Search

The effect of stoichiometry on the dynamics of moisture absorption in epoxy networks is reported. The results show that formulations containing an excess of epoxide groups are characterized by reduced water absorption. For all the considered formulations, water absorption is a reversible process. Water absorption reduces the glass transition temperature, T g , with each 1 wt% of absorbed water reducing T g by ~10 °C. We attribute this effect to water molecules masking network polar sites and, thereby, inhibiting the formation of intra- or inter-segmental hydrogen bonds. Dielectric spectroscopy reveals that a significant fraction of the absorbed water exhibits relaxation processes similar to the relaxation of the network polar groups and, thus, results in strengthening the existing relaxation peaks within the system. This phenomenon may be employed to magnify the presence of material polar groups, particularly when these groups have low concentration or cannot produce a detectable effect on the dielectric spectra. The presence of distributed absorbed water enhances charge transport, an effect reflected in a proportional reduction in DC breakdown strength. Conversely, AC breakdown strength is not critically affected for the levels of water absorption seen here.

© 2020, Electromagnetics Academy. All rights reserved. Millimetre-wave thermography is used to image through the soles of shoes as proof of principle study into the application of such an approach for security inspection. Current airport security screening practice necessitates the removal of shoes prior to x-ray screening for potential threats or other concealments, for example explosive or explosive precursor materials; narcotic substances or small weapons. The authors demonstrate that thermography at ~ 250 GHz is able to reveal a variety of objects concealed within the soles of typical shoes, and that such an approach might be applied to rapidly screen passengers without necessitating the removal of their footwear.

A half-space electromagnetic model of human skin over the band 30-300 GHz was constructed and used to model radiometric emissivity. The model showed that the radiometric emissivity rose from 0.4 to 0.8 over this band, with emission being localized to a layer approximately one millimeter deep in the skin. Simulations of skin with differing water contents associated with psoriasis, eczema, malignancy, and thermal burn wounds indicated radiometry could be used as a non-contact technique to detect and monitor these conditions. The skin emissivity of a sample of 30 healthy volunteers, measured using a 95 GHz radiometer, was found to range from 0.2 to 0.7, and the experimental measurement uncertainty was ±0.002. Men on average were found to have an emissivity 0.046 higher than those of women, a measurement consistent with men having thicker skin than women. The regions of outer wrist and dorsal forearm, where skin is thicker, had emissivities 0.06-0.08 higher than the inner wrist and volar forearms where skin is generally thinner. Recommendations are made to develop a more sophisticated model of the skin and to collect larger data sets to obtain a deeper understanding of the signatures of human skin in the millimeter wave band. Bioelectromagnetics. 2017;9999:XX-XX. © 2017 The Authors. Bioelectromagnetics published by Wiley Periodicals, Inc.

Millimeter-wave imaging may be used to assist in\ud the problem of wastepaper recognition during the sorting and\ud recycling process. Currently, there are a variety of techniques,\ud which are used within materials recovery facilities to sort waste\ud paper and card; however, improvements in sorting capability can\ud be realized using a multiple sensors to provide complementary\ud (orthogonal) data. This paper discusses the issues related to\ud the processes, which are currently used for sorting of recycled\ud wastepaper and the phenomenology for millimeter-wave imaging\ud of a wastepaper material. Experimental measurements and\ud images in the millimeter wave band are presented, which support\ud the proposed application.

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.

This paper describes how information about the electro- magnetic structure of targets can be obtained from direct detection radar techniques, where the relative phase of the transmitted and re- ceived signals is not measured. A comparison is made between the re- solved structure of a simple test target from an ultra wide band, pulse synthesis direct detection radar system at 14{40GHz and an equiv- alent heterodyne radar receiver where phase information is recorded. The test targets employed are wax sheet of thickness 20mm and 80mm which are illuminated alone and in contact with the human body. A vector network analyser is used as the radar system. The simplicity of constructing ultra wide band direct detection radar systems combined with their cost makes the use of such radar systems appealing for appli- cations such as concealed threat detection and non-destructive testing, where absolute range to the target, if required, can be determined by other methods.

