This PDF contains front matter associated with SPIE Proceedings Volume 8022, including Title Page, Copyright Information, Table of Contents, and the Conference Committee listing.

Millimeter wave detection and imaging is becoming increasingly important with the proliferation of hostile, mobile millimeter wave threats from both weapons systems and communication links. Improved force protection, surveillance, and targeting will rely increasingly on the interception, detection, geo-sorting, and the identification of sources, such as point-to point communication systems, missile seekers, precision guided munitions, and fire control radar systems. This paper describes the Naval Research Laboratory's (NRL) demonstration broadband passive millimeter wave (mmW) interferometric imaging system. This Ka-band system will provide a capability for meter-precision geolocation for imaged objects. The interferometer uses a distributed array of 12 antenna elements to synthesize a large aperture. Each antenna is packaged into an individual receiver, from which a baseband signal is recorded. The correlator is software-based, utilizing signal processing techniques for visibilities, and image formation via beamforming methods. This paper presents first results from an interferometer flight campaign.

Currently, brownout is the single largest contributor to military rotary-wing losses. Millimeter-wave radiation penetrates these dust clouds effectively, thus millimeter-wave imaging could provide pilots with valuable situational awareness during hover, takeoff, and landing operations. Herein, we detail efforts towards a passive, video-rate imager for use as a brownout mitigation tool. The imager presented herein uses a distributed-aperture, optically-upconverted architecture that provides real-time, video-rate imagery with minimal size and weight. Specifically, we detail phenomenology measurements in brownout environments, show developments in enabling component technologies, and present results from a 30-element aperiodic array imager that has recently been fabricated.

Against a background of newly emerged security threats the well-established idea of utilizing submillimeter-wave radiation for personal security screening applications has recently evolved into a promising technology. Possible application scenarios demand sensitive, fast, flexible and high-quality imaging techniques. At present, best results are obtained by passive imaging using cryogenic microbolometers as radiation detectors. Building upon the concept of a passive submillimeter-wave stand-off video camera introduced previously, we present the evolution of this concept in a practical application-ready imaging device. This has been achieved using a variety of measures such as optimizing the detector parameters, improving the scanning mechanism, increasing the sampling speed, and enhancing the camera software. The image generation algorithm has been improved and an automatic sensor calibration technique has been implemented taking advantage of redundancy in the sensor data. The concept is based on a Cassegrain-type mirror optics, an opto-mechanical scanner providing spiraliform scanning traces, and an array of 20 superconducting transition-edge sensors (TES) operated at a temperature of 450-650 mK. The TES are cooled by a closed-cycle cooling system and read out by superconducting quantum interference devices (SQUIDs). The frequency band of operation centers around 350 GHz. The camera can operate at an object distance of 7-10 m. At 9m distance it covers a field of view of 110 cm diameter, achieves a spatial resolution of 2 cm and a pixel NETD (noise equivalent temperature difference) of 0.1-0.4 K. The maximum frame rate is 10 frames per second.

The cylindrical millimeter-wave imaging technique, developed at Pacific Northwest National Laboratory (PNNL) and commercialized by L-3 Communications/Safeview in the ProVision system, is currently being deployed in airports and other high-security locations to meet person-borne weapon and explosive detection requirements. While this system is efficient and effective in its current form, there are a number of areas in which the detection performance may be improved through the use of other reconstruction algorithms and sensing configurations. PNNL and Northeastern University (NEU) have teamed together to investigate higher-order imaging artifacts produced by the current cylindrical millimeter-wave imaging technique using full-wave forward modeling and laboratory experimentation. Based on imaging results and scattered-field visualizations using the full-wave forward model, a new imaging system is proposed. The new system combines a multistatic sensor configuration with the generalized synthetic aperture focusing technique (GSAFT). Initial results show an improved ability to image in areas of the body where target shading, specular reflections, and higher-order reflections occur.

We are developing a 350 GHz cryogenic passive video imaging system for use in standoff security applications. This demonstration system uses 800 photon-noise-limited superconducting transition edge sensor bolometers, read out using a time-division multiplexed readout system. It will image a 1 m x 1 m field of view at a standoff distance of 16 m to a resolution of approximately 1 cm at video frame rates (20 frames per second). High spatial resolution is achieved by the use of an f/2.0 Cassegrain optical system with 1.3 m primary mirror. Preliminary dark and optical testing of prototype detectors indicates that we can achieve a noise equivalent temperature difference (NETD) below 100 mK for the fully sampled 1 m x 1 m image at 20 frames per second. We report on the current status of development of this system.

