This PDF file contains the front matter associated with SPIE Proceedings Volume 6948, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

In the wake of the September 11, 2001 terrorist attack on America, our security and defense industry was instantly tasked with delivering technologies that could be used to help prevent future terrorist activities. The general public world wide is asking for solutions that will foster a safe society and travel environment. Our best defenses rest in our talents within a free open society to prevent dangerous individuals from boarding planes, entering buildings, courthouses, transportations hubs and military bases with weapons capable of causing damage and bodily harm in the first place. Passive millimeter wave (PMMW) whole body imaging systems are based upon the principle that every physical entity emits, reflects, and/or absorbs electromagnetic energy. The term "passive" means that this approach does not bombard the test subject with energy radiation to further induce the discovery of hidden objects. PMMW whole body imaging systems focus on the human body's natural emission and reflection of millimeter wavelength energy. In physics, "millimeter waves" (MMW) are defined as extremely high-frequency (30-300 GHz) electromagnetic oscillations. On the electromagnetic spectrum these waves are just larger than infrared waves, but smaller than radio waves. The wavelength of a MMW is between 1 millimeter and 10 millimeters. That is approximately the thickness of a large paperclip up to the diameter of an "AAA" battery.

It is well known that millimeter-wave technology provides an important imaging capability through clothing and adverse weather conditions, among others. Alfa Imaging has undertaken a project to study the different applications of mm-wave imaging. An important part of this project is the measurement of material properties of a number of clothing and packaging samples in the frequency range from 40 to 306GHz. This task has been undertaken by the Antenna Group at the Public University of Navarra using an ABmm Network Analyser. The resulting data has been analysed and is presented in this paper along with example images and conclusions on the ideal operating frequency for the various applications studied.

Millimeter-wave imaging has the unique potential to penetrate through poor weather and atmospheric conditions and create a high-resolution image. In pursuit of this goal, we have implemented a far-field imaging system that is based on optical upconversion techniques. Our imaging system is passive, in which all native blackbody radiation that is emitted from the object being scanned is detected by a Cassegrain antenna on a rotating gimbal mount. The signal received by the Cassegrain is passed to an optical modulator which transfers the radiation onto sidebands of a near-infrared optical carrier frequency. The signal is then passed to a low-frequency photodetector that converts remaining sideband energy to a photocurrent. Even though optical upconversion can produce loss, our system demonstrates low noise equivalent powers (NEP) due to the low-noise of the photodetection process. Herein, we present our experimental results and images obtained by using the far-field scanning system, which was assembled with commercially available components. In addition, we detail efforts to increase the resolution of the image and to compact the imaging system as a whole.

We present ultrawideband imagery obtained with modular, 8-element, superconducting Nb microbolometer arrays. Conically scanned images are presented and compared with raster-scanned images obtained on the same arrays and from similar NbN arrays at VTT. Statistical data on detector non-uniformity, and methods for mitigating and compensating it are described. Low-noise readout is accomplished with room-temperature electronics using the transimpedance scheme of Pentilla et al. Characterization of spatial resolution, noise-equivalent temperature difference, and spectral response is done using metrology tools - standard targets, mm-wave blackbodies, and variable filters - that have been developed at NIST for this purpose.

A W-band unamplified direct detection radiometer module is described that provides a wideband response and is scalable to large arrays. The radiometer design is intended to provide sufficient sensitivity for millimeter wave imaging applications with a goal of 2K noise equivalent temperature difference (NETD) at a 30 Hz frame rate. This effort leverages previously reported device scaling to increase sensitivity. We present a radiometer module designed for 60 GHz RF bandwidth that utilizes HRL's antimonide-based backward tunnel diode. An impedance matching circuit with on- and off-chip elements, as well as ridged waveguide, provides a wideband match to the detectors. Modules were designed with two different microwave substrates: 125 micron thick quartz and 100 micron thick alumina. flip-chip bonding of the detectors is amenable to automated pick-and-place for high volume manufacturing. The modular nature of the array approach allows large arrays to be manufactured in a straightforward manner. We present the design approach along with both electromagnetic simulations and measured performance of the modules. This work was supported by phase II of DARPA's MIATA program.

Recently, our group has developed a high-sensitivity millimeter-wave (mmW) imaging system based on optical upconversion. In such a system, native mmW radiation of objects is first collected by a broadband horn antenna, which feed the mmW signal to Co-Planar Waveguide (CPW) on a Lithium Niobate(LN) Electro Optical (EO) modulator. The mmW power is then transferred to the sidebands of an optical carrier due to phase modulation. Detection is realized by measuring the optical power transferred to the sidebands. The overall performance of the imaging system is highly dependent on the conversion efficiency of the EO modulator, which is a function of the frequency of the collected millimeter-wave energy. In this paper, we present the design, fabrication and experimental results towards realizing LN EO modulators for use in the 95 GHz imaging band.

