This PDF file contains the front matter associated with SPIE Proceedings Volume 9078, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

The challenge for any security screening system is to identify potentially harmful objects such as weapons and explosives concealed under clothing. Classical border and security checkpoints are no longer capable of fulfilling the demands of today’s ever growing security requirements, especially with respect to the high throughput generally required which entails a high detection rate of threat material and a low false alarm rate. TeraSCREEN proposes to develop an innovative concept of multi-frequency multi-mode Terahertz and millimeter-wave detection with new automatic detection and classification functionalities. The system developed will demonstrate, at a live control point, the safe automatic detection and classification of objects concealed under clothing, whilst respecting privacy and increasing current throughput rates. This innovative screening system will combine multi-frequency, multi-mode images taken by passive and active subsystems which will scan the subjects and obtain complementary spatial and spectral information, thus allowing for automatic threat recognition. The TeraSCREEN project, which will run from 2013 to 2016, has received funding from the European Union’s Seventh Framework Programme under the Security Call. This paper will describe the project objectives and approach.

Freight transportation service by truck is an extremely growing market all over the world. Consequently, optimization of truck’s capacity utilization by in-situ estimation of load distribution with a fast and stand-off monitoring sensor is useful. MWs or MMWs used in radars and radiometers can penetrate thin dielectric walls like synthetic truck canvas. Such systems can deliver also valuable information for security applications, e.g. about illegal transportation attempts. This paper describes the application of DLR’s experimental MW radar and MMW radiometers used for estimation of truck load under controlled driving conditions of a test truck. Experimental imaging results of both systems are presented.

Millimeter wavelength radiation holds promise for detection of security threats at a distance, including suicide bomb belts and maritime threats in poor weather. The high sensitivity of superconducting Transition Edge Sensor (TES) detectors makes them ideal for passive imaging of thermal signals at these wavelengths. We have built a 350 GHz video-rate imaging system using a large-format array of feedhorn-coupled TES bolometers. The system operates at a standoff distance of 16m to 28m with a spatial resolution of 1:4 cm (at 17m). It currently contains one 251-detector subarray, and will be expanded to contain four subarrays for a total of 1004 detectors. The system has been used to take video images which reveal the presence of weapons concealed beneath a shirt in an indoor setting. We present a summary of this work.

Sub-millimeter wave 3D imaging radar is a promising technology for the stand-off detection of threats concealed on people. The IRAD 340 GHz 3D imaging radar uses polarization intensity information to identify signatures associated with concealed threats. We report on an extensive trials program which has been carried out involving dozens of individual subjects wearing a variety of different clothing to evaluate the detection of a wide range of threat and benign items. We have developed an automatic algorithm to run on the radar which yields a level of anomaly indication in real time. Statistical analysis of the large volume of recorded data has enabled performance metrics for the radar system to be evaluated.

This paper discusses the design of an improved passive millimeter wave imaging system intended to be used for base security in degraded visual environments. The discussion starts with the selection of the optimum frequency band. The trade-offs between requirements on detection, recognition and identification ranges and optical aperture are discussed with reference to the Johnson Criteria. It is shown that these requirements also affect image sampling, receiver numbers and noise temperature, frame rate, field of view, focusing requirements and mechanisms, and tolerance budgets. The effect of image quality degradation is evaluated and a single testable metric is derived that best describes the effects of degradation on meeting the requirements. The discussion is extended to tolerance budgeting constraints if significant degradation is to be avoided, including surface roughness, receiver position errors and scan conversion errors. Although the reflective twist-polarization imager design proposed is potentially relatively low cost and high performance, there is a significant problem with obscuration of the beam by the receiver array. Methods of modeling this accurately and thus designing for best performance are given.