In this paper we given an overview of the design and predicted performance of a passive video-rate THz camera intended for stand-off and walk-by concealed weapons and explosives detection. The system is based on previously reported work, and it utilizes a linear array of superconducting antenna-coupled microbolometers. Our present efforts have focussed on improving the performance, stability, set-up time and cost of production of the camera. The system is designed to acquire near video frame rate (~10 Hz) passive THz imagery of objects at ~5 meters from the system, with a field-of-view of 2 m x 1 m and a spatial resolution of 1 cm. The system will be readily integrated to other security systems as it provides encrypted stream of THz imagery over conventional LAN interface that also allows for remote operation.

Measuring many people within a short time is still a great challenge to scanners in security related areas, e.g. airports or stations. A new approach for fast and high resolution scanning is presented, based on the so called "circular synthetic aperture radar" (CSAR), applied to a walkway. Due to the circular movement, each of the receiving antennas creates a circular synthetic aperture itself. By reconstructing the complex valued SAR-images for each receiver channel, the ability to perform interferometric SAR-Analysis of a tested person is given. The screened persons do not have to stand still, thus, a measurement in a passenger flow is possible.

In this work, we present the development of a multi-sensor system for the detection of objects concealed under clothes using passive and active millimeter-wave (mmW) technologies. This study concerns both the optimization of a commercial passive mmW imager at 94 GHz using a phase mask and the development of an active mmW detector at 77 GHz based on synthetic aperture radar (SAR). A first wide-field inspection is done by the passive imager while the person is walking. If a suspicious area is detected, the active imager is switched-on and focused on this area in order to obtain more accurate data (shape of the object, nature of the material ...).

An active system for stand-off imaging operating in a frequency range from 234 GHz to 306 GHz is presented. Imaging is achieved by combining a line array consisting of 8 emitters and 16 detectors with a scanning cylindrical mirror system. A stand-off distance of 7-8 m is achieved using a system of mirrors with effective aperture of 0.5 x 0.5 meter. Information about range and reflectivity of the object are obtained using an active FMCW (frequency modulated continuous wave) radar operation principle. Data acquisition time for one line is as short as 1 ms. Synthetic image reconstruction is achieved in real-time by an embedded GPU (Graphical Processing Unit).

As millimeter-wave imaging technology becomes more mature, several applications are emerging for which this technology may be useful. However, effectively predicting the nuances of millimeter-wave phenomenology on the usefulness for a given application remains a challenge. To this end, an accurate millimeter-wave scene simulator would have tremendous value in predicting imager requirements for a given application. Herein, we present a passive millimeter-wave scene simulator built on the open-source 3d modeling software Blender. We describe the changes made to the Blender rendering engine to make it suitable for this purpose, including physically accurate reflections at each material interface, volumetric absorption and scattering, and tracking of both s and p polarizations. In addition, we have incorporated a mmW material database and world model that emulates the effects of cold sky profiles for varying weather conditions and frequencies of operation. The images produced by this model have been validated against calibrated experimental imagery captured by a passive scanning millimeter-wave imager for maritime, desert, and standoff detection applications.

We present experimental results obtained from a scanning passive W-band fully polarimetric imager. Passive millimeter wave imaging offers persistent day/nighttime imaging and the ability to penetrate dust, clouds and other obscurants, as well as thin layers of clothing and even dry soil. The selection of the W-band atmospheric window at 94 GHz offers a compromise as there is sufficient angular resolution for imaging applications using modestly-sized reflectors appropriate for mobile as well as fixed location applications. The imager is based upon an F/2.1 off-axis parabolic reflector that exhibits -34 dB of cross polarization suppression. The heterodyne radiometer produces a 6 GHz IF with 4 GHz of bandwidth resulting in an NEDT of < 200 mK. Polarimetric imaging reveals the presence of man-made objects due to their typically anisotropic nature and the interaction of these objects with incident millimeter wave radiation. The phenomenology studies were undertaken to determine the richest polarimetric signals to use for exploitation. In addition to a conventional approach to polarimetric image analysis in which the Stokes I, Q, U, and V images were formed and displayed, we present an alternative method for polarimetric image exploitation based upon multivariate image analysis (MIA). MIA uses principal component analysis (PCA) and 2D scatter or score plots to identify various pixel classes in the image compared with the more conventional scene-based image analysis approaches. Multivariate image decomposition provides a window into the complementary interplay between spatial and statistical correlations contained in the data.