The 94GHz imaging band is the most commercially focused of the mm-wave imaging "windows". However, the commercial uptake of imaging systems has been limited due to the production costs involved, of which a significant proportion is due to the front-end receivers. Conventionally, the receiver is machined from metal and made-up of either several modules or a single more integrated module containing the RF and DC circuitry. Even with the more integrated approach the cost is prohibitive, due to the cost of the MMICs, the machining of the metal and integration of different materials during assembly. The front end receiver cost is a potential limiting factor in the deployment of imaging systems. LCP multi-layer substrates remove the requirement for expensive metal machining and because the RF and DC circuitry is integrated in the same substrate the assembly cost of the module is also reduced. Cost is not the only consideration, LCP has excellent properties which are especially attractive for high-performance microwave applications. These properties include low permittivity, low loss tangent, low water-absorption coefficient and most importantly low cost. By means of heat treatments, their coefficients of thermal expansion can be tailored to make them more amenable to integration into packages that include other materials. The LCP is manufactured in large sheet/panel form allowing batch manufacture of circuits which ensures circuit to circuit repeatability and a high yield. LCP has a dielectric constant of 3.16±0.05 and a dielectric loss tangent of 0.0049 to 100GHz. These properties have resulted in measured line loss of 0.2dB/mm at 110GHz. This level of loss makes this material system a viable approach for low noise integrated imaging receivers and will allow sensitivities of <0.8°K NETD to be achieved. This paper describes the design, measurement and characterisation of the first 94GHz receiver manufactured using LCP reported in the literature.

As Passive Millimeter Wave (PMMW) imagers mature, users have located them in more and more challenging locations. Because PMMW systems typically use natural sky coldness as a contrast source, indoor installations can become problematic. A number of semi-active illumination systems have been proposed, with various degrees of success. A problem many of them suffer from is reversed contrast - the illumination source is considerably hotter than the ambient scene, causing the cameras to act in unexpected ways. The relatively narrow extent of the illumination source is another problem, with speckle and glint often dominating the image. There are a number of unintended illumination sources in indoor locations, and all are poorly understood. This paper will examine several of them, as well as their polarametric properties, and discuss their effects on image quality.

At Pacific Northwest National Laboratory, wideband antenna arrays have been successfully used to reconstruct three-dimensional images at microwave and millimeter-wave frequencies. Applications of this technology have included portal monitoring, through-wall imaging, and weapons detection. Fractal antennas have been shown to have wideband characteristics due to their self-similar nature (that is, their geometry is replicated at different scales). They further have advantages in providing good characteristics in a compact configuration. We discuss the application of fractal antennas for holographic imaging. Simulation results will be presented. Rectennas are a specific class of antennas in which a received signal drives a nonlinear junction and is retransmitted at either a harmonic frequency or a demodulated frequency. Applications include tagging and tracking objects with a uniquely-responding antenna. It is of interest to consider fractal rectenna because the self-similarity of fractal antennas tends to make them have similar resonance behavior at multiples of the primary resonance. Thus, fractal antennas can be suited for applications in which a signal is reradiated at a harmonic frequency. Simulations will be discussed with this application in mind.

We describe a computational imaging technique to extend the depth-of field of a 94-GHz imaging system. The technique uses a cubic phase element in the pupil plane of the system to render system operation relatively insensitive to object distance. However, the cubic phase element also introduces aberrations but, since these are fixed and known, we remove them using post-detection signal processing. We present experimental results that validate system performance and indicate a greater than four-fold increase in depth-of-field from 17" to greater than 68".

Conventional material measurements of transmission and reflection in the millimeter-wave and terahertz frequency range do not differentiate between scattering and absorption, grouping effects from both mechanisms together into "loss". Accurate knowledge of the balance between scattering and absorption is critical in applications such as radiometric scene modeling for concealed object detection, where evaluation of object detectability depends strongly on the amount of scattering due to concealers such as clothing. We describe an experimental setup for the measurement of spatial bidirectional reflectance distribution function (BRDF). Previous measurements have shown extremely low-level grating lobes from periodic clothing materials such as corduroy, around 30 dB below the transmitted beam. To adequately address this issue of high dynamic range, we utilize a cryogenic antenna-coupled microbolometer for detection. We present data on several types of expanded polystyrene, a common structural material for systems and experiments in this frequency range. In these measurements of BRDF, transmission agrees with previous measurements, and the balance between low and high angle scattering, specular reflectance, and absorption is examined.

We have developed a novel approach to performing automatic detection of concealed threat objects in passive MMW imagery of people scanned in a portal setting. It is applicable to the significant class of imaging scanners that use the protocol of having the subject rotate in front of the camera in order to image them from several closely spaced directions. Customary methods of dealing with MMW sequences rely on the analysis of the spatial images in a frame-by-frame manner, with information extracted from separate frames combined by some subsequent technique of data association and tracking over time. We contend that the pooling of information over time in traditional methods is not as direct as can be and potentially less efficient in distinguishing threats from clutter. We have formulated a more direct approach to extracting information about the scene as it evolves over time. We propose an atypical spatio-temporal arrangement of the MMW image data - to which we give the descriptive name Row Evolution Image (REI) sequence. This representation exploits the singular aspect of having the subject rotate in front of the camera. We point out which features in REIs are most relevant to detecting threats, and describe the algorithms we have developed to extract them. We demonstrate results of successful automatic detection of threats, including ones whose faint image contrast renders their disambiguation from clutter very challenging. We highlight the ease afforded by the REI approach in permitting specialization of the detection algorithms to different parts of the subject body. Finally, we describe the execution efficiency advantages of our approach, given its natural fit to parallel processing. mage

