The Use of Terahertz Imaging Technology in Counter-Terrorism

Guest author: Salih Ergün

| Part 4

In this part we will look at terahertz technology as an imaging technology, together with its uses in counter-terrorism.

What Is Terahertz Imaging?

THz waves can be used for imaging applications and non-harmful imaging. Because of their proximity to the infrared range, optical approaches can be adapted for designing and building THz imaging systems. For example, lenses and mirrors can be used to focus, steer and collimate a THz beam. THz imaging applications have gained a very important place in the field of security. Because THz imaging can see behind and inside objects and has good resolution, it stands out in security applications for screening people (the passenger scanners widely used at airports, however, mostly use the adjacent millimetre-wave band).

Compared with X-ray devices it is less harmful to the human body and to the operator. Terahertz imaging is implemented as active (a system containing a source that can emit terahertz wavelengths) and passive (a system that measures the terahertz radiation naturally emitted by the target and its surroundings, without an external source) imaging systems.

Passive applications rely mostly on the emission of the human body, which lies mainly in the infrared but also falls into the THz band. Because low THz frequencies (0.1 THz or 100 GHz) pass through fabric and other dielectric coverings, it becomes possible to detect suspicious material or material that could pose a potential threat hidden under coverings. Active THz imaging systems can also be designed to illuminate the target and measure the reflection.

A reflection with a structure different from the reflections coming from the body and the covering on it may indicate the presence of a suspicious object.[i] In both active and passive applications, target detection is affected by atmospheric conditions, the state of the target, the target’s covering and its surroundings[ii].

The key element in developing THz imaging systems is setting up the system for the application. Since THz wavelengths range from sub-millimetre up to the millimetre range, they are much longer than the wavelengths of visible light. In imaging systems, once the positions of the THz receiver and transmitter have been set for the desired resolution and field of view, the devices used for detection are deployed either singly or in arrays, depending on whether point or two-dimensional detection is used.

For example, a sample can be scanned by moving the receiver while focusing the THz beam on the sample. By using an array of detectors instead of a point detector, or an array of sources and a single detector, it is possible to acquire images in 2D simultaneously. THz imaging has been used in combination with tomographic techniques, with effective results in distinguishing materials[iii]. These techniques are still developing, and methods capable of acquiring images at video rate have recently been developed as well.

Applying THz Imaging in Counter-Terrorism

Being able to detect and identify a threat in advance is vital for taking measures to prevent and neutralise it. Detecting the threat correctly therefore comes first. The effect of a terrorist attack can be minimised especially through detection and identification at greater distances. This is exactly where THz technology comes in. Its ability to detect materials such as weapons and explosives that terrorists hide on their bodies or elsewhere, without harming the human body, will bring entirely new technological applications to the field of security. With its distinctive features, THz radiation differs from other imaging techniques, but it is a technology still under development.

With THz imaging it may also become possible to detect suicide bombers at long and short range, to spot weapon systems and to carry out chemical and biological screening of critical packages and letters[iv].

Considerable progress has been made in imaging since 1979. Thanks to THz imaging, the detection of materials carried or hidden on the body, especially illegal materials, is developing rapidly[v].

Detecting land mines using THz spectroscopy would be a significant advance. The number of land mines remaining in the ground worldwide is not reliably known; the estimates of around 100 million once cited are now considered outdated. Compared with other imaging and detection systems, THz technology is assessed to be better able to distinguish anti-personnel mines in particular from small pieces of rock and to detect them. An image[vi] that could serve as a proof of concept for imaging a non-metallic land mine under dry soil has been produced with THz technology[vii].

From a technology perspective, THz imaging is a developing, newly growing and very costly field. It is assessed that THz spectroscopy and imaging technology could offer new and more comprehensive solutions for improving existing reconnaissance and surveillance applications in counter-terrorism. We can summarise them as follows:

It is assessed that work on the THz gap may yield better results in active and passive THz imaging in the future. Current THz imaging technology provides a suitable and usable capability for security applications.

(To be continued)

[i] Zomega Terahertz Corporation, The Terahertz Band, (2012) The Terahertz E-book, Technical Overview, p.44

[ii] Cummins H., (2006)  Challenges to Terahertz Counter-Terrorism and Security-Related Applications, NATO Science for Peace and Security Series, Springer, Spiez, p.283

[iii] Truscott W., (2006). Systems Requirements for A Multi-Channel Terahertz Contraband Scanner, Terahertz Frequency Detection and Identification of Materials and Objects, p.188

[iv] Cummins H., (2006)  Challenges to Terahertz Counter-Terrorism  and Security-Related Applications, NATO Science for Peace and Security Series, Springer, Spiez, p.283

[v] Appleby R., Coward P.R., Sinclair G. N., (2006) Terahertz Detection of Illegal Objects, p.225-228

[vi] Fitch M. J., Schauki D., Kelly C. A., Osiander, R. (2004) Terahertz Imaging and Spectroscopy for Landmine Detection Proc. SPIE 5354, p.45

[vii] Federici, J.F., Schulkin B., Huang F., Gary D., Barat R., Oliveira F., Zimdars D., (2005) THz Imaging and Sensing for Security Applications: Explosives, Weapons and Drugs, Semiconductor Science and Technology, 20, pp. S266–S280