The Jet Propulsion Laboratory's 675 GHz, 25 m standoff imaging radar can achieve >1 Hz real time frame rates over 40x40 cm fields of view for rapid detection of person-borne concealed weapons. In its normal mode of operation, the radar generates imagery based solely on the time-of-flight, or range, between the radar and target. With good clothing penetration at 675 GHz, a hidden object will be detectable as an anomaly in the range-to-surface profile of a subject. Here we report on results of two modifications in the radar system that were made to asses its performance using somewhat different detection approaches. First, the radar's operating frequency and bandwidth were cut in half, to 340 GHz and 13 GHz, where there potential system advantages include superior transmit power and clothing penetration, as well as a lower cost of components. In this case, we found that the twofold reduction in range and cross-range resolution sharply limited the quality of through-clothes imagery, although some improvement is observed for detection of large targets concealed by very thick clothing. The second radar modification tested involved operation in a fully polarimetric mode, where enhanced image contrast might occur between surfaces with different material or geometric characteristics. Results from these tests indicated that random speckle dominates polarimetric power imagery, making it an unattractive approach for contrast improvement. Taken together, the experiments described here underscore the primary importance of high resolution imaging in THz radar applications for concealed weapons detection.

The actual and continuous threat by international terrorism and the increasing number of terroristic attacks raise the danger to the public and create a new and more complex dimension of threat. This evolution must and can only be combatted by the application of new counter-measures like advanced imaging technologies for wide-area surveillance and the detection of concealed dangerous objects. 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 and observation areas, hence fully avoiding health risks. Furthermore the acquisition of polarimetric object characteristics can increase the detection capability by gathering complementary object information. The recent development and construction of a fully-polarimetric receiver at W band allows the acquisition of a new dimension of information compared to former imaging capabilities. The new receiver can be part of various imaging systems used at DLR over the years. This paper will show some imaging results recorded recently from different sceneries.

A pulsed terahertz imaging system modified to perform angular detection of light for quantitatively characterizing reflection and scattering from samples is reported. Reflection from a gold mirror shows that the full width half maximum (FWHM) of the terahertz beam angular spread is < 1° with signal-to-noise of 65 dB. Two samples, a paper index card and a corduroy cloth sample were tested. The index card reflects ca. 1% of the incident terahertz energy with similar angular spreading while the corduroy sample reflected approximately 0.01% of the incident terahertz energy with FWHM of 5 - 10°. The corduroy sample also exhibits temporal pulse scattering as a function of angle which correlates with direct frequency domain measurements.

Cylindrical millimeter-wave imaging systems and technology have been under development at the Pacific Northwest National Laboratory (PNNL) for several years. This technology has been commercialized, and systems are currently being deployed widely across the United States and internationally. These systems are effective at screening for concealed items of all types; however, new sensor designs, image reconstruction techniques, and image rendering algorithms could potentially improve performance. At PNNL, a number of specific techniques have been developed recently to improve cylindrical imaging methods including wideband techniques, combining data from full 360-degree scans, polarimetric imaging techniques, calibration methods, and 3-D data visualization techniques. Many of these techniques exploit the three-dimensionality of the cylindrical imaging technique by optimizing the depth resolution of the system and using this information to enhance detection. Other techniques, such as polarimetric methods, exploit scattering physics of the millimeter-wave interaction with concealed targets on the body. In this paper, calibration, reconstruction, and three-dimensional rendering techniques will be described that optimize the depth information in these images and the display of the images to the operator.

We present a Hadamard transform based imaging technique and have implemented it on a single-pixel passive millimeter-wave imager in the 146-154 GHz range. The imaging arrangement uses a set of Hadamard transform masks of size p x q at the image plane of a lens and the transformed image signals are focused and collected by a horn antenna of the imager. The cyclic nature of Hadamard matrix allows the use of a single extended 2-D Hadamard mask of size (2p-1) x (2q-1) to expose a p x q submask for each acquisition by raster scanning the large mask one pixel at a time. A total of N = pq acquisitions can be made with a complete scan. The original p x q image may be reconstructed by a simple matrix operation. Instead of full N acquisitions, we can use a subset of the masks for compressive sensing. In this regard, we have developed a relaxation technique that recovers the full Hadamard measurement space from sub-sampled Hadamard acquisitions. We have reconstructed high fidelity images with 1/9 of the full Hadamard acquisitions, thus reducing the image acquisition time by a factor of 9.

In the absence of detector arrays, a single pixel coupled with a spatially selective mask has been shown to be a practical solution to imaging problems in the terahertz and sub-millimeter wave domains. In this paper we demonstrate real-time two-dimensional imager for sub-millimeter waves that is based on a spatially selective image plane mask. The imager consists of a heterodyne source and receiver pair, image forming optics, a spatially selective mask, data acquisition hardware, and image reconstruction software. The optics form an image onto the spatially selective mask and linear measurements of the image are made. The mask must be designed to ensure maximum transmission, measurement linearity, and measurement to measurement independence and our design parameters are presented. Once enough linearly independent measurements are made, the image is reconstructed by solving a system of linear equations that is generated from the mask patterns and the corresponding measurements. We show that for image sizes envisioned for many current applications, this image reconstruction technique is computationally efficient and can be implemented in real time. We present images collected using this system, discuss the results, and discuss other applications for some components of the imager.

