In recent years the security of people and critical infrastructures is of increasing interest. Passive microwave sensors in
the range of 1 - 100 GHz are suitable for the detection of concealed objects and wide-area surveillance through poor
weather and at day and night time. The enhanced extraction of significant information about an observed object is
enabled by the use of a spectral sensitive system. For such a spectral radiometer in the microwave range also some depth
information can be extracted. The usable frequency range is thereby dependent on the application. For through-wall
imaging or detection of covert objects such as for example landmines, the lower microwave range is best suited. On the
other hand a high spatial resolution requires higher frequencies or instruments with larger physical dimensions. The
drawback of a large system is the required movement of a mirror or a deflecting plate in the case of a mechanical scanner
system, or a huge amount of receivers in a fully-electronic instrument like a focal plane array. An innovative technique
to overcome these problems is the application of aperture synthesis using a highly thinned array. The combination of
spectral radiometric measurements within a wide frequency band, at a high resolution, and requiring a minimum of
receivers and only minor moving parts led to the development of the ANSAS instrument (Abbildendes Niederfrequenz-Spektrometer mit Apertursynthese). ANSAS is a very flexible aperture synthesis technology demonstrator for the
analysis of main features and interactions concerning high spatial resolution and spectral sensing within a wide
frequency range. It consists of a rotated linear thinned array and thus the spatial frequency spectrum is measured on
concentric circles. Hence the number of receivers and correlators is reduced considerably compared to a fully two-dimensional
array, and measurements still can be done in a reasonable time. In this paper the basic idea of ANSAS and
its setup are briefly introduced. Some first imaging results showing the basic capabilities are illustrated. Possible error
sources and their impacts are discussed by simulation and compared to the measured data.
We have built a passive millimeter wave imaging and spectroscopy system with a 15-channel filter bank in the 146-154
GHz band for terrestrial remote sensing. We had built the spectroscopy system first and have now retrofitted an imaging
element to it as a single pixel imager. The imaging element consisted of a 15-cm-diameter imaging lens fed to a
corrugated scalar horn. Image acquisition is carried out by scanning the lens with a 2-axis translation stage. A
LabVIEW-based software program integrates the imaging and spectroscopy systems with online display of spectroscopic
information while the system scans each pixel position. The software also allows for integrating the image intensity of
all 15 channels to increase the signal-to-noise ratio by a factor of ~4 relative to single channel image. The integrated
imaging and spectroscopy system produces essentially 4-D data in which spatial data are along 2 dimensions, spectral
data are in the 3rd dimension, and time is the 4th dimension. The system performance was tested by collecting imaging
and spectral data with a 7.5-cm-diameter and 1m long gas cell in which test chemicals were introduced against a liquid
The performance of stand-off imaging systems of concealed weapons in the mm-wave range remains limited by the
relatively poor angular resolution using practical aperture sizes. For this reason, increasing the operating frequency of the
systems is desired, but in practice is hard to realize due to the lack of affordable, low noise amplifiers well beyond 100
GHz. In this paper we present a passive terahertz imaging system which acquires passive terahertz (~200 GHz - ~1 THz)
imagery near video frame rate. The system, one copy of which is built in Finland and the other in the U.S., is based on a
64 pixel linear array of superconducting antenna-coupled microbolometers operated within a commercial cryogen-free
closed cycle cryocooler, and utilizes conical scanning Schmidt optics. Quantitative measurements on the imager
resolution metrics (thermal, spatial and temporal) will be presented. The results from field tests at the Helsinki-Vantaa
airport will be presented.
As reported before,1, 2 Safe VISITOR (Safe VISible, Infrared and Terahertz Object recognition) is a German
project to build a passive security camera which visualizes sub-mm wavelengths using cooled bolometer arrays.
This camera could be used for a variety of application scenarios, such as airport screenings or to protect military
camps. In all cases, a practical instrument requires ease of use, in particular a flexible installation and a
straightforward usage by the security personnel.
Here we present a new generation of Safe VISITOR designed to meet these requirements. The main condition
for an effective operation is a high frame rate of the imager. Safe VISITOR is able to record videos up to 10 Hz,
using a small array of superconducting bolometers in combination with an opto-mechanical scanner. The required
cooling of the detector array is provided by a commercial pulse tube cooler with a second, self-contained cooling
stage. The cooling cycle is completely automated; after 10 hours of initial cooling from room temperature the
system can operate quasi-continuously.
