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A new wideband millimeter wave holographic imaging technique is under development for use in concealed weapons detection systems. This new wideband technique provides far superior images than single frequency holographic techniques on thick objects such as the human body. The wideband technique obtains fully focused images over a designated volume and provides excellent lateral and depth resolution. Using this method, a "3-D" volumetric hologram is gathered with a millimeter wave linear arrays, a mechanical scanner and a sweep frequency transceiver. The "3-D" volumetric hologram is then processed by high speed computational processors to reconstruct the fully focused image.Two prototype wide band millimeter wave holographic arrays have been developed at the Pacific Northwest Laboratory. The two arrays consist of sequentially switched 2 x 37 Ku Band (12.5 -18 0Hz) and 2 x 64 Ka Band (26.5 -40 GHz) systems which are coupled to high speed sweep frequency heterodyne transceivers. The arrays are used to obtain volumetric imaging data at high speeds by electronically sequencing arid frequency sweeping the array antennas along one dimension while performing a mechanical scan along the other dimension. The current prototype system scans an aperture the size of a large human body in about one second. Extensive laboratory testing has been performed with people carrying various concealed weapons and innocuous items with both imaging arrays during the first quarter of 1995. INTRODUCTIONMany significant milestones have been accomplished in the development of this new personnel screening technology. It was demonstrated early on that millimeter waves could readily penetrate clothing barriers and that holographic processing could obtain very high resolution images of concealed plastic and metallic weapons". An early, significant milestone was the development of a single frequency holographic scanned linear array that operated at 35 GHz. This first-of-a-kind surveillance system, which was field tested at Sea-Tac International Airport in the Spring of 1993, was capable obtaining images of humans in near real-time. There were, however, shortcomings to the engineering prototype and further development was required. One shortcoming was the narrow depth of focus in which one part of the body was in focus while other parts were left out of focus.More recently, a wideband holographic image reconstruction algorithm has been developed to solve the problem of narrow depth of focus. This new wideband algorithm focuses over a designated three-dimensional volume so that each image plane is fully in focus. Other recent improvements include the development of 12.5 -18 GHz and 26.5 -40 GHz wideband holographic arrays and high-speed sweep frequency heterodyne transceivers. These systems have been used to obtain 3600 images of humans taken at 5 to 10 degree increments. Unlike X-rays, millimeter waves will not penetrate the human body, therefore full 360 degree coverage is required for complete surveillance. The 5 to 10 degree incremental images ha...

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AB&RACTMillimeter wave holographic imaging systems capable of imaging through clothing to detect contraband, metal, plastic, or ceramic weapons may provide a practical solution to personnel inspection needs in mass transportation centers. TraditiOnal inspection systems, such as metal detectors and x-ray imaging systems, have limitations for the detection of concealed weapons. Metal detectors are limited because they cannot detect plastic weapons and x-ray imaging systems are limited in use due to radiological health considerations. A prototype millimeter wave holographic surveillance system has been developed and demonstrated at the Pacific Northwest Laboratory (PNL). The prototype millimeter wave holographic surveillance system developed at PNL consists of a sequentially switched 2 x 64 elemsnt array coupled to a 35 GHz bi-static transceiver. The sequentially Switched array of antennas can be used to obtain the holographic data at high s by electronically sequencing the antennas along one dimension and performing a mechanical scan along the other dimension. A one-dimensional mechanical scan can be performed in about one second. The prototype system scans an aperture of 0.75 by 2.05 m. This system has been demonstrated and images have been obtained on volunteers at Sea-Tac InternatiOnal airport in Seattle, Washington.