Proceedings of Meetings on Acoustics Volume 19, 2013 http://acousticalsociety.org/ ICA 2013 Montreal Montreal, Canada 2-7 June 2013 Signal Processing in Acoustics Session 1pSPc: Miscellaneous Topics in Signal Processing in Acoustics (Poster Session) 1pSPc11. Security screening using ultrasound David Hutchins*, Lee Davis and Sheldon Tsen *Corresponding author's address: School of Engineering, University of Warwick, Coventry, CV4 7AL, Warwickshire, United Kingdom, D.A.Hutchins@warwick.ac.uk This work will demonstrate that it is possible to produce images of hidden objects, using ultasound transmitted through air. For example, it can be shown that a knife can be imaged, when hidden behind a layer of clothing fabric. To achieve this, it is necessary to use coded waveforms and signal recovery techniques, in order to retrieve small signals in the presence of a much larger reflection from the outer fabric surface. In addition, ultrasound can be used in through-transmission to detect hidden objects within thin packages. This and other examples of the use of air-coupled ultrasound for security work will be demonstrated. Published by the Acoustical Society of America through the American Institute of Physics 2013 Acoustical Society of America [DOI: 10.1121/1.4799225] Received 15 Jan 2013; published 2 Jun 2013 Proceedings of Meetings on Acoustics, Vol. 19, 055033 (2013) Page 1
INTRODUCTION There are many approaches that can be considered for personals security screening, and some of these are already in operation at airports, other transport hubs, and in secure areas 1. Of these, two main types are used. The first is back-scattered X-rays, which is able to give good images of objects hidden beneath clothing layers. However, it is a form of ionizing radiation, and even though it is used at very low dosage levels, there is still concern in terms of long-term radiological exposure 2,3. An alternative is the use of THz (or mm-wave) imaging 4,5. This can penetrate through many different types of clothing, and has the potential for stand-off imaging in both active and passive modes. Some materials are difficult to penetrate however, such as wool, and there are issues with attenuation by water vapor and hence limitations on range. Both X-rays and THz imaging have are currently widely-used in screening, but this does not mean that alternatives should not be sought that are either non-ionising, or which can be used outdoors or indoors at significant stand-off distances. This paper looks at the possibility of ultrasonic imaging through clothing as a means of personal security screening. The generation and detection of ultrasound in air at increased bandwidths can be achieved using several different approaches 6. Transducers are available that can generate high intensity signals across a significant bandwidth, especially in the 0.1 1 MHz range. These signals have wavelengths that vary from approximately 0.3 3 mm, an interesting range when compared to those used for THz and mm-wave imaging in security screening. The measurements across this frequency range were made using electrostatic (capacitive) transducers as both the source and receiver of ultrasound 7. An alternative approach is to use much lower frequencies, using widely-available resonant piezoelectric devices 8. Both approaches have been investigated, as will now be described. EXPERIMENTS ON CLOTHING SAMPLES AT LOW FREQUENCY In these experiments, piezoelectric transducers were used as both emitters and receivers, each device being highly resonant at 40 khz in their unmodified state. These devices are typically used for distance ranging in air, as a send-receive pair. The first set of experiments were performed in a through-transmisson configuration, so that the transmission properties across different types of fabric could be studied. Ultrasonic signals were transmitted across clothing samples as shown in Figure 1. The two ultrasonic transducers, shown in Figure 1, were separated in air by a gap of approximately 150 mm, and were aligned axially. FIGURE 1. Schematic diagram of the apparatus used to study ultrasonic transmission through clothing. photograph of a pair of resonant 40 khz transducers, showing the casing, protective grill and resonator. Waveforms were obtained both with and without clothing samples being present. In the absence of a sample, Figure 2, the expected result from an impulsive drive signal isseen, namely a tone-burst at 40 khz, with a narrow spectral range. Proceedings of Meetings on Acoustics, Vol. 19, 055033 (2013) Page 2