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

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.

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.

The detection and identiflcation of conducting objects using electromagnetic pulses to excite circulating eddy currents within the object is demonstrated by numerical simulation using a flnite element time domain electromagnetic solver. The ability to discriminate between objects is based on the decay rate of the induced currents in the object, typically » 100"S. The decay rates are difierent for a wide variety of everyday objects, allowing threat objects such as handguns, grenades and knives to be discriminated from benign objects such as mobile phones handsets, watches, keys, etc.. Crucially, the time constant characterising an object depends only upon the electrical properties of the object (conductivity) and the shape and size of the object; the orientation of the object is irrelevant. This aspect independence of temporal current decay rate forms the basis of a potential object detection and identiflcation system. By application of an algorithm based on the generalized pencil of function method, the authors demonstrate the ability to efiectively count and indentify multiple objects carried in close proximity providing that the objects do not have very similar time constants and that signal to noise ratio is high.

The authors discuss and demonstrate the feasibility of using ultra wide band microwave radar to detect and identify small arms flre. Detection and tracking is by standard radar techniques, but identiflcation is carried out by exciting the projectiles Complex Natural Resonances and using this aspect independent information to assign a caliber to the incoming projectiles. The typical sizes of small arms projectiles (calibers 5.56mm through to 13mm) imply that ultra wide band illumination in the microwave region of the spectrum between 1.5{5.5GHz is required to excite these object's fundamental resonances. The authors give a discussion of the efiects of motion on the quality of the complex natural resonance data obtainable and present both simulated and laboratory data for the radar cross section of three difierent caliber projectiles (5.56mm, 7.62mm and 13mm).

A free-wave, transmission only technique for the determination of complex permittivity in the millimetre (mm)-wave band 14-40 GHz of planar samples of textiles is presented. With this method an accurate alignment of source and detector horns is not required and time gating methods to reduce or remove standing wave interference between horns is replaced by a data smoothing process. The technique is novel as phase information from the transmitted wave is not required to determine the complex permittivity. The textiles investigated are denim (cotton) and cow leather (two colours, red and beige). This method is shown to be simple to set up, easy to use and fast when compared with other methods such as free-wave reflectance and transmittance or Fabry-Perot cavity and gives results which are accurate enough for most of the practical applications. A significant difference in the absorption of mm-wave power between the two leather samples is observed. This can be explained by the different chemical composition of the two leather samples, investigated using a scanning electron microscope with electron dispersive spectrometry and which is almost certainly a result of the colouring process employed.

Measurements of the temperature dependence of the dark current for standard commercial photomultipliers with multialkali photocathodes (type S20) have been contrasted with data from new prototype tubes with greatly enhanced long wavelength performance. The analyses suggest that for the larger structures part of the cathode is closer to a bialkali composition. This reduces the activation energy of the dark current from ~1.1 eV for normal S20 cathodes down to ~0.8 eV. Many tubes show a lower activation energy component near 0.6 eV which may represent a sensitization process from monolayers of molecular oxygen. Similarly, monolayers of water can play a significant role, but do so by reducing cathode sensitivity. Several tubes display a sharp transition in gain at the limiting temperature for water desorption (i.e. a sublimation transition). The commercial structured cathodes can show evidence for up to 70% improvements in quantum efficiency by operation at low temperature which can be immediately exploited. Prototype tubes with greatly improved performance were studied but for very small area illumination spots there can be problems of local variations in gain and wavelength response across the structural features of pyramidal modified cathode windows. Overall the data suggest that not only is improved performance already available with existing photomultiplier tubes but also with structured photocathode windows, and/or the introduction of surface 'defects' in the window, even further improvements are feasible.

A simple and general method of using a photon detector to directly distinguish between two or three different polarizations is described. The method is suitable for a pre-selected limited wavelength region, for example as with encoded fibre optic communication. Alternatively, wavelength discrimination via wavelength selective polarization is possible.