We present design and simulations of a reconfigurable reflectarray suitable for use as a sub-reflector within a confocal millimeter-wave imaging system. The performance of various reflectarray designs for use within a confocal imaging system are evaluated, e.g. phase quantization, feeding field. A quantized-particle-swarm optimization algorithm is discussed and applied to the reflectarray phase distribution in order to circumvent common challenges, such as the effects of the periodic phase error across the aperture, in addition to the generation of grating lobes due to aperture discontinuities. The reflectarray elements are based on a microstrip structure integrated with MEMS-based phase shifters in order to yield dynamic reconfigurability of the aperture. Both 1- and 2-bit phase quantization is considered. A reflectarray is characterized in a near-field measurement range; reflections from individual reflectarray elements are imaged at a distance on the order of a wavelength. Both the realized reflection phase and magnitude of the specular reflection are measured, yielding accurate characteristics of reflectarray element performance when compared to standard beam pattern measurements. A near-field measurement system is configured and tested at 120 GHz. The lateral resolution and phase measurement accuracy were measured to be 0:56λ and ±4° respectively. A static reflectarray is tested in the measurement system, and the reflection coefficient of the individual elements measured.

Since 2007 we are developing passive submillimeter-wave video cameras for personal security screening. In contradiction to established portal-based millimeter-wave scanning techniques, these are suitable for stand-off or stealth operation. The cameras operate in the 350GHz band and use arrays of superconducting transition-edge sensors (TES), reflector optics, and opto-mechanical scanners. Whereas the basic principle of these devices remains unchanged, there has been a continuous development of the technical details, as the detector array, the scanning scheme, and the readout, as well as system integration and performance. The latest prototype of this camera development features a linear array of 128 detectors and a linear scanner capable of 25Hz frame rate. Using different types of reflector optics, a field of view of 1×2m² and a spatial resolution of 1–2 cm is provided at object distances of about 5–25m. We present the concept of this camera and give details on system design and performance. Demonstration videos show its capability for hidden threat detection and illustrate possible application scenarios.

We discuss the application of recently developed 670 GHz low-noise amplifiers based on InP HEMTs to passive indoor imaging. Packaged LNAs were integrated with commercial zero-bias diodes, and accurate measurements of system noise-equivalent temperature difference (NETD) made, using blackbody sources. The NETD values are compared with independent prior measurements (Deal et al. 2011) of LNA gain, noise figure, and bandwidth, and with cryogenic bolometer measurements made in the same test conditions. Currently, the LNA gain is not sufficient to render the ZBD noise negligible; measurements are presented that separate the two components. Low-frequency noise measurements are also presented that display the effects of 1/f noise in the ZBD and gain variations in the LNA. The implications of the low-frequency noise are discussed in terms of scanning or beam-steering strategies for an imager based on the LNAs. Raster-scanned, single-pixel images of indoor scenes are presented. They are quantitatively interpreted in terms of NETD, and angular resolution and coupling efficiency of the optics.

Successful development of remote sensing and communication systems in the terahertz band requires a better understanding of the scattering behavior of various structures. Materials that could be considered homogeneous and smooth at microwave frequencies may begin to display surface and volumetric scattering behavior in the terahertz band. The co-polarization backscattering coefficient of several types of metal and dielectric structures were measured in indoor compact radar ranges operating at 100 GHz, 160 GHz, 240 GHz, and 1.55 THz. These structures consisted of roughened aluminum plates, as well as homogeneous and inhomogeneous dielectric surfaces. The roughness and inclusions of the measured samples were tailored in order to systematically investigate various scattering effects. Polarimetric backscattering measurements of these materials were collected at elevation angles from 5 to 75 degrees. Analysis of the backscatter data supports a better understanding of surface and volumetric scattering behavior of materials at terahertz frequencies.

Many groups are developing submillimeter cameras that will be used to screen human subjects for improvised explosive devices (IEDs) and other threat items hidden beneath their clothing. To interpret submillimeter camera images the scattering properties, specifically the bidirectional scattering distribution function (BSDF) must be known. This problem is not trivial because surfaces of man-made objects and human skin have topographic features comparable to the wavelength of submillimeter radiation—thus simple, theoretical scattering approximations do not apply. To address this problem we built a goniometer instrument to measure the BSDF from skin surfaces of live human subjects illuminated with a beam from a 650 GHz synthesized source. To obtain some multi-spectral information, the instrument was reconfigured with a 160 GHz source. Skin areas sampled are from the hand, interior of the forearm, abdomen, and back. The 650 GHz beam has an approximately Gaussian profile with a FWHM of approximately 1 cm. Instrument characteristics: angular resolution 2.9^⍛; noise floor -45 dB/sr; dynamic range ˃ 70 dB; either s or p-polarization; 25^⍛ bidirectional-scattering-angle ≤ 180^⍛ ; The human scattering target skin area was placed exactly on the goniometer center of rotation with normal angle of incidence to the source beam. Scattering power increased at the higher frequency. This new work enables radiometrically correct models of humans.

