In short range applications such as in the outdoor screening of people for security purposes, passive millimetre wave imaging has several benefits: it is not necessary to artificially irradiate subjects, clothing is transparent, and images have similarities to those obtained visibly. Passive millimetre wave imaging has been developed largely with defence funding and recent research has outlined video frame rate imager concepts and produced demonstrators, which may be used for security screening. This paper will outline the application of these concepts to security screening.A Ka band mechanically scanned imaging system, with a frame rate of up to 12 Hz, designed for outdoor security screening is discussed. This system is based on folded conical scan technology which has been reported previously. It is constructed from low cost materials such as polystyrene and printed circuit board. The advantages of multiple over single frame imagery are discussed. It is concluded that multiple frame imagery offers considerable advantages because weapons are varied in size and shape, and may be positioned and oriented in many ways. A human observer can more easily interpret images when a series of images is presented showing the subject from different viewing angles.The optical properties of clothing samples have been determined and examples of their spectral reflectivity and transmission are presented. Transmission is close to constant for many samples from 60 to 150GHz, above which it decreases for some clothing materials. For example, the transmission of a cotton T-shirt is typically 95% and of a leather jacket up to 85% at lower frequencies. A model is presented for calculating the contrast of objects concealed under clothing and it indicates contrasts as large as 200 K for a weapon can be achieved outdoors. This contrast has been realised with a prototype system and example images are presented.
This paper describes a novel real time mechanically scanned passive millimetre wave imager. This imager produces a field of view of 40°x20° with diffraction limited performance and a 25Hz frame update rate. It is relatively inexpensive because the scene is imaged using 32 direct detection receivers with a frequency of operation from 28-33GHz. The compact antenna uses polarisation techniques to fold the beam and is constructed from readily available low cost materials.
No abstract
The threat in modern life necessitates the use of security systems in many areas. Systems, whether manual search or automated, need to be able to detect concealed munitions beneath clothing and in baggage. Systems which scan people, unlike baggage, must be safe to avoid damaging those who must be repeatedly scanned. Passive millimetre wave (mmw) systems have the ability to scan people through clothing to detect concealed objects without irradiating the individual. The performance of such systems is dependent on operating frequency, which is a trade-off between resolution, clothing transmission, and material visibility. Transmission and reflection spectra of clothing, skin, and other materials which may be worn under clothing, over the frequency range 60 to 500 GHz, are presented with their implications for operating frequency. The practicalities of imaging are discussed, the differences between the indoor and outdoor situation highlighted, and the limitations of the indoor case described. Imagery of persons with concealed objects, obtained using DERA's MITRE 94 GHz mmw imager, are presented.
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