The complex permittivity values of four S20 photocathodes are reported and contrasted. Values were obtained indirectly by measuring the reflectance of the cathode and then using non-linear fitting software to obtain the best estimates of the real and imaginary parts of the complex permittivity, together with the thickness of the cathode layer. The permittivity values span the wavelength range from 250 to 950 nm. The values are broadly similar for the 4 sets of data. However, the variations may reflect the fact that the S20 is believed to consist of alkali compounds with at least two different stoichiometric compositions and the stoichiometry may not be identical between cathodes. For comparison purposes data were recorded for a simpler composition bi-alkali cathode and compared with earlier data derived from an alternative analysis and there was good agreement between the values from the two methods.

A variant route to lock-in amplifier detection of optical emission from radio pulsars is considered. The method uses the radio pulsar frequency signals as a timing clock but splits the optical signals into equal portions of one third the pulsar period. Differences in their intensities offer evidence for an optical signal at the same frequency as the pulsar without the need to know the phase differences between optical and radio signals. The route benefits from sensitivity to pulsed optical signals seen against a steady background and also allows detection of periodic reductions in intensity. The method is insensitive to changes in observational conditions. When used to acquire star images with the viewing field phase split onto three detector areas, the phase block method should provide intensity differences which identify the location of an optical signal with the frequency of the radio pulsar. Supportive data for the method from a simple laboratory based proof of concept experiment are cited. The three ...

Conventional S20 multialkali photocathodes have a wide wavelength coverage from 850 nm, but their high transparency and the surface work function result in low quantum efficiencies at longer wavelengths. Theoretical modelling of the photon and excited electron interactions that define the cathode performance provides a realistic prediction of the measured response. The theory emphasizes that the basic light absorption is strongly sensitive to the cathode thickness, wavelength, polarization and incident angle. Parameters can be selected which predict that even at long wavelengths (e.g. 900 nm), absorption may be increased from ~1% to ~100%. Cathode topographies can be designed to exploit these responses and offer increased absorption at the longer wavelengths. Alternative designs, which include waveguiding of light within the cathode window, or in structured surfaces, can similarly lead to almost total absorption of the incident light by increasing the number of interactions. These concepts of optimal incidence and waveguiding have been both theoretically modelled and demonstrated in newly fabricated cathode designs. The methods have variously reached quantum efficiencies in excess of 50% at wavelengths in the range from 200 to > 750 nm under different operational conditions. The improvement factors relative to normal incidence on planar cathodes increase for longer wavelengths, and examples of 20–50 times by ~900 nm were noted. Whilst the absolute S20 efficiency values at long wavelengths are still small, the improvements offer a usable sensitivity even beyond 1 µm, as demonstrated by spectroscopy data up to at least 1140 nm.

Multialkali photocathodes are used from 200 to 850 nm, but high transparency at longer wavelengths reduces the quantum efficiencies. Theoretically, routes to enhance absorption could achieve high efficiency over most of this spectral range. Realisations of some of the concepts have been successfully attempted leading to quantum efficiencies of ~50%, with examples from 200 to 750 nm. Improvement factors of up to 50 times occur by ~900 nm, giving an extension of the useful wavelength operating range.

By considering a monochromatic plane wave obliquely incident upon a planar layer of S-20 photocathode material, deposited upon a non-absorbing glass substrate, the distribution of optical power absorbed within the layer can be resolved. This is important to the question of photocathode efficiency, as the absorbed light excites photoelectrons within the photocathode which then may pass from the photocathode into the vacuum of the photomultiplier tube and be collected and multiplied. The calculation uses the measured complex permittivity of an extended red S-20 photocathode in the wavelength range, 375–900 nm. The results show that thin film effects are important within the photocathode, as they give rise to interesting power absorption profiles. This information is invaluable in predicting optimum photocathode thickness for wavelength selective applications. Electromagnetic waves that are obliquely incident upon the photocathode are also considered in both transverse electric and transverse magnetic polarizations.

