depends on the image resolution of the system.
A good example of a concealed object imaging system that exhibits high image clarity is the cabinet x-ray system used at airports to screen carry-on luggage. Although very effective for certain security tasks, X-ray imaging can pose a serious health risk to living organisms due to X-ray exposure, and is therefore unacceptable to the public. On the other hand, RF radiation in the microwave and millimeter-wave (e.g., 5 GHz to 1 THz) range offers a possible solution for concealed weapon detection and imaging, because the RF radiation can easily penetrate clothing and also represents no known health threat to humans at moderate power levels (see, for example, U.S. Pat. No. 6,791,487 to Singh; U.S. Pat. No. 6,876,322 to Keller; U.S. Pat. No. 6,992,616 to Grudkowski et al; and U.S. Pat. No. 6,965,340 to Maharav at al.).
Specifically, U.S. Pat. No. 6,791,487 describes imaging methods and systems for concealed weapon detection. In an active mode, a target can be illuminated by a wide-band RF source. A mechanically scanned antenna, together with a highly sensitive wide-band receiver can then collect and process the signals reflected from the target. In a passive mode, the wide-band receiver detects back-body radiation emanating from the target and possesses sufficient resolution to separate different objects. The received signals can then be processed via a computer and displayed on a display unit thereof for further analysis by security personnel.
A wideband millimeter-wave imaging system is described in the article titled “Concealed explosive detection on personnel using a wideband holographic millimeter-wave imaging system,” by Sheen et al., Proceedings of SPIE—The International Society for Optical Engineering, V. 2755, 1996, PP. 503-513. To form an image, Sheen et al. use a linear array of 128 antennas that can 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 image reconstruction algorithm that 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.
U.S. Pat. No. 6,965,340 describes a security inspection system including a portal through which a human subject is capable of walking and a scanning panel including an array of antennas that are programmable with a respective phase delay to direct a beam of microwave illumination toward a target on the human subject. The antennas are further capable of receiving reflected microwave illumination reflected from the target. A processor is operable to measure an intensity of the reflected microwave illumination to determine a value of a pixel within an image of the human subject. Multiple beams can be directed towards the human subject to obtain corresponding pixel values for use by the processor in constructing the image.
U.S. Pat. No. 6,992,616 describes active imaging systems including an antenna apparatus configured to transmit toward and receive from a subject in a subject position, electromagnetic radiation. The antenna apparatus may move in a partial or continuous loop around the subject, toward or away from the subject, or in an opposite direction to an associated antenna apparatus. Antenna units in the antenna apparatus may be oriented at different angular positions along an array. Antenna arrays may also be formed of a plurality of array segments, and a group of arrays may be combined to form an antenna apparatus.