10.1117/2.1200808.1219

A low cost alternative toterahertz imaging for securityand defense applicationsGeoff Diamond

A novel near-infrared imaging technique could catch threats usingportable equipment.

A new near-infrared (NIR) imaging technique has been devel-oped that can be used for applications previously consideredpossible to image only by using terahertz (THz) waves. The tech-nique developed at the University of Warwick uses NIR beamsof light—around the wavelengths found in ordinary domes-tic remote controls—and signal recovery techniques commonlyused in astronomy.

This alternative technique can be realized using simple, inex-pensive electronics and is far more portable and easier to usethan THz imaging systems because no special power sourcesare required. Already successfully used in industrial environ-ments and security applications, it can emulate the performanceof commercial THz systems at a fraction of the cost and withmuch quicker image processing times.

Most images acquired via NIR are indistinguishable fromtheir THz-derived counterparts (see Figure 1). However, un-like THz methods, this technique can penetrate bulk water andhigh humidity atmospheres. Moreover, NIR be used in transmis-sion mode on biological and medical samples to produce non-ionizing images akin to x-rays that can differentiate betweendifferent types of soft tissue. Another advantage is that this tech-nique affords the means to provide simultaneous in situ chem-ical bond analysis at a distance of certain chemical signatures,such as those found in drugs and explosives.

Conventional NIR spectroscopy and imaging are based onthe different wavelength-dependent absorption and scatteringproperties of the sample. By measuring the properties of trans-mitted and reflected near-infrared light, information concerningthe internal distribution of the sample can be obtained. Contin-uous wave systems measure the intensity of the transmitted orreflected light. Alternatively, a short laser pulse can be used as a

Figure 1. Direct comparison of terahertz (THz) and lock-in near-IR(NIR) imaging.1 The terahertz (THz) system is costly to build, main-tain, and run, as well as being large and requiring specialist equipmentand highly trained staff. In contrast, the lock-in NIR system is small,portable, and much less expensive.

probing signal, and in these cases the distribution of the photonsis usually measured.

We have looked at much simpler approaches to infraredimaging, which could have a far wider application to securityissues. The optimum NIR band is between 700–900nm as pho-tons of these energies cause virtually no electronic or molecular

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Figure 2. Revealing bone within flesh. The NIR beam used less than1mW of unfocused source power. Shading corresponds to differing den-sities of bone at the joints.

interaction, and so transmission through a specimen is almosttotally a function of thickness (absorption) and intra-specimenparticle size (scattering). The lock-in signal recovery technique,which can detect a signal from a noisy environment, does notproduce time resolved images but merely variations in the trans-mitted intensity and resorts to an entirely analog implemen-tation of the synchronous detection process. Hence, no analogto digital (A/D) conversion, numerical computation, or digi-tal signal processing (DSP) is involved and as a result the im-age processing and acquisition times are extremely rapid and inreal-time.

Examples of the range of applications are presented in Figures2 to 5. Figure 2 illustrates the ability of this technique to penetrateflesh and image bone in a non-ionizing way, which means thelight beam does not interact with the specimen, unlike x-rays.Figure 3 shows a cardboard box that contains several specimensmade of wood and cotton (cotton bud, or Q-tip), plastic (pen lid)and metal (tack and paper clip). Note that it shows several mate-rials in shot at the same time, three of which are normally highlyradiolucent (plastic, cotton and wood are not well imaged byx-rays). Figure 5 illustrates the extreme selectivity of this tech-nique because it reveals not only the hidden powder contents ofa sealed envelope but the writing on the letter. Results similarto those illustrated in Figure 3 have been obtained for clothingand, unlike when using THz methods, wet clothing can some-times enhance the penetrative power of the NIR beam.

Whilst terahertz spectroscopy has the potential for standoffdetection through some materials, such as clothing, that mightbe used to conceal explosives for example, the THz approach re-quires the development of significant higher power sources. At-mospheric absorption primarily from water vapor is also a pri-mary obstacle for efficient THz spectroscopy.2

Figure 3. Box contents (top left), sealed box (top right), NIR scan of theclosed box taken in through-transmission mode, showing the contents(bottom).

Figure 4. Lock-in NIR through hollow-fiber ’fleece’ garment revealsobscured objects.

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Figure 5. Concealed powder within a sealed envelope is revealed. Alsovisible are the written contents of letter.

NIR spectroscopy on the other hand is a well establishedtechnique that has historically been used for many applica-tions including remote sensing for the detection and identi-fication of chemical materials. For instance, the NIR spectrafor explosives, both molecular- and oxidizer-based, would beoxygen-hydrogen (OH), carbon-hydrogen (CH) and nitrogen-hydrogen (NH) bonds. For real-time imaging, conventional de-tectors would be ineffective in this important task.

The use of lock-in NIR imaging systems to detect overtone andcombination bonds in explosives makes detection possible. Eventhe biochemical composition of certain classes of bacteria andspores (such as anthrax and fungal spores) gives rise to NIR vi-brational transitions and these characteristic overtone and com-bination bands and could possibly be used for the classificationand identification of different strains of pathogen.3

The lock-in NIR technique offers the opportunity to power-fully combine spectral identification with imaging. We predictthat these two features could be united to produce a conven-tional image that shows the chemical and biological data of thespecimen superimposed in false colors generated by imagingsoftware.

Author Information

Geoff DiamondSchool of EngineeringUniversity of WarwickCoventry, United Kingdom

Dr. Diamond is a physicist with an extensive history in the fieldof novel instrumentation design and development.

References

1. http://www.thznetwork.org/wordpress/index.php/thz-images/2. Adrian Dobroiu1, Chiko Otani, and Kodo Kawase1, Terahertz-wave sources andImaging Applications, Meas. Sci. Technol. 17, pp. R161–R174, 2006.3. L. E. Rodriguez-Saona, F. M. Khambaty, F. S. Fry, and E. M. Calvey, Rapid De-tection and Identification of Bacterial Strains by Fourier Transform Near – Infrared Spec-troscopy, J. Agric. Food Chem. 49, pp. 574–579, 2001.

c© 2008 SPIE