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Transport operators to the UK have responsibility for ensuring that they do not transfer unauthorised people into Great Britain. A common way for people to try and enter the UK is by passage in the freight compartments of lorries. In Europe 90% of lorries have non-metallic sides which are transparent to millimetre wave (MMW) radiation. A 35 GHz linescan imaging system has been used to detect stowaways hidden in the rear of lorries. A video based sensor is used to provide vehicle speed information to the passive MMW linescan system, allowing a MMW image to be built up as the vehicle passes. Contrast is increased by the use of a large reflector panel on the opposite side of the vehicle to the sensor. This panel reflects the sky, which is cold millimetrically, through the lorry in the direction of the sensor. The linescan operation of the system does not require passing vehicles to stop, allowing all non-metallic sided vehicles to be scanned with minimal interference to flow patterns. The system has achieved considerable success since it entered operation, detecting several hundred stowaways per month.

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.