Outdoor passive millimetre wave security screening

In short range applications such as 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. The paper outlines 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 150 GHz, above which it decreases for some clothing materials. 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.