For imaging, a 50 cm diameter optics is used which is able to provide an object resolution of approximately
1.5 cm at 8 m distance. For a flexible installation, the object distance can be tuned manually between 7 and
10 m. Additionally, video streams from two commercial cameras are fused with the sub-mm stream: a CCD for
visible light and a microbolometer for far infrared (14 μm). This combines the ability of identification of the
person under test with the unprecedented temperature resolution at infrared and the almost perfect transmission
at sub-mm. To assist a security official, all image data are displayed in various graphic renditions by a unified
Present applications of microwave remote sensing systems cover a large variety. One utilisation of the frequency range
from 1 - 300 GHz is the domain of security and reconnaissance. Examples are the observation of critical infrastructures
or the performance of security checks on people in order to detect concealed weapons or explosives, both being frequent
threats in our world of growing international terrorism. The imaging capability of concealed objects is one of the main
advantages of microwave remote sensing, because of the penetration performance of electromagnetic waves through
dielectric materials in this frequency domain. The main physical effects used in passive microwave sensing rely on the
naturally generated thermal radiation and the physical properties of matter, the latter being surface characteristics,
chemical and physical composition, and the temperature of the material. As a consequence it is possible to discriminate
objects having different material characteristics like ceramic weapons or plastic explosives with respect to the human
body. Considering the use of microwave imaging with respect to people scanning systems in airports, railway stations, or
stadiums, it is advantageous that passively operating devices generate no exposure on the scanned objects like actively
operating devices do. For frequently used security gateways it is additionally important to have a high through-put rate in
order to minimize the queue time. Consequently fast imaging systems are necessary. In this regard the conceptual idea of
a fully-electronic microwave imaging radiometer system is introduced. The two-dimensional scanning mechanism is
divided into a frequency scan in one direction and the method of aperture synthesis in the other. The overall goal here is
to design a low-cost, fully-electronic imaging system with a frame rate of around one second at Ka band. This frequency
domain around a center frequency of 37 GHz offers a well-balanced compromise between the achievable spatial
resolution for a given size, and the penetration depth of the electromagnetic wave, which are conflictive requirements.
A millimeter-wave radar designed for landing helicopters in brown-out conditions is described and data is presented
from an initial flight test. The radar operates in a frequency modulated continuous wave architecture, determining range
to target by calculating the difference between transmitted and returned frequencies. The millimeter-wave frequency
band provides sand and dust penetration and allows for small apertures appropriate for helicopter mounting. This radar
also uses a flat panel phased-array receive antenna and phase processor to sample multiple antenna beams
simultaneously, an architecture that has previously been successfully used in passive millimeter-wave imaging systems.
The radar presents a wide field-of-view image to the operator at a 3 Hz frame rate where range to the ground and
obstacles is depicted in grayscale. The flight test showed the radar to be capable of depicting terrain height variations
and obstacles such as buildings, vehicles, building materials, and even power lines. Reductions in noise and symbology
improvements are necessary developments for a viable landing system.
The sub-millimeter (sub-mm) wave frequency band from 300 - 1000 GHz is currently being developed for standoff
concealed weapon detection imaging applications. This frequency band is of interest due to the unique combination of
high resolution and clothing penetration. The Pacific Northwest National Laboratory (PNNL) is currently developing a
350 GHz, active, wideband, three-dimensional, radar imaging system to evaluate the feasibility of active sub-mm
imaging for standoff detection. Standoff concealed weapon and explosive detection is a pressing national and
international need for both civilian and military security, as it may allow screening at safer distances than portal
screening techniques. PNNL has developed a prototype active wideband 350 GHz radar imaging system based on a
wideband, heterodyne, frequency-multiplier-based transceiver system coupled to a quasi-optical focusing system and
high-speed rotating conical scanner. This prototype system operates at ranges up to 10+ meters, and can acquire an
image in 10 - 20 seconds, which is fast enough to scan cooperative personnel for concealed weapons. The wideband
operation of this system provides accurate ranging information, and the images obtained are fully three-dimensional.
During the past year, several improvements to the system have been designed and implemented, including increased
imaging speed using improved balancing techniques, wider bandwidth, and improved image processing techniques. In
this paper, the imaging system is described in detail and numerous imaging results are presented.
We present an active THz-imaging technique, which utilizes holographic process in image retrieval. In this technique,
information of the target is stored in an interference pattern. The pattern is formed with a reference field and a field
reflected from the target. This technique, called indirect holographic imaging, involves only amplitude detection. The
image of the target is formed computationally from the complex field given by the holographic process. An experimental
imaging system operated at 310 GHz is described. Millimeter-wave images of different targets are presented. The imager
performance is described with image signal-to-noise ratio and noise equivalent reflectivity difference, as well as with the
cross-range resolution. The indirect holographic imaging method is assessed with variable system signal-to-noise ratios.
A knife-edge method is utilized to approximate the point spread function of the imaging system. Cross-range resolution
of 0.18° and noise equivalent reflectivity level of 0.002 is achieved with an experimental imager at
310 GHz with 40-cm aperture.
This paper describes an active millimeter-wave (MMW) holographic imaging system used for the study of compressive
measurement for concealed weapons detection. We record a digitized on-axis, Gabor hologram using a single pixel
incoherent receiver that is translated at the detector plane to form an image composite. Capturing measurements in the
MMW regime can be costly since receiver circuits are expensive and scanning systems can be plagued by their long data
acquisition times. Thus, we leverage recent advances in compressive sensing with a traditional holographic method in
order to estimate a 3D (x,y,z) object distribution from a 2D recorded image composite. To do this, we minimize a convex
quadratic function using total variation (TV) regularization. Gabor holograms are recorded of semi-transparent objects,
in the MMW, mimicking weapons and other objects. We present preliminary results of 3D reconstructions of objects at
various depths estimated from a 2D recorded hologram. We compare backpropagation results with our decompressive
inference algorithm. A possible application includes remote concealed weapons detection at security checkpoints.
