Stuart Harmer

A Review of Nonimaging Stand-Off Concealed Threat Detection with Millimeter-Wave Radar [Application Notes]

There is now, more than ever before, a need for technologies that enable the screening of people from a distance. A wide variety of weapons can be easily concealed under clothing and carried into crowded public sites to target national infrastructure, spread fear, and inflict mass murder and casualties. The most feared and devastating terrorist weapon is the suicide bomb or person borne improvised explosive device (PBIED). Such devices are relatively simple to conceal on the body, and successful detection is required at considerable distance or stand-off range before the bomber reaches the target area.

ON BODY CONCEALED WEAPON DETECTION USING A PHASED ANTENNA ARRAY

The detection and identiflcation of metal items and, in particular weapons, of linear size ‚ 10cm, concealed upon the human body, is demonstrated as being entirely feasible by using a phased array of suitably ultra wide band transceivers. The complex natural resonances and especially the fundamental resonance, are excited by ultra wide band, stepped frequency continuous wave illumination of the target, using a phased array of antennae to focus the radiation. Broadband illumination of the target with microwave radiation of suitable frequency range (Typically 0.3{3GHz for handgun sized objects) excites low order complex natural resonances and the late time response of the concealed item can be spatially located using phased array imaging techniques. Further processing of the late time response enables classiflcation of the concealed object, based on the complex natural resonant frequencies of the object, so that threat items such as handguns and knives can be difierentiated from benign items such as mobile phone handsets and cameras.

Luminescence characterization of lattice site modifications of Nd in Nd:YAG surface layers

Abstract Cathodoluminescence spectra provide an effective means of recording the changes in surface quality of Nd:YAG crystals resulting from polishing or ion beam amorphization. There are spectral shifts in wavelength and changes in intensity linked to imperfections caused by surface preparation such as polishing, chemical etching or by ion beam bombardment. The wavelength shifts between the Nd ion spectra from surface damaged and bulk material can approach 10 nm. Cathodoluminescence intensity is suppressed by factors of >3 as the result of polishing damage, and by up to 1000 times after amorphization of the surface. The depth profiles of these changes and their wider implications for losses in surface waveguide structures, waveguide lasers and cathodoluminescence imaging are discussed.

Optical constants for the S20 photocathode, and their application to increasing photomultiplier quantum efficiency

Abstract The complex refractive index of an S20 multi-alkali photocathode has been determined for the first time over the whole visible and near infrared wavelength range using reflectance measurements. The real part n is around 3 and shows little variation with wavelength. The imaginary part k falls from 1.2 in the blue towards

A swept millimeter-wave technique for the detection of concealed weapons and thin layers of dielectric material with or without fragmentation

Active millimetre wave systems, operating at frequencies up to 110 GHz have been used to detect the presence of both concealed dielectric and metallic objects at standoff distances. Co- and cross-polarized superheterodyne or direct detectors are used to differentiate between metallic and purely dielectric objects. The technique determines the thickness of a dielectric target and detects the presence of concealed handguns or fragmentation by utilising the pattern of the responses from both the co- and cross-polarized detectors. The returned signals are processed and analysed by an artificial neural network, which classifies the responses according to their correspondence to previous training data.

Ultra wide band detection of on body concealed weapons using the out of plane polarized late time response

A method of detecting concealed handguns and knives, both on and off body, has been developed. The method utilizes aspect-independent natural, complex resonances (poles) excited by illuminating the target with frequency swept, ultrawide band microwaves in the range 0.5 - 18 GHz. These natural resonances manifest as a Late Time Response (LTR) that extends significantly (~ 5 ns) beyond the direct reflections from the human body (the Early Time Response) and are of the form of a superposition of exponentially decaying sinusoidal waveforms. Two handguns are examined, both on the human body and in isolation, by the established methodology of applying the Generalised-Pencil-Of-Function to the late time response data of the target. These poles allow the weapon to be effectively classified. Out of plane polarized (cross-polarized) scattered response is used here as this gives improved discrimination between the early and late time responses. Determination of the presence or absence of particular weapons concealed under clothing, on the human body, is demonstrated. A novel bow-tie slot antenna is described which has good pulse and frequency response over the range 0.3-1 GHz and which is suitable for excitation of the fundamental natural resonances.

