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This paper discusses the design of an improved passive millimeter wave imaging system intended to be used for military forward operating base security in degraded visual environments. A simple end-to-end model of such an imager is described, including a simple scene model based on transformations applied to visible and infrared imagery, optical aberrations, focal plane sampling, scan conversion, receiver performance and image processing algorithms. The use of such a model as a design tool is discussed, especially with regard to optimizing scan conversion and image processing algorithms. The expected performance of the latest imager design is predicted.

This paper discusses the design of an improved passive millimeter wave imaging system intended to be used for base security in degraded visual environments. The discussion starts with the selection of the optimum frequency band. The trade-offs between requirements on detection, recognition and identification ranges and optical aperture are discussed with reference to the Johnson Criteria. It is shown that these requirements also affect image sampling, receiver numbers and noise temperature, frame rate, field of view, focusing requirements and mechanisms, and tolerance budgets. The effect of image quality degradation is evaluated and a single testable metric is derived that best describes the effects of degradation on meeting the requirements. The discussion is extended to tolerance budgeting constraints if significant degradation is to be avoided, including surface roughness, receiver position errors and scan conversion errors. Although the reflective twist-polarization imager design proposed is potentially relatively low cost and high performance, there is a significant problem with obscuration of the beam by the receiver array. Methods of modeling this accurately and thus designing for best performance are given.

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.

This paper describes the compromise necessary between image sampling and thermal sensitivity in the design of passive millimetre wave imaging systems. The use of linear arrays of receivers in fast ( f/0.5) conical scanned systems, for real time imagery at 35GHz and 94GHz, presents particular difficulties. Analysis is presented which shows that it is not possible, with a single ro w of receivers, to achieve both high optical efficiency which eq uates to thermal sensitivity and good sampling of the image. Two methods are discussed for overcoming this limitation. In the first method, good efficiency and sampling are achieved simu ltaneously with multiple rows of receivers; however this method uses more receivers than predicted by basic theory. In the second method, the focal length in image space is increased by introducing an additional mirror in a Cassegrain configuration. This allows a single row of receivers to sample the image at the Nyquist rate, but results in an efficiency loss. The choice of which method to use will depend on the application. If the application requires the spatial resolution to be maximized and there is a high contrast scene, the Cassegrain method could be chosen. However, if high efficiency and resolution are both required a multiple row solution would be preferred. A 90GHz imager which uses 5 rows is discussed and contrasted to an alternative design that uses the Cassegrain approach. Keywords : Millimetre-wave imaging, spill over, gain, sampling, 94GHz.

It is well known that the image detected by a camera is only a subset of the information available in a scene. The finite aperture of an optical system leads to a certain amount of diffraction and image blurring, as described by Rayleigh's diffraction criterion. The image of the scene is then sampled with a finite spatial frequency by the detector(s), be they individual elements in a CCD sensor, light-sensitive cells in the eye, or grains on a photographic film. It is important, when designing a camera system, to ensure that as much of the available image information as possible is collected by the sensor. This involves an analysis of the spatial frequencies that are present in the blurred image, and the design of the image plane detectors in order to best sample the image information. This paper describes the image sampling requirements of millimetre-wave imagers, in particular mechanically scanned imagers that use a conical scanning technique. The trade-off between the requirement to make optimum use of limited image resolution with the high cost of millimetre-wave receiver arrays is considered. Different array layouts used in existing millimetre-wave imagers are presented.

It is well known that millimetre wave systems can penetrate poor weather and battlefield obscurants far better than infrared or visible systems. Imaging in this band offers the opportunity for passive surveillance and navigation allowing military operations in poor weather. We have previously reported a novel prototype real time mechanically scanned passive millimetre wave imager operating at 94GHz. This 94GHz imager has diffraction limited performance over the central two thirds of the 30 x 60 degrees field of view with and a 25Hz frame update rate. This paper reports the redesign of the prototype to operate in a helicopter environment. The vibration resulting from the helicopter rotors can typically range up to 3g peak at specific frequencies and is the major challenge for imaging systems. The mechanical design of the new imager is based on a rigid space frame utilising expanded polystyrene as a structural element to support the receiver array. The optical design of the imager has also been toleranced so that the impact of vibration on image quality is minimised. The use of novel design techniques and materials support a development path to a low cost low mass production unit.

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.

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.

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.

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.

A method of detecting weapons concealed under clothing using passive millimeter wave imaging is described. The optical properties of clothing are discussed and examples given of the spectral reflectivity and transmission. The transmission tends to be constant from 60 to 150 GHz above which it decreases for some clothing materials. 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 a metallic concealed weapon when hidden under clothing and it indicates contrasts as large as 200 K can be realized outdoors. The advantages of real time over static frame imagery are discussed. It is concluded that real time imagery offers considerable advantages as weapons can be very varied in size, position and orientation and movement offers vital clues to the human observer which aid the recognition process.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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.

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.

The long-term potential of terrestrial passive millimeter imaging is contingent upon demonstrating real-time imaging with a system that can be conveniently a deployed at an acceptable cost. For small apertures with modest resolution, imaging can be readily achieved using fully-staring focal plane arrays, but as in the infrared, the high cost per pixel means that scanning (preferably electronic) of a smaller number of detectors across the image is attractive. Aperture synthesis using sparse and filled arrays of antennas offers high sensitivity and resolution from a small number of antennas that can be conformal to vehicle shape, but requires complicated beam-forming technologies. Alteratively, electronic scanning can be accomplished by electronic modulation of a filled antenna. This paper will discuss novel approaches and technology requirements for achieving electronic beam-steering.

This paper discusses the issues that need to be addressed in the design and development of real time millimeter wave (MMW) imaging systems. The advantage offered by MMW imagers under adverse weather conditions are considered as are their thermal and spatial resolution. It is shown that it is difficult to use fully integrated focal plane arrays of receivers in large aperture high performance systems since the focal plane is so large. Instead scanned systems are considered to be preferable. Mechanical and electronic scanning techniques are described as is the use of image processing techniques which may increase the sharpness of an image by up to a factor of four. The paper introduces a new figure of merit for image quality which can be used to compare imaging systems.

Passive millimeter wave (mm-wave) imaging systems have attracted an increasing interest over the past years due to their superior poor weather performance compared with visible and infrared systems. In the UK the Defence Evaluation and Research Agency Malvern developed its first mm-wave radiometers in the late 1950s. These systems were bulky and had poor spatial resolution and low thermal sensitivity, but the considerable advances in semiconductor solid state devices have allowed the size and weight of imagers to be reduced. Advantage can also be taken of sophisticated on-line signal processing and of complex theoretical modeling and analysis. This paper examines the merits of the different operating frequencies in terms of atmospheric transmission vs. resolution and also discusses issues such as image processing. High quality images are presented to demonstrate the potential of this emerging technology.

Passive millimetre wave (MMW) imaging systems have attracted an increasing interest due to their superior poor weather performance compared with visible and infrared (IR) systems. In the UK, the Defence Evaluation and Research Agency at Malvern developed its first MMW radiometers in the 1950s. These systems were bulky, had low thermal sensitivity and poor spatial resolution but the considerable advances in semiconductor solid state devices have allowed the size and weight of imagers to be reduced and sensitivity to be increased. Advantage can also be taken of sophisticated on-line signal processing to increase spatial resolution and of complex theoretical device modelling and analysis. This paper examines the impact of these advances in technology and identifies new methods to overcome the difficulties of poor spatial resolution and slow response time. High quality images are presented which demonstrate the potential of this emerging technology to military applications.