Trex Enterprises Corporation has developed a full body passive millimeter-wave security screening imager. The system images naturally occurring W-band blackbody radiation, which penetrates most types of clothing. When operated indoors, the primary mechanism for image formation is the contrast between body heat radiation and the room temperature radiation emitted or reflected by concealed objects that are opaque at millimeter-wave wavelengths. Trex Enterprises has previously demonstrated that an imager noise level of 0.25 to 0.5 K is necessary to detect and image small concealed threats indoors. Achieving this noise level in a head-to-toe image required image collection times of 24 seconds using the previous imager design. This paper first discusses the measurement of the noise temperature of the MMW detectors employed. The paper then explores reducing the image collection times through a new front-end amplifier design and the addition of more imaging units. By changing the orientation and direction of travel of the imaging units, the new design is able to employ more detectors and collect imagery from a subject's front and sides. The combination of lower noise amplifiers and a new scanning architecture results in an imager appropriate for high throughput security screening scenarios. Imagery from the new configuration is also presented.

Microwaves in the range of 1-300 GHz are used in many respects for remote sensing applications. Besides radar sensors particularly passive measurement methods are used for two-dimensional imaging. The imaging of persons and critical infrastructures for security purposes is of increasing interest particularly for transportation services or public events. Personnel inspection with respect to weapons and explosives becomes an important mean concerning terrorist attacks. Microwaves can penetrate clothing and a multitude of other materials and allow the detection of hidden objects by monitoring dielectric anomalies. Passive microwave remote sensing allows a daytime independent non-destructive observation and examination of the objects of interest under nearly all weather conditions without artificial exposure of persons or areas. The performance of millimeter-wave radiometric imaging with respect to wide-area surveillance is investigated. Measurement results of some typical critical infrastructure scenarios are discussed. Requirements for future operational systems are outlined exploring a radiometric range equation.

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 object 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 scattered 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 Baum¹ 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.

Imaging in the sub-millimeter wave range of 300 - 1000 GHz is useful for a variety of applications including security screening, imaging through obscurations, and non-destructive evaluation. Waves in this frequency range have wavelengths ranging from 0.3 to 1.0 mm and are able to penetrate many optical obscurants. The ability to form high-resolution images that penetrate clothing makes imaging in this frequency range particularly interesting for personnel security screening at standoff distances. The Pacific Northwest National Laboratory (PNNL) has previously developed portal screening systems that operate at the lower end of the millimeter-wave frequency range around 30 GHz. These systems are well suited for screening within portals and can achieve resolution of about 5 mm at ranges of less than 1 meter. However, increasing the range of these systems would dramatically reduce the resolution due to diffraction at their relatively long wavelength. Operation at much higher frequencies, for example 350 GHz, will enable an order of magnitude improvement of the resolution at a given range, while still achieving adequate clothing penetration. PNNL's portal imaging systems have relied on wavefront reconstruction, or holographic, imaging techniques to mathematically focus the imagery. In the sub-millimeter-wave, this may not always be practical due to sensitivity of the system to slight changes in the position of the imaging target during data collection. In this case, physical focusing using lenses or reflectors may be more practical. In this paper, we examine the effectiveness of imaging near 350 GHz for security screening applications. Imaging results are presented using the holographic wavefront reconstruction technique, as well as a focused reflector-based imaging system.

Sensors used for security purposes have to cover the non-invasive control of men and direct surroundings of buildings and camps to detect weapons, explosives and chemical or biological threat material. Those sensors have to cope with different environmental conditions. Ideally, the control of people has to be done at a longer distance as standoff detection. The work described in this paper concentrates on passive radiometric sensors at 0.1 and 0.2 THz which are able to detect non-metallic objects like ceramic knifes. Also the identification of objects like mobile phones or PDAs will be shown. Additionally, standoff surveillance is possible, which is of high importance with regard to suicide bombers. The presentation will include images at both mentioned frequencies comparing the efficiency in terms of range and resolution. In addition, the concept of the sensor design showing a Dicke-type 220GHz radiometer using new LNAs and the results along with image enhancement methods are shown. 2.1 Main principle

Passive imaging of concealed objects at stand-off distances in excess of a few meters requires both excellent spatial, thermal and temporal resolution from the terahertz imaging system. The combination of these requirements while keeping the overall system cost at a reasonable level has been the motivation for this joint work. The THz imaging system under development is capable of sub-Kelvin NETD at video frame rates. In this paper we report the first imaging results from a 16-pixel array of superconducting antenna-coupled NbN vacuum-bridge microbolometers, operated within a cryogen-free, turn-key refrigerator. In addition to the system overview, we shall also address the uniformity of the detectors and present passive indoors raster-scanned imagery.