In this paper we demonstrate the use of compressive sensing to form an image with an image plane random mask and a single pixel sub-millimeter wave receiver. This type of imaging device is a practical solution in domains where focal plane arrays do not exist. The imager consists of a heterodyne source and receiver pair, image forming optics, a spatially selective mask, and data acquisition and post-processing hardware and software. The spatially selective mask modulates the signal measured by the receiver which is then sampled by an analog to digital converter and is post-processed to reconstruct the image. The spatially selective mask can produce image samples at full video rates. The post-processing used for this research consists of a sparseness inducing transformation on the measurements and application of compressive sensing reconstruction algorithms. We show several images acquired and reconstructed using this system. While the data acquisition of this system is real time, the processing currently must be done online. We comment on the performance improvement by using compressive sensing methods.

Stand-off THz imaging to detect concealed treats is a coming technique for security applications. A THz sensor can provide high resolution 3D imagery of a scene. However, efficient scene scanning and management of the THz sensor is a challenging task due to the limited field of view of the sensor and physical scanning limitations. In this paper we discuss the requirements on a scene scanning solution and present a scene scanning technique using a multi-camera system with 3D positioning capabilities. A visual hull method is used to position subjects in the scene. The presented technique relaxes the requirements on the scanning speed of the THz sensor and facilitates an efficient scene scanning solution.

Recent developments in millimetre to submillimetre-wave imaging radars with excellent ranging resolution provide an attractive route towards stand-off imaging of concealed explosives at ranges up to several tens of meters. Present systems typically rely on only one transceiver, coupled with an optomechanical scanning system for image formation. This limits the image acquisition speed to several seconds/frame. Frame rate can in principle be increased with increasing the channel count but this adds substantially to the system complexity and cost, while only providing a modest speed increase. In this paper we present preliminary designs for rapid electronic beam steering system that could provide a way towards real-time millimetre-wave to submillimetre-wave imaging radars.

The goal of this project is to enable light-weight, durable, and portable systems capable of performing standoff detection of person-borne improvised explosive devices (PB-IEDs) through the development of millimeter-wave reflection-type phased arrays. Electronic beam steering eliminates the need for complex mechanical scanners that are commonly implemented with millimeter-wave imaging systems and would reduce overall system size and weight. We present a concept study of a 220 GHz reflection-type phased array for the purpose of performing beam scanning of a confocal reflector system. Requirements for effective imaging of the desired target region are established, including spatial resolution, total scan angle, and number of image pixels achievable. We examine the effects of array architecture on beam characteristics as it is scanned off broadside, including Gaussicity and encircled energy. Benchmark requirements are determined and compared with the capabilities of several potential phase shifter technologies, including MEMS-based variable capacitor phase shifters, switches, and varactor diode-based phase shifters.

At this paper we report on a W-band direct detection radiometer cascading a single-pole four-throw switch with integrated 50 Ω load as a reference noise source, a 3 x 20 dB low-noise amplifier chain, and a broadband Schottky-diode detector. All components are designed and fabricated in 100 nm metamorphic high electron mobility transistor (mHEMT) technology and use waveguide packaging. By using 2 channels of the switch module the Dicke-principle is implemented to drastically reduce the inherent amplifier noise. The multi-throw switch insertion loss is less than 3.5 dB on the chip level and 4.4 dB on the module level. The entire W-band direct detection radiometer chain is also integrated on a single chip and packaged into a waveguide module, which was successfully tested and is now ready for system integration.

At the moment, body scanners based on terahertz and millimeter-wave technologies are implemented at airports around the world. Thus, challenges of acceptance and acceptability become pressing. In this context, we present the results of an ethical research project on the development and implementation of body scanners. We will show which requirements concerning the system, its developers, and its users should be met in order that the scanners can be acceptable from an ethical point of view. These requirements involve, among others, questions of privacy, health, data protection, and security processes. A special ethical challenge for body scanners, however, still remains: Automatic anonymization processes are based on the assumption of "normal" bodies. Certain groups of persons with "deviant bodies" (e.g. persons with hidden disabilities, persons with aberrant sex characteristics, etc.) are affected in a special way: Their deviation from the standard (for instance their disability) is socially hidden, but eventually exposed by body scanners, even (and even more) if the produced pictures are anonymized. Here, we address the question how the possible discrimination against and exclusion of people with "deviant bodies" could be mitigated or prevented.