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Defense & Security

Terahertz imaging safely detects concealed objects

Interferometric imaging provides both spatial and spectral information about screened objects with a limited number of expensive detectors.
1 February 2007, SPIE Newsroom. DOI: 10.1117/2.1200701.0539

Safety screening is standard at airports, railway stations and other public places in response to the threat of terrorism. However, such systems must be significantly improved in order to address new and emerging threats. Modern security systems must detect not only guns, knives, and other metal weapons, but also recognize many kinds of explosives and dangerous chemical and biological agents—and they must do so while presenting minimal health risk to both the operator and the person being scanned.

Terahertz (THz) radiation, a form of long wavelength infrared electromagnetic energy, has great potential for such security applications. First, THz waves—located in the spectrum between microwaves and the infrared optical band—can propagate through most nonmetallic, non-polar materials, enabling the detection of weapons and dangerous agents concealed behind barriers such as clothing and book bags. In addition, many materials of interest for security applications, such as the explosives C-4, HMX, RDX, and Trinitrotoluene (TNT), have characteristic transmission/reflection spectra in the THz range.2,3 Therefore, these materials appear as different ‘colors’ to a THz detector as compared to non-hazardous items. Also, unlike X-rays, THz waves are non-ionizing, thus ensuring the safe screening of people and animals.

Closely spaced detectors are not available for THz light as they are for visible, such as in digital cameras. Instead, THz imaging can be performed with a limited number of detectors by using the principles of interferometry.5 Also, continuous wave (CW) THz radiation used for interferometric imaging can be generated by the photomixing of two semiconductor lasers. Using these methods, compact imaging systems can be designed inexpensively, making them highly desirable for security applications where both physical space and budgetary concerns are significant.

Figure 1. Two-dimensional interferometric images of a metal object.

Introduced in radio astronomy, interferometric imaging requires an array of detectors. The target reflects a portion of the incident CW THz radiation towards the detector array. The amplitude and phase of the reflected electric field depends on the shape and the reflectivity of the target. With the amplitude and phase of the reflected THz electric field obtained at each individual detector, all detector pairs are correlated. The image is then reconstructed through the following Fourier inversion: where σE is the time-averaged intensity of the source, k is the wave number, and N is the number of detectors in the array.4,1

For N detectors, there are N(N-1)/2 pairs of detectors to be correlated, corresponding to N(N-1)/2 independent measurements. These measurements are used in the above equation to construct an image with only a limited number of detectors. However, the interferometric image quality depends on the number of detectors and their arrangement: the larger the number of detectors, the better the reconstructed image quality.

In our experiments, we used only one THz detector, mounted on an X-Y-Z computer-controlled stage. Scanning of this stage changes the position of the THz detector, thus providing the THz amplitude and phase at several points. As a result, we can simulate the performance of an array.

Figure 1 shows 2D images of a metal object: Figure 1 (left) shows the unobstructed object, while Figure 1 (right) shows the same object detected in the correct position despite being hidden behind a nylon book bag. The book bag is ∼1cm thick and opaque to visible light. These images were obtained in reflection mode at 0.3THz, with a distance of ∼45cm between the object and the detector array. The object size is 3×3cm.

Figure 2 shows images of scanned RDX samples: Figure 2 (left) shows the unobstructed RDX, while Figure 2 (right) shows the sample concealed behind a book bag barrier. The size of the RDX sample is ∼3cm, shown as the white circle in the image. The images are taken in reflection mode at 0.26THz, with a distance of ∼45cm between the sample and the detector array. Again, the concealed RDX sample is detected at the correct position. For effective RDX detection in the field, it is necessary to obtain images at several THz frequencies in order identify the explosive using its spectral signature.2 We plan to study such spectral interferometric imaging in the future.

Figure 2. Two-dimensional interferometric images of an RDX sample.

With our current setup, two hours are needed to acquire a 2D image. This is a result of low generated THz power and the mechanical scanning of the single detector. However, in a practical stand-off interferometric detection, N detectors will be used simultaneously. In addition, we are also exploring several modulation methods that will reduce the imaging time to the order of several seconds, making it much more appropriate for real-time security applications.

Alexander Sinyukov
New Jersey Institute of Technology
Department of Physics, USA