The authors present an analysis of compressive sensing (CS) as applied to millimeter wave and optical
imaging systems, showing that the technique inherently reduces detection efficiency due to reflection and diffraction
effects of the underlying electromagnetics. The results show that single-detector imaging approaches that rely on
simultaneous detection of multiple spatial modes (i.e., image pixels) require an electrically large detector to maintain
high detection efficiency.
Devices using electromagnetic (EM) waves in the GHz range are evolving rapidly. The advancement of this technology
for security applications, such as explosives detection and personnel screening, requires an understanding of the optical
properties of various materials. Using terahertz time-domain spectroscopy and free space millimeter-wave
measurements, the dielectric constant of explosives have been measured. Methods used to standardize the experimental
measurement and characterize the EM/material interaction are described. These results have enabled the development of
mixtures of benign substances as simulants for testing. A comparison of the anticipated signal returns are presented for
the range 100 - 500 GHz for a limited set of explosives and simulants.
The brightness of radiation escaping a two-dimensional slab of material under ambient illumination is characterized
in terms of its complex dielectric constant. Transmission and reflection coefficients derive from wave
optics and the application of Beer's law; the emissivity follows from detailed balancing using Kirchoff's law. The
solutions are compared with intensities measured with a commercial millimeter wave imaging system. The results
show that millimeter wave imaging of semi-transparent materials can be described by optical physics based
on dielectric material properties. In addition, analysis of millimeter wave images of materials could provide
information about their dielectric properties.
Belgium leads the tenth initiative in the CNAD Programme of Work for the Defense Against Terrorism (PoW
DAT), dealing with Critical Infrastructure Protection (CIP). The BELCOAST 09 event, comprising a series of
technology demonstrations, was organized to tackle the need for an event that brings together the operational,
armaments and technological communities in the field of CIP. A counter terrorism scenario has been created:
Terrorist with body-borne IED approaching the entrance of an installation, and a millimeter-wave imager's ability to
detect IEDs has been demonstrated. The results of this scenario-based demonstration are presented in this paper.
This paper addresses the registration and the fusion techniques between passive millimeter wave (MMW) and visual
images for concealed object detection. The passive MMW imaging system detects concealed objects such as metal and
man-made objects as well as small liquid and gel containers. The registration and fusion processes are required to
combine information from both of visual and MMW images. The registration process is composed of feature extraction
and matching stages. The body areas in two images are adjusted to each other in scale, rotation, and location. The image
fusion method is based on discrete wavelet transform and a fusion rule, which emphasizes the person's identity and the
hidden object together. The experimental and simulation results show the proposed technique can detect a concealed
object and fuse two different types of images in a fully automated way.
The THz range offers the possibility of measuring water content. This can be useful in wine industry to
control plants water levels and also to decrease irrigation costs. This paper presents a THz imaging
system used to characterise water content in leaves using frequency and time domain methods from 0.14
to 0.22 THz. Our results show the possibility of getting useful information out of the preformed
In this paper we present a single mode active device for sub-millimeter wave line imaging. The illuminated scene
is imaged through focusing optics onto a device we have developed and have dubbed a spatially selective mask
(SSM). This device transmits parts of the image onto a heterodyne receiver. Currently the SSM is capable of
transmitting user-selectable parts of one line of the image that is focused on it. Multiple patterns are used to
sample a line in the image. The voltage in the receiver resulting from each pattern constitutes an independent
measurement of the illuminated scene along a line. A one dimensional image is reconstructed from the measurement
results and a priori knowledge of the patterns using methods derived from the theory of compressive
sensing. The theory behind the device and the design principles we use are reviewed. We show line images
obtained at 640 GHz. Extension of this technique to two dimensional imaging is discussed.
We are developing a 350 GHz cryogenic passive video imaging system. This demonstration system uses 800
photon-noise-limited superconducting transition edge sensor bolometers. It will image a 1 m x 1 m area 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 testing of prototype detectors indicates that we can achieve a noise equivalent temperature difference
(NETD) of 70 mK for the fully sampled 1 m × 1 m image at 20 frames per second.
A fully-integrated silicon-based 94-GHz direct-detection imaging receiver with on-chip Dicke switch and baseband
circuitry is demonstrated. Fabricated in a 0.18-μm SiGe BiCMOS technology (fT/fMAX = 200 GHz), the receiver chip
achieves a peak imager responsivity of 43 MV/W with a 3-dB bandwidth of 26 GHz. A balanced LNA topology with an
embedded Dicke switch provides 30-dB gain and enables a temperature resolution of 0.3-0.4 K. Initial imaging
measurements using the chip along with off-chip antennas are also presented. The imager chip consumes 200 mW from
a single 1.8-V power supply.
We describe progress towards an Oroton-based sub-millimeter-wave source with a design frequency of 500 GHz. Key
features of the devices are a microfabricated, carbon nanotube field-emission-based electron gun which creates a sheet-beam
at the required current density without the need for beam compression, and a microfabricated Smith-Purcell
grating, and a uniform Z-direction magnetic field confinement.