Laser surface patterning has recently been used to form black silicon. For photon energies above the band gap energy (1.07 eV corresponding to a wavelength of 1.1 μm) the origin of the very low reflectivity can be accurately modelled as arising from enhanced absorptance from the micro-structure on the silicon. However, at lower photon energies the decreased reflectance cannot be attributed to the structured surface alone. For these lower photon energies an excellent theoretical description of the experimentally observed absorptance can be achieved by the addition of an increased imaginary part of the complex permittivity. This term increases linearly with wavelength in the energy range 1.07 eV down to 0.496 eV (corresponding to a wavelength of 2.5 μm). The implication is that the structuring has introduced a wide range of new surface and sub-surface states (potentially from impurities) which absorb the light.

Excitation of surface plasmons on a metal substrate, via the attenuated total reflection method can theoretically offer preferential absorption of light at one particular wavelength, whilst reflecting the nearby spectrum. Normally this `filtering' action is limited to removal of p-polarized light, and the acceptance angle of such a filtering device is very narrow, which limits practical applications, such as separation of fundamental and laser harmonics. The possibility of avoiding this angular precision is explored by considering the complex permittivity of metal composites. By using a two or more layer structure, as opposed to a single metal substrate, the acceptance angle of the device can be broadened, by a factor of about 15 times. An example is discussed for separation of the fundamental and harmonics from a Nd : YAG laser. Variants of the structure allow the design of an in-line transmission filter for the various wavelengths with sufficient angular tolerance to include focusing lenses. Avoidance of laser ablation of the metal is discussed.

A dispersive antenna, formed by locating a dielectric sheet waveguide in the near-field zone of a millimetric scale diffraction grating, is described for the application of passive millimeter-wave imaging in the W-band. Two dimensional radiometric contrast images are formed by frequency scanning in the radial direction of the image and mechanically scanning in the azimuthal direction. Mechanical scanning is by means of rotation of the antenna at a uniform angular speed, while frequency scanning is achieved by careful selection of antenna parameters to provide a near linear dependence of beam angular position with frequency of ∼ 1 degree per GHz. The imaging system operates over the frequency range 84–100 GHz and utilizes a single millimeter-wave LNA (Low Noise Amplifier) and mixer to produce a microwave frequency intermediate frequency; further down conversion stages result in 64 baseband channels. In this way, the 16 GHz operational bandwidth is divided into 64 equal width frequency channels, giving the system a field of view of ∼ 16 degrees.

The risk of attacks on travelling public has led to changes in security screening at airports. After the shoe bomber in 2001, each passenger must remove shoes, which might contain concealed threats to be x-rayed individually, which is inconvenient to passengers as well as airlines. An alternative approach is an Ultra Wide Band (UWB) Microwave Footwear Scanner swept between 15 to 40 GHz, which detects threats concealed within the shoes, while they are still worn. A prototype system is tested, which demonstrates the ability to detect concealed metal and dielectric objects. Results are compared with Finite Element Modelling simulations.

This paper investigates the use of Late Time Response (LTR) analysis for detecting multiple simulated objects using via Finite Element Analysis (FEA) in concealed object detection. The LTR results from a pair of simulated targets are presented individually and present within the same simulation. The simulation used a plane wave to represent the Ultra Wide-Band (UWB) signal from a Vector Network Analyser (VNA). The simulated targets are suspended in air and the perimeter of the environment is a perfectly matched layer. To allow for the de-convolution of the signal and the removal of background clutter post processing has been applied.

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

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.

The poor quantum efficiency of alkali photocathodes in the near infrared can be improved by multiple total internal reflection. A limit for the improvement attainable by this method is calculated and the maximum ratio of the enhanced over the conventional quantum efficiency is found to be the inverse of the absorptance at normal incidence. A novel, simple way of measuring the absorptance of standard tubes is described. Experimental results for the quantum-efficiency enhancement and the absorptance are presented and agree with the theoretical considerations. The measured ratio of the enhanced over the conventional quantum efficiency for the extended S20 photocathode ranges from 2 at 400 nm to more than 12 at 980 nm. The authors believe that, by combining multiple total internal reflection techniques with the use of dedicated photocathodes, the quantum efficiency in the near infrared can be increased by factors of 20-30.

