A low-cost alternative to terahertz imaging for security and defense applications

A new near-infrared (NIR) imaging technique has been developed that can be used for applications previously considered possible to image only by using terahertz (THz) waves. The technique developed at the University of Warwick uses NIR beams of light—around the wavelengths found in ordinary domestic remote controls—and signal recovery techniques commonly used in astronomy.

This alternative technique can be realized using simple, inexpensive electronics and is far more portable and easier to use than THz imaging systems because no special power sources are required. Already successfully used in industrial environments and security applications, it can emulate the performance of commercial THz systems at a fraction of the cost and with much quicker image processing times.

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

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

Conventional NIR spectroscopy and imaging are based on the different wavelength-dependent absorption and scattering properties of the sample. By measuring the properties of transmitted and reflected near-infrared light, information concerning the internal distribution of the sample can be obtained. Continuous wave systems measure the intensity of the transmitted or reflected light. Alternatively, a short laser pulse can be used as a probing signal, and in these cases the distribution of the photons is usually measured.

We have looked at much simpler approaches to infrared imaging, which could have a far wider application to security issues. The optimum NIR band is between 700–900nm as photons of these energies cause virtually no electronic or molecular interaction, and so transmission through a specimen is almost totally a function of thickness (absorption) and intra-specimen particle size (scattering). The lock-in signal recovery technique, which can detect a signal from a noisy environment, does not produce time resolved images but merely variations in the transmitted intensity and resorts to an entirely analog implementation of the synchronous detection process. Hence, no analog to digital (A/D) conversion, numerical computation, or digital signal processing (DSP) is involved and as a result the image processing and acquisition times are extremely rapid and in real-time.

Figure 2. Revealing bone within flesh. The NIR beam used less than 1mW of unfocused source power. Shading corresponds to differing densities of bone at the joints.

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

Examples of the range of applications are presented in Figures 2 to 5. Figure 2 illustrates the ability of this technique to penetrate flesh and image bone in a non-ionizing way, which means the light beam does not interact with the specimen, unlike x-rays. Figure 3 shows a cardboard box that contains several specimens made of wood and cotton (cotton bud, or Q-tip), plastic (pen lid) and metal (tack and paper clip). Note that it shows several materials in shot at the same time, three of which are normally highly radiolucent (plastic, cotton and wood are not well imaged by x-rays). Figure 5 illustrates the extreme selectivity of this technique because it reveals not only the hidden powder contents of a sealed envelope but the writing on the letter. Results similar to those illustrated in Figure 3 have been obtained for clothing and, unlike when using THz methods, wet clothing can sometimes enhance the penetrative power of the NIR beam.

Figure 4. Lock-in NIR through hollow-fiber ‘fleece’ garment reveals obscured objects.

Whilst terahertz spectroscopy has the potential for standoff detection through some materials, such as clothing, that might be used to conceal explosives for example, the THz approach requires the development of significant higher power sources. Atmospheric absorption primarily from water vapor is also a primary obstacle for efficient THz spectroscopy.²

NIR spectroscopy on the other hand is a well established technique that has historically been used for many applications including remote sensing for the detection and identification of chemical materials. For instance, the NIR spectra for explosives, both molecular- and oxidizer-based, would be oxygen-hydrogen (OH), carbon-hydrogen (CH) and nitrogen-hydrogen (NH) bonds. For real-time imaging, conventional detectors would be ineffective in this important task.

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

The lock-in NIR technique offers the opportunity to powerfully combine spectral identification with imaging. We predict that these two features could be united to produce a conventional image that shows the chemical and biological data of the specimen superimposed in false colors generated by imaging software.

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