Imaging systems in millimeter waves are required for applications in medicine, communications, homeland security, and space technology. This is because there is no known ionization hazard for biological tissue, and atmospheric attenuation in this range of the spectrum is low compared to that of infrared and optical rays. The lack of an inexpensive room temperature detector makes it difficult to give a suitable real time implement for the above applications. A 3D MMW imaging system based on chirp radar was studied previously using a scanning imaging system of a single detector. The system presented here proposes to employ a chirp radar method with Glow Discharge Detector (GDD) Focal Plane Array (FPA of plasma based detectors) using heterodyne detection. The intensity at each pixel in the GDD FPA yields the usual 2D image. The value of the I-F frequency yields the range information at each pixel. This will enable 3D MMW imaging. In this work we experimentally demonstrate the feasibility of implementing an imaging system based on radar principles and FPA of inexpensive detectors. This imaging system is shown to be capable of imaging objects from distances of at least 10 meters.

Compressive sensing has been identified as a significant technique to reduce the volume of data collected in sensing applications to a minimum. Prior art has empirically demonstrated the effectiveness of a spinning disk for reconstruction of TeraHertZ (THZ) images. Prior empirical data has demonstrated reconstruction artifacts that are associated, in part, with the statistical Probability Density Function (PDF) of the randomly distributed transmission holes in the rotating plate. Empirical demonstration at other wavelengths such as the InfRared (IR) has also been suggested. This document summarizes the statistical requirements for artifact minimization for the previously reported spinning disk system. Consideration is given to the impact of operation at non-THZ wavelengths such as the IR.

A miniature neon indicator lamp, also known as a Glow Discharge Detector (GDD), costing about 50 cents, was found to be an excellent room temperature THz radiation detector. Polarization effects on heterodyne detection were investigated in this work. In heterodyne detection, because of the dot product relationship between signal electric field (E_S) and local oscillator (LO) electric field (E_lo), optimal operation of heterodyne detection is obtained when E_S and E_lo are of the same polarization. Preliminary results at 300 GHz showed better sensitivity by a factor of 20 with only 56 microwatt local oscillator power using heterodyne compared to direct detection. Further improvement of the detection sensitivity can be achieved if the LO power (P_lo) is increased. In this work investigation of polarization effects in heterodyne detection using neon indicator lamp GDD was carried-out. Experimental results of heterodyne detection at 300 GHz showed that an intermediate frequency (IF) signal was obtained for orthogonal polarization of the LO and signal, in contradiction to the theory. Also, our latest imaging results using Glow Discharge Detector at millimeter wavelengths will be shown in this work.

Sequentially-switched linear arrays are an enabling technology for a number of near-field microwave imaging applications. Electronically sequencing along the array axis followed by mechanical scanning along an orthogonal axis allows dense sampling of a two-dimensional aperture in near real-time. The Pacific Northwest National Laboratory (PNNL) has developed this technology for several applications including concealed weapon detection, groundpenetrating radar, and non-destructive inspection and evaluation. These techniques form three-dimensional images by scanning a diverging beam swept frequency transceiver over a two-dimensional aperture and mathematically focusing or reconstructing the data into three-dimensional images. Recently, a sparse multi-static array technology has been developed that reduces the number of antennas required to densely sample the linear array axis of the spatial aperture. This allows a significant reduction in cost and complexity of the linear-array-based imaging system. The sparse array has been specifically designed to be compatible with Fourier-Transform-based image reconstruction techniques; however, there are limitations to the use of these techniques, especially for extreme near-field operation. In the extreme near-field of the array, back-projection techniques have been developed that account for the exact location of each transmitter and receiver in the linear array and the 3-D image location. In this paper, the sparse array technique will be described along with associated Fourier-Transform-based and back-projection-based image reconstruction algorithms. Simulated imaging results are presented that show the effectiveness of the sparse array technique along with the merits and weaknesses of each image reconstruction approach.