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.

The feasibility of screening hand luggage for concealed threat items such as Person-Borne Improvised Explosive Devices (PBIED's) both metallic and non-metallic, together with handguns and at millimetre wavelengths is investigated. Previous studies by the authors and others indicate that hand baggage material and fabric is much more transmissive and has less scattering at lower millimetre wave frequencies and the ability to use K-band active imaging with high spatial resolution presents an opportunity to image and hence recognise concealed threats. For this feasibility study, a 1.6 m aperture, 35 GHz security screening imaging system with a spatial resolution of 2.5 cm and a depth of field of around 5 cm is employed, using spatially incoherent illuminating panels to enhance image contrast. In this study, realistic scenarios using backpacks containing a realistic range of threat and non-threat items are scanned, both carried and standalone. This range of items contains large vessels suitable for containing simulated home-made PBIED’s and handguns. The comprehensive list of non-threat items includes laptops, bottles, clothing and power supplies. For this study, the range at which imaging data at standoff distances can be acquired is confined to that of the particular system in use, although parameters such as illumination and integration time are optimised. However, techniques for extrapolating towards effective standoff distances using aperture synthesis imagers are discussed. The transmission loss through fabrics and clothing that may form, or be contained in baggage, are reported over range of frequencies ranging from 26 to 110 GHz.

At airports, security screening can cause long delays. In order to speed up screening a solution to avoid passengers removing their shoes to have them X-ray scanned is required. To detect threats or contraband items hidden within the shoe, a method of screening using frequency swept signals between 15 to 40 GHz has been developed, where the scan is carried out whilst the shoes are being worn. Most footwear is transparent to microwaves to some extent in this band. The scans, data processing and interpretation of the 2D image of the cross section of the shoe are completed in a few seconds. Using safe low power UWB radar, scattered signals from the shoe can be observed which are caused by changes in material properties such as cavities, dielectric or metal objects concealed within the shoe. By moving the transmission horn along the length of the shoe a 2D image corresponding to a cross section through the footwear is built up, which can be interpreted by the user, or automatically, to reveal the presence of concealed threat within the shoe. A prototype system with a resolution of 6 mm or less has been developed and results obtained for a wide range of commonly worn footwear, some modified by the inclusion of concealed material. Clear differences between the measured images of modified and unmodified shoes are seen. Procedures for enhancing the image through electronic image synthesis techniques and image processing methods are discussed and preliminary performance data presented.

This paper investigates the use of Late Time Response (LTR) analysis for detecting multiple objects in concealed object detection. When a conductive object is illuminated by an ultra-wide band (UWB) frequency radar signal, the surface currents induced upon the object give rise to LTR signals. The LTR results from a number of different targets are presented. The distance between the targets within the same radar beam has been adjusted in increments of 5cm to determine the point at which the individual objects can be distinguished from each other. The experiment was performed using double ridged horn antennas in a pseudo-monostatic arrangement. Vector network analysers (VNA) are used to provide the UWB stepped frequency continuous wave radar signal. The distance between the transmitting antenna and the target object is kept at 50cm for all the experiments performed and the power level at the VNA was set to 2dBm. The targets in the experimental setup are suspended in isolation in a non-anechoic environment. To allow for the de-convolution of the signal and the removal of background clutter Matlab was used in post processing. The Fast Fourier Transform (FFT) and Continuous Wavelet Transform (CWT) are used to process the return signals and extract the LTR features from the noise clutter. A Generalized Pencil-of-Function (GPOF) method was then used to extract the complex poles of the signal. In the case of a single needle these poles can be found around 1.9GHz.