FIGURE 2. Waveform and spectrum for a pair of unmodified 40 khz transducers transmitting across an air gap of 15 cm. However, it is found that this is transmitted very efficciently across many different types of clothing. Some examples are shown in Figure 3. It can be seen that good transmission through cotton shirt material and wool is possible, but that denim is more of a problem. This is due to the coarse nature of the weave, and the lack of porosity in the material, compared to the other samples tested. This is examined further below at broader bandwidths. (c) FIGURE 3. Waveforms transmitted across cotton shirt material, denim and (c) wool, using a pair of 40 khz transducers. BROAD BANDWIDTH EXPERIMENTS AT HIGHER FREQUENCIES Experiments were now performed with broad bandwidth capacitive transducers, driven by a swept-frequency linear chirp signal. Figure 2 shows a typical frequency spectrum for a chirp signal in air, with useable signal across the approximate range 0.25 0.6 MHz. The insertion of a clothing sample provided a modified spectrum, and division of the two spectra then gave a form of a deconvolution. This represented signal loss as a function of ultrasonic frequency. An example for an acrylic woven fabric is given in Figure 2. The relative amplitude decreases steadily with frequency, indicating that higher frequencies were less easily transmitted through the single Proceedings of Meetings on Acoustics, Vol. 19, 055033 (2013) Page 3
layer of clothing. This would be expected from mechanisms due to attenuation and scattering from the fibers of the material. The greater transmission at lower frequencies was a general result for all fabrics tested. FIGURE 4. Frequency spectrum of ultrasonic signal transmitted through air. Result of deconvolution of a signal transmitted through a polyester fabric, showing amplitude transmission as a function of frequency. The above test was performed on a range of clothing samples, including man-made and natural fibers. Each material had its own characteristics, in terms of fiber characteristics, weave pattern and porosity 6. Various tests were performed, and relative transmission efficiencies were measured using air-coupled ultrasound. The results are presented in terms of transmission coefficient (averaged across the available bandwidth from the chirp signals) in Table 1. The Table also shows the porosity of the samples, measured separately via an analytical method involving gas pressure drop across the samples for a given air flow rate. It can be seen that the transmission coefficient of each type of clothing correlates approximately with the porosity of the fabric in other words, the lower the porosity, the smaller the transmission coefficient, although the relationship is not linear. Note the interesting result that transmission through wool is particularly good. This is interesting in that other technologies for security screening, such as THz signals, tend to have difficulties with this type of fabric material. TABLE 1. Transmission coefficient of single-layers of selected fabrics. Sample Thick Cotton Towel Thick Cotton Acrylic White Polyester Wool Transmission coefficient 0.02 0.04 0.15 0.33 0.44 Porosity 0.2 2.2 9.5 13.0 17.2 PULSE-ECHO IMAGING USING AIR-COUPLED ULTRASOUND In a practical security scan, it will be necessary to produce images in reflection mode. This introduces a much greater problem the fact that there will be a strong reflection from the front surface of the fabric, which will tend to mask the much smaller signals that are derived from the far side of the fabric. In ultrasound imaging, either an array or a scanned transducer pair will be needed. Two approaches were investigated: a pair of 40 khz transducers, in both the as-received state and modified so as to improve the bandwidth, and a pair of wider bandwidth capacitive devices, in this case using a focused oblique source and a 10 mm diameter ultrasonic receiver. In all cases, both transducers in each pair were at the same distance from the clothing sample (approximately 100 mm), but angled slightly so as to be pointing at the same approximate area of the sample surface. An object could be placed behind the fabric layer, and the transducers defocussed slightly to optimize detection of the signal reflected from the hidden object. Consider first wide bandwidth imaging, where the wider bandwidth would allow clearer separation between signals reflected from the front surface of the fabric and any object hidden behind it. By using a window function, it Proceedings of Meetings on Acoustics, Vol. 19, 055033 (2013) Page 4
