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.
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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