The authors discuss a phased array antenna design consisting of multiple, modular, flat panels the number of which is determined by specific application requirements. Each panel has 512 antenna elements which are driven by their own Application Specific Integrated Circuits, in order to control the signal phase needed for electronic beam steering. Furthermore, the antenna elements can transmit (or receive) in orthogonal polarisation states. In the first embodiment, Ku-band microwave signals (receive over 10.95-12.75 GHz and transmit over 14.0-14.5 GHz) are phase-aligned, down converted to baseband and summed, permitting the receiver and the digital control circuitry to be fabricated on one low-cost SiGe ASIC. It is envisaged that this approach will be a cost-effective alternative to mechanically steered dish antennas for radar, satellite communication and point to point communications, while bearing the advantages of flat or conformal structure and no moving parts.

Security screening of airline passengers is commonplace and very often requires that, in addition to screening of baggage and body, the passenger's shoes should be examined. The process of shoe screening is currently carried out by use of x-ray imaging, whereby the passenger's shoes are placed into an x-ray machine and inspected by a skilled operator for signs of any concealments, for example explosive materials; weapons or narcotics. Exposure to x-ray radiation is tightly regulated for health and safety reasons and passengers must remove their shoes prior to the screening process and then replace them after; this comes at the cost of reduced passenger throughput and is one of the most cited inconveniences by air passengers. Additionally, screening of 100% of passengers' shoes is not always acceptable with the current x-ray approach and therefore shoe screening is not necessarily encompassing.

A millimeter wave (75 - 110 GHz) polarimetric radar system (MiRTLE) has been developed for the detection of threat objects, such as guns, knives, or explosive devices, which have been concealed under clothing upon the human body. The system uses a Gaussian lens antenna to enable operation at stand-off ranges up to 25 meters. By utilizing ultra-wideband Swept Frequency Continuous Wave Radar very high range resolution (~ 10mm) is realized. The system is capable of detecting objects positioned in front of the body and of measuring the range of a target. By interpretation of the scattered waveform, the presence of a wide spectrum of threat items concealed on the human body may be detected. Threat detection is autonomously rendered by application of a neural network to the scattered time domain, polarimetric radar returns and the system may be taught to alarm or reject certain classes of objects; this allows for highly specific or broad spectrum threat detection. The radar system is portable and operator steerable allowing standoff monitoring of moving human targets in real time. Rapid (1ms) sweep times and fast signal acquisition and processing allow decisions to be made at video frame rates (30 fps) and integrated directly to a video feed providing the operator with a field of view and facilitating aiming. Performance parameters for detection of guns and simulated explosive devices are presented for ranges up to 25 meters.

The use of active millimeter wave radar has proven successful in the field of Concealed Weapon Detection. Time resolved signals acquired from the radar scans are pre-processed and classified using an Artificial Neural Network. A problem with this method occurs in the training of the ANN, where the network must be trained on each weapon type that it is to reliably classify. Any deviation from the training weapon sub-class type leads to a decrease in the classifier's performance. This is illustrated in the experimental results. A possible improvement on this method is outlined in the form of including signals reconstructed using Principal Component Analysis into the training set.

A millimetre wave (75 - 110 GHz) polarimetric RADAR system is demonstrated for the detection of threat objects concealed under clothing upon the human body at stand-off ranges of up to 25 metres. The system implements Swept Frequency Continuous Wave RADAR with low cost components to deliver a compact, UWB, high resolution (~ 1 cm) RADAR system capable of detecting, resolving and discriminating a wide spectrum of threat items concealed on the human body. Threat detection is autonomously rendered by application of a neural network to the scattered time domain polarimetric radar return, the system may be taught to alarm or reject certain classes of objects; allowing for highly specific through to broad spectrum threat detection. The authors present data for some simple envisaged threat scenarios at stand off ranges out to 25 metres.

