Peter R. Coward

Development of an illumination chamber for indoor millimeter-wave imaging

Passive millimetre-wave imaging provides the capability to detect objects concealed beneath clothing. In the past, this has been accomplished successfully outdoors, using the cold sky illumination to provide the majority of the scene contrast. However, many applications, such as airport security scanning, require a technology that can operate in an indoor environment. This paper describes work done to develop a safe and effective illumination source for indoor millimetre-wave imaging. Results from a prototype indoor millimetre-wave security scanner are presented, clearly showing hidden weapons and other objects.

Compact real-time (video rate) passive millimeter-wave imager

This paper describes a novel real time mechanically scanned passive millimeter wave imager. This imager produces a field of view of 40 degree(s) X 20 degree(s) with diffraction limited performance and a 25 Hz frame update rate. It is relatively inexpensive because the scene is imaged using 32 direct detection receivers with a frequency of operation from 28 - 33 GHz. The compact antenna uses polarization techniques to fold the beam and is constructed from readily available low cost materials.

Mechanically scanned real-time passive millimeter-wave imaging at 94 GHz

It is well known that millimetre wave systems can penetrate poor weather and battlefield obscurants far better than infrared or visible systems. Thermal imaging in this band offers the opportunity for passive surveillance and navigation, allowing military operations in poor weather. We have previously reported a novel real time mechanically scanned passive millimetre wave imager operating at 35GHz and in this paper a 94GHz variant will be described. This 94GHz imager has a field-of-view of 60° x 30° and has diffraction limited performance over the central two thirds of this field-of-view. It is relatively inexpensive because the scene is imaged using a linear array of direct detection receivers and compact folded optics. The receiver array has been constructed using indium phosphide monolithic microwave integrated circuits (MMICs) allowing high gain and low noise figure to be achieved. The compact optics consist of a polarisation sensitive mirror and a Faraday rotator. readily The mirror is constructed from expanded polystyrene, supporting a printed copper grid etched onto a PTFE/glass fibre substrate. These materials are low cost and readily available. The Faraday rotator is made from a commercial grade plasto-ferrite sandwiched between antireflection coatings. The optics produce a conical scan pattern and image processing is used to generate a raster scan pattern and to perform gain and offset corrections.

Passive millimeter-wave imaging in security scanning

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 millimeter 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 DERAs MITRE 94 GHz mmw imager, are presented.

Whole-body 35-GHz security scanner

A 35GHz imager designed for Security Scanning has been previously demonstrated. That imager was based on a folded conical scan technology and was constructed from low cost materials such as expanded polystyrene and printed circuit board. In conjunction with an illumination chamber it was used to collect indoor imagery of people with weapons and contraband hidden under their clothing. That imager had a spot size of 20mm and covered a field of view of 20 x 10 degrees that partially covered the body of an adult from knees to shoulders. A new variant of this imager has been designed and constructed. It has a field of view of 36 x 18 degrees and is capable of covering the whole body of an adult. This was achieved by increasing the number of direct detection receivers from the 32 used in the previous design to 58, and by implementing an improved optical design. The optics consist of a front grid, a polarisation device which converts linear to circular polarisation and a rotating scanner. This new design uses high-density expanded polystyrene as a correcting element on the back of the front grid. This gives an added degree of freedom that allows the optical design to be diffraction limited over a very wide field of view. Obscuration by the receivers and associated components is minimised by integrating the post detection electronics at the receiver array.

Image analysis for object detection in millimetre-wave images

Video-frame-rate millimetre-wave imaging has recently been demonstrated with a quality similar to that of a low-quality uncooled thermal imager. In this paper we will discuss initial investigations into the transfer of image processing algorithms from more mature imaging modalities to millimetre-wave imagery. The current aim is to develop body segmentation algorithms for use in object detection and analysis. However, this requires a variety of image processing algorithms from different domains, including image de-noising, segmentation and motion tracking. This paper focuses on results from the segmentation of a body from the millimetre-wave images and a qualitative comparison of different approaches is presented. Their performance is analysed and any characteristics which enhance or limit their application are discussed. While it is possible to apply image processing algorithms developed for the visible-band directly to millimetre-wave images, the physics of the image formation process is very different. This paper discusses the potential for exploiting an understanding of the physics of image formation in the image segmentation process to enhance classification of scene components and, thereby, improve segmentation performance. This paper presents some results from a millimetre-wave image formation simulator, including synthetic images with multiple objects in the scene.

Evaluation of a Passive Millimeter Wave (PMMW) imager for wire detection in degraded visual conditions

Ground vehicle tests have been performed to evaluate the performance of a Passive Millimeter Wave (PMMW) imager in reduced visibility conditions and in particular, the ability to detect power lines and cables. A PMMW imager was compared with Long Wave Infrared (LWIR) and visible imaging cameras. The three sensors were mounted on a Land Rover, together with GPS and digital recording system. All three sensors plus the GPS data were recorded simultaneously in order to provide direct comparisons. The vehicle collected imagery from a number of sites in the vicinity of Malvern, UK, in January, 2008. Imagery was collected both while the vehicle was stationary at specific sites and while it was moving. Weather conditions during the data collection included clear, drizzle, rain and fog. Imagery was collected during the day, at night, and during dusk/dawn transition periods. The PMMW imager was a prototype which operated at 94 GHz and was based on a conically scanned folded Schmidt camera and the LWIR and visible sensors were commercial off the shelf items.

Security scanning at 94GHz

It is well known that millimetre waves can pass through clothing. In short range applications such as in the scanning of people for security purposes, operating at W band can be an advantage. The size of the equipment is decreased when compared to operation at Ka band and the equipments have similar performance. In this paper a W band mechanically scanned imager designed for imaging weapons and contraband hidden under clothing is discussed. This imager is based on a modified folded conical scan technology previously reported. In this design an additional optical element is added to give a Cassegrain configuration in image space. This increases the effective focal length and enables improved sampling of the image and provides more space for the receivers. This imager is constructed from low cost materials such as polystyrene, polythene and printed circuit board materials. The trade off between image spatial resolution and thermal sensitivity is discussed.

Security scanning at 35 GHz

It has been known for some time that millimeter waves can pas through clothing. In short range applications such as in the scanning of people for security purposes, operating at Ka band can be an advantage. The penetration through clothing is increased and the cost of the equipment when compared to operation at W band. In this paper a Ka band mechanically scanned imager designed for security scanning is discussed. This imager is based on the folded conical scan technology previously reported. It is constructed from low cost materials such as polystyrene and printed circuit board. The trade off between image spatial resolution and the number of receivers will be described and solutions, which minimize this number discussed.

Detection of illegal passengers in lorries using a passive millimetre wave scanner

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.