Realisation of 50% quantum efficiency from photomultiplier cathodes

Standard trialkali red sensitive photomultiplier (PM) tubes have cathode quantum efficiencies which typically fall from ∼25% at 400 nm to ∼1% at 800 nm, partly because the material has lower optical absorption coefficients at long wavelengths. Increasing cathode thickness benefits long wavelength response but overall reduces electron extraction efficiency. In the present work modelling of the interaction of light within the dielectric materials of the windows indicates that considerably greater quantum efficiency (QE) is feasible under a variety of conditions. Some of these possibilities have been explored with standard PM tubes giving data where the blue response is increased by a factor of 2, to ∼50%. Much higher improvement factors, of more than 10 times, are realised at longer wavelengths. Current work suggests that the enhancement methods can be exploited further, and at least 50% performance obtained across most of the range of spectral response. A secondary feature of the enhancement is that the useful operating range of the tube is extended further into the near infrared.

A multifaceted active swept millimetre-wave approach to the detection of concealed weapons

The effective detection of concealed handguns and knives in open spaces is a major challenge for police and security services round the world. Here an automated technique for the detection of concealed handguns that relies on active swept illumination of the target to induce both scattered fields and aspect independent responses from the concealed object is presented. The broad frequency sweep permits information about the objects size to be deduced from transformations into the time/distance domain. In our experiments we collect multiple sweeps across the frequency range at very high speed, which produces a time evolved response from the target, from both normal and cross polarized detectors. From this we extract characteristic signatures from the responses that allow those from innocent objects (e.g. mobile phones, keys etc) to be distinguished from handguns. Information about the optical depth separation of the scattering corners and the degree and shape of cross polarization allows a neural network to successfully concealed handguns. Finally this system utilizes a range of signal processing techniques ranging from correlation between cross and normally polarized scattering through to a neural network classifier to deduce whether a concealed weapon is present.

On the Feasibility of Assessing Burn Wound Healing Without Removal of Dressings Using Radiometric Millimetre-Wave Sensing

The authors present transmission data, taken at Ka (36GHz) and W (95 GHz) bands in the millimetre-wave region of the electromagnetic spectrum, for various dressing materials used in the treatment and management of burn wounds. The results show that such materials are highly transparent (typically > 90% transmission) and, in their dry state, will permit the sensing of the surface of the skin through the thick layers (> 2 cm) of different dressings typically applied in medical treatment of burn wounds. Furthermore, the authors present emissivity data, taken at the same frequency bands, for different regions of human skin on the arm and for samples of chicken flesh with and without skin and before and after localised heat treatment. In vivo human skin has a lower emissivity than chicken flesh samples, 0.3–0.5 compared to 0.6–0.7. However, changes in surface emissivity of chicken samples caused by the short-term application of heat are observable through dressing materials, indicating the feasibility of a millimetre-wave imaging to map changes in tissue emissivity for monitoring the state of burn wounds (and possibly other wounds) non-invasively and without necessitating the removal of the wound dressings.

Active Millimeter Wave Sensor for Standoff Concealed Threat Detection

We present a millimeter radar for threat level evaluation developed for the detection concealed threats, such as guns and person borne improvised explosive devices (PBIED). The system uses a Gaussian optic lens antenna to achieve stand-off ranges up to 25 m. Ultra-wideband swept frequency radar, using direct detection receivers, is implemented to achieve short radar range resolution . The system is capable of detecting a wide range of objects positioned in front of the body by interpretation of the scattered waveform. Threat detection is rendered autonomously by a neural network that processes the scattered polarimetric, depth domain radar waveforms. The system may be configured to alarm or reject certain classes of objects, allowing for the detection of specific or broad spectrum threats. The radar system is portable and manually steered by the operator to enable standoff monitoring of walking human targets in real time. A video feed provides the operator with a wide field of view that allows tracking of persons and greatly facilitates aiming of the sensor. Rapid (1 ms) radar sweep times and fast signal acquisition and processing are implemented to provide threat detection at video frame rates (30 fps). Performance parameters for the detection of hand-guns and simulated PBIED are presented for ranges up to 25 m.