Photomultiplier tubes are widely used detectors of low level light signals; however their performance is often limited, especially at long wavelengths. Input signals are reduced both by surface reflection and by transmission through the photocathode layer. Earlier methods of overcoming these weaknesses are summarized. New predictive modelling of the reflectivity and absorption reveals dependencies that are a function of angle of incidence, cathode thickness and polarization. Improvements on normal usage using extremely simple and low cost techniques are effective. These are demonstrated using retrofits that can improve the overall sensitivity of many types of photomultiplier. Examples include a simple external conical torch reflector, which has raised the efficiency of an S20 multialkali photocathode by between 20 to 10% across the blue to red spectral range. A second example, of a semi-cylindrical glass coupler, improved the absorption efficiency by exploiting 60 degree, rather than normal incidence of the light. Enhancements are up to 500% at longer wavelengths. Such gains are particularly valuable as this is the region of lowest quantum efficiency for the standard operation of the tubes.

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, ultra-wide band microwaves in the range 0.5 - 18 GHz. These natu ral 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.

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.

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 object's 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.

An active technique for the standoff detection and identification of concealed conducting items such as handguns and knives is presented. This technique entails illuminating an ob ject with wide range stepped millimetre wave radiation and inducing a local electromagnetic field comprised of a superposition of modes. The coupling to these modes from the illuminating and scattered fields is, in general, frequency dependent and this forms the basis for the detection and identification of conducting items. The object needs to be fully illuminated if a full spectrum of modes and therefore a full frequency response are to be excited and collected. The s cattered EM power is measured at “stand off” distance of several metres as the illuminating field is frequency swept and patterns in frequency response characteristic to the target item being sought are looked for. This system relies on contributions from the aspect independent late time responses employed by Baum 1 together with aspect independent information derived specifically from gun barrels and polarisation from scattering effects. This technique is suitable for a deployable gun and concealed weapons detection system and does not rely on imaging techniques for determining the presence of a gun. Experimental sets of responses from typical metal or partially conducting objects such as keys, mobile phones and concealed handguns are presented at a range of frequencies. Keywords: Concealed object detection, microwaves, standoff detection, aspect independent.

Photomultipliers are fast, sensitive and low noise light detectors which operate across the ultraviolet‐optical‐near infra red region of the electromagnetic spectrum. Sensitivity is determined by the composition of the photocathode layer in which incident photons excite photoelectrons and the thickness of this layer. Incident light is partially reflected from and partially transmitted through the photocathode layer, which is typically ∼20nm thick, and this energy is unavailable for photoelectron excitation, limiting sensitivity. Typical reflectance and transmittance values at 500nm are 21% and 33%, respectively, for KCsSb; 27% and 24% for RbCsSb and 36% and 35% for Na2SbK:Cs. These substantial losses can be reduced by the addition of one or more transparent impedance matching layers between the photomultiplier tube (PMT) window and the photocathode, resulting in enhanced sensitivity without effecting the PMT geometry, photocathode deposition process or altering the acceptance angle of the photomultiplier. The impedance matching serves to reduce reflectance losses, increasing cathode absorptance. By using published measurements of the dispersive properties of bialkali and trialkali (S20) photocathode compositions, accurate modelling of the electromagnetic field distribution within the photocathode layer is possible. This model facilitates the prediction of the enhancement of sensitivity obtainable with an anti-reflective layer of zirconium dioxide (zirconia). The authors present the enhancement factors possible with standard bialkali and trialkali photocathode compositions using the material zirconia for impedance matching. Enhancement factors up to 27% and 38% are predicted for KCsSb and RbCsSb photocathodes, respectively, while an enhancement factor of up to 44% is predicted for the S20 photocathode. 1. Background Photomultiplier tubes (PMTs) are the detector of choice for applications that require low noise, high sensitivity and short response time measurements. PMTs are particularly suited to photon counting applications and much use is made of them in scintillators for detection of nuclear radiation, particle physics experiments and in medical apparatus, including the recent advances in optical biopsy. In these applications semiconductor photodiode devices have still not displaced the venerable PMT and considerable advancement in photodiode technology will be required before this occurs. However