Roger Appleby

Millimeter-Wave and Submillimeter-Wave Imaging for Security and Surveillance

Passive equipments operating in the 30-300 GHz (millimeter wave) band are compared to those in the 300 GHz-3 THz (submillimeter band). Equipments operating in the submillimeter band can measure distance and also spectral information and have been used to address new opportunities in security. Solid state spectral information is available in the submillimeter region making it possible to identify materials, whereas in millimeter region bulk optical properties determine the image contrast. The optical properties in the region from 30 GHz to 3 THz are discussed for some typical inorganic and organic solids. In the millimeter-wave region of the spectrum, obscurants such as poor weather, dust, and smoke can be penetrated and useful imagery generated for surveillance. In the 30 GHz-3 THz region dielectrics such as plastic and cloth are also transparent and the detection of contraband hidden under clothing is possible. A passive millimeter-wave imaging concept based on a folded Schmidt camera has been developed and applied to poor weather navigation and security. The optical design uses a rotating mirror and is folded using polarization techniques. The design is very well corrected over a wide field of view making it ideal for surveillance and security. This produces a relatively compact imager which minimizes the receiver count.

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.

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.

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.

High-performance passive millimeter-wave imaging

We discuss the current status of passive millimeter-wave radiometry as a thermal imaging technique. The major problems are poor spatial resolution and slow response time. Techniques for overcoming these difficulties are identified, including the use of aperture synthesis, multichannel receivers, correlation, and inverse transform techniques. A comparison is made with infrared imaging.

Millimeter-Wave and Terahertz Imaging in Security Applications

The relatively short wavelength of mm-wave and THz radiation coupled with good transmission through many dielectric materials allows images to be formed of concealed objects. This chapter gives an overview of the detectors, their associated circuitry, and system developments over the past 10 years, focussing on personnel security screening. We will discuss the phenomenology of imaging at these wavelengths, introduce the reader to the basic architectures being used and developed for image forming instruments, show examples of systems, and also discuss the feasibility of spectroscopic THz imaging for security screening applications.

The design of a real-time 94-GHz passive millimetre-wave imager for helicopter operations

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.