was found possible to separate the large reflected signal from the front surface of the fabric from that reflected from a hidden object. The amplitude of the signal could then be detected as the transducer pair was scanned across the fabric sample 7. The chosen object was a plastic knife, difficult to detect with back-scattered X-rays for example, which was placed behind a cotton shirt material (shown in Figure 5). The resulting image from this broad bandwidth scan is shown in Figure 5, and presents a clear image of the knife. The fringing in the image is caused by interference between waveforms scattered from different areas of the sample, and subsequent propagation through the fabric (where the fringing can be thought of as a form of speckle). FIGURE 5. A plastic knife and a cotton clothing sample used in these experiments. A pulse-echo ultrasound image, where the knife was hidden behind the clothing fabric. Interference patterns due to scattering from the fabric can be seen. Experiments were now performed with the 40 khz devices. Experiments showed that it was possible to increase their bandwidth using viscous damping liquids, attached to the resonant flexible structure on their front surface. An example is shown in Figure 6. This improved the image quality that could be obtained in pulse-echo mode. FIGURE 6. The frequency response of a modified 40 khz piezoelectric transducer, where the resonant outer flexible structure has been coated in Vaseline (petroleum gel). Note the increased bandwidth when compared to that obtained from an unmodified device (shown earlier in Figure 2). Images of a knife hidden behind the same cotton fabric have been obtained with 40 khz transducers, in both their original and modified state. The results are shown in Figure 7. It will be seen that the use of a lower ultrasonic frequency, and hence decreased spatial resolution, has caused the images to be less distinct than those obtained with the higher frequency capacitive devices (Figure 5). In addition, it can be seen that the increased bandwidths of the modified devices (Figure 7) has led to some improvement, in that the variations caused by interference processes have been reduced over the result from the more resonant response of the unmodified transducers (Figure 7). Proceedings of Meetings on Acoustics, Vol. 19, 055033 (2013) Page 5
FIGURE 7. Pulse-echo ultrasound images of a knife hidden behind a layer of cotton clothing. The images were obtained using the 40 khz piezoelectric transducers in the as-received state, and modified using a petroleum gel coating to increase the available bandwidth. DISCUSSION AND CONCLUSIONS The research has shown that air-coupled ultrasound can be used to detect hidden objects, when concealed behind layers of fabric. This may be of use to various government agencies, who wish to improve detectability of hidden weapons. These results are very much from a preliminary investigation. For such a system to be implemented, much more investigation needs to take place. In particular, the creation of an imaging array, coupled with a new set of coded algorithms for noise reduction, should lead to big improvements in future research. REFERENCES 1. J. E. Bjarnason, T. L. J. Chan, A. W. M. Lee, M. A. Celis and E. R. Brown, "Millimeter-wave, terahertz, and mid-infrared transmission through common clothing," Appl. Phys. Lett. 85, 519-21 (2004). 2. E. Hindie, and David J. Brenner. "Backscatter X-ray Machines at Airports Are Safe," Medical Physics 39, 4649-652 (2012). 3. J.E. Moulder, "Risks of Exposure to Ionizing and Millimeter-Wave Radiation from Airport Whole-Body Scanners", Radiation Research 177, 723-26 (2012). 4. A. Hommes, D. Nüssler, P. Warok, C. Krebs, S. Heinen, and H. Essen. "Inspection of samples using a fast millimetre wave scanner", Journal of Physics: Conference Series 307, 012033 (2011). 5. D.M. Sheen, D.L. McMakin, and T.E. Hall. "Three-dimensional millimeter-wave imaging for concealed weapon detection." IEEE Trans. Microwave Theory and Techniques 49,1581-592 (2001). 6. R. T. Ogulata and S. Mavruz, "Investigation of porosity and air permeability values of plain knitted fabrics", Fibres & Textiles in Eastern Europe 18, 71-5 (2010). 7. T.H. Gan, D.A. Hutchins, D.R. Billson and D.W. Schindel, The use of broadband acoustic transducers and pulse compression techniques for air-coupled ultrasonic imaging, Ultrasonics 39, 181-194 (2001). 8. M. Se-yuen, Wave experiments using low-cost 40 khz ultrasonic transducers, Phys. Educ. 38, 441-446 (2003). Proceedings of Meetings on Acoustics, Vol. 19, 055033 (2013) Page 6