Image processing techniques can be used to improve the cost-effectiveness of future interferometric Passive MilliMetre Wave (PMMW) imagers. The implementation of such techniques will allow for a reduction in the number of collecting elements whilst ensuring adequate image fidelity is maintained. Various techniques have been developed by the radio astronomy community to enhance the imaging capability of sparse interferometric arrays. The most prominent are Multi- Frequency Synthesis (MFS) and non-linear deconvolution algorithms, such as the Maximum Entropy Method (MEM) and variations of the CLEAN algorithm. This investigation focuses on the implementation of these methods in the defacto standard for radio astronomy image processing, the Common Astronomy Software Applications (CASA) package, building upon the discussion presented in Taylor et al., SPIE 8362-0F. We describe the image conversion process into a CASA suitable format, followed by a series of simulations that exploit the highlighted deconvolution and MFS algorithms assuming far-field imagery. The primary target application used for this investigation is an outdoor security scanner for soft-sided Heavy Goods Vehicles. A quantitative analysis of the effectiveness of the aforementioned image processing techniques is presented, with thoughts on the potential cost-savings such an approach could yield. Consideration is also given to how the implementation of these techniques in CASA might be adapted to operate in a near-field target environment. This may enable a much wider usability by the imaging community outside of radio astronomy and thus would be directly relevant to portal screening security systems in the microwave and millimetre wave bands.

Calculations are presented of vibrational resonance structure at THz frequencies for a 38 molecule cluster of H2O using density functional theory (DFT). This resonance structure is due to coupling of molecular vibrational modes. In particular, the coupling among resonance modes provides a reasonable molecular level interpretation of spectral features associated with THz excitation of molecular clusters. THz excitation is associated with frequencies that are characteristically perturbative to molecular electronic states, in contrast to frequencies that can induce appreciable electronic state transition. Owing to this characteristic of THz excitation, one is able to make a direct association between local oscillations about ground-state minima of molecules comprising a cluster and THz excitation spectra. The DFT software GAUSSIAN was used for the calculations of vibrational resonance structure presented here.

As it is well-known, application of the passive THz camera for the security problems is very promising way. It allows seeing concealed object without contact with a person and this camera is non-dangerous for a person. Efficiency of using the passive THz camera depends on its temperature resolution. This characteristic specifies possibilities of the detection of concealed object: minimal size of the object, maximal distance of the detection, image detail. One of probable ways for a quality image enhancing consists in computer processing of image. Using computer processing of the THz image of objects concealed on the human body, one may improve it many times. Consequently, the instrumental resolution of such device may be increased without any additional engineering efforts. We demonstrate new possibilities for seeing the clothes details, which raw images, produced by the THz cameras, do not allow to see. We achieve good quality of the image due to applying various spatial filters with the aim to demonstrate independence of processed images on math operations. This result demonstrates a feasibility of objects seeing. We consider images produced by THz passive cameras manufactured by Microsemi Corp., and ThruVision Corp., and Capital Normal University (Beijing, China).

In this paper, we review automatic concealed object recognition with multi-channel passive millimeter wave images. A four-channel passive millimeter wave imaging system operates in the 8 and 3 mm wavelength regimes with linear vertical and horizontal polarization directions. Registration between multi-channel images and segmentation of concealed objects are addressed. Multi-channel image registration is performed by means of the affine transform derived by the geometric feature matching. Gaussian mixture models are adopted to cluster hidden object pixels in the images. Multi-level segmentation separates the human body region from the background, and concealed objects from the body region, sequentially. In the experiments, the metallic and non-metallic objects concealed under clothing are captured and processed.