Thermal human phantom for testing of millimeter wave cameras

Norbert Palka, Radoslaw Ryniec, Marek Piszczek, Mieczyslaw Szustakowski, Marek Zyczkowski, Marcin Kowalski

Military University of Technology, Institute of Optoelectronics 2 Kaliski Str., 00-908 Warsaw, Poland

ABSTRACT

Screening cameras working in millimetre band gain more and more interest among security society mainly due to their capability of finding items hidden under clothes. Performance of commercially available passive cameras is still limited due to not sufficient resolution and contrast in comparison to other wavelengths (visible or infrared range). Testing of such cameras usually requires some persons carrying guns, bombs or knives. Such persons can have different clothes or body temperature, what makes the measurements even more ambiguous. To avoid such situations we built a moving phantom of human body. The phantom consists of a polystyrene manikin which is covered with a number of small pipes with water. Pipes were next coated with a silicone "skin". The veins (pipes) are filled with water heated up to 37 C degrees to obtain the same temperature as human body. The phantom is made of non-metallic materials and is placed on a moving wirelessly-controlled platform with four wheels. The phantom can be dressed with a set of ordinary clothes and can be equipped with some dangerous (guns, bombs) and non-dangerous items. For tests we used a passive commercially available camera TS4 from ThruVision Systems Ltd. operating at 250 GHz. We compared the images taken from phantom and a man and we obtained good similarity both for naked as well as dressed man/phantom case. We also tested the phantom with different sets of clothes and hidden items and we got good conformity with persons. Keywords: terahertz radiation, screening camera, thermal phantom, image processing

1. INTRODUCTION The law enforcement agencies are currently faced with the problem of countering the terrorism-related threats. Of particular interests are improvised explosive devices and weapon hidden beneath the clothing. Therefore, many research and development institutions are looking for safe and accurate stand-off technologies. Electromagnetic waves at terahertz frequencies (0.1-3.0 THz) seem to be a promising candidate for this purpose. Development and implementation of terahertz technology in security area is connected with unique features of terahertz radiation. Many explosives, e.g. Hexogen (RDX), Penthryte (PETN), and Octogen (HMX) have characteristic transmission and reflection features in the THz range1,2 that could help distinguish them from other common materials like clothes, human skin or metal materials. Moreover, THz electromagnetic radiation can be transmitted through clothes with small attenuation and is strongly reflected by metallic objects like knives or guns. THz waves pose minimal health risk to human beings or the system’s operation because photon energy is very small (4,4 meV @ 1THz)2. The main factor that limits propagation through air is strong molecular absorption by water vapor1,3. The above mentioned features cause that THz radiation can be applied in two security-orientated areas: explosives detection and people screening.

As far as second application is concerned, first portals and cameras for people screening, both passive and active, working mainly in the 0.1-0.3THz range can be found on the market4,5 and simultanously they still atract attention of many research groups, what results in many projects and papers6,7. Pure passive systems working in the millimetre wave region can detect hidden on persons objects thanks to difference in naturally emitted power between human body and the objects. Human skin emits more power than hidden items because its radiometric temperature is usually higher than the radiometric temperature of the items. Moreover, emissivity of human skin, which consists of 70% of water is ussually different than emissivity of hidden dangerous metallic or dielectric materials. The combination of these two features connected with high transmission of ordinary clothes would enable a camera to look through clothes and find hidden objects like weapons, bombs etc.

*npalka@wat.edu.pl; phone 48 22 683 99 36, www.wat.edu.pl

Passive and Active Millimeter-Wave Imaging XV, edited by David A. Wikner, Arttu R. Luukanen, Proc. of SPIE Vol. 8362, 83620K · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.917904

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Testing of such cameras usually requires some persons carrying hidden guns, bombs or knives. Such persons can have different clothes or body temperature, what makes the measurements ambiguous. To avoid the time-consuming, tedious and expensive testing on human subjects we built a moving platform with the human phantom, which mimics a moving human being.

2. CONSTRUCTION OF THERMAL PHANTOM During design of the phantom we took into account the human body, whose skin consists of many veins filled up with circulating heated blood. The veins, through the arteries, are connected with the heart, which pump the blood into all parts of the body. In practice, it is hard to mimic such a complicated structure, but for the testing of cameras, we built a substitute, which in our opinion fulfill the requirements. The phantom is made of non-metallic materials and can be heated up to 37 C degree to obtain the same temperature as human body. It is placed on the moving wirelessly-controlled platform with four wheels. It can be dressed with a set of ordinary clothes and can be equipped with some dangerous (e.g. guns, bombs) and non-dangerous (e.g. wallet) items.

2.1 Phantom

The skeleton of the phantom consists of a polystyrene manikin (Fig. 1a) which was vertically covered with about 300 plastic pipes with internal and external diameter of 4mm and 5mm, respectively (Fig. 1b). The distance between the pipes is not longer than 3 mm to avoid inhomogeneities in heat transfer. As expected, we observed temperature differences between the upward and downward water streams thanks to heat transfer mainly to the silicone skin. Therefore, to minimize this phenomenon we applied a complicated arrangements of pipes (Fig. 2).

The upper ends of the pipes are connected to a water reservoir (a head of the phantom), while the lower ends of the pipes through a set of pipes with wider diameters (rings: I-V) reach inlet and outlet arteries (Fig. 2b). We determined experimentally, that water from the inlet artery should be directed upward through the 80% of pipes to the reservoir. Only 20% of pipes is exploited to direct water gravitationally from the reservoir downward. For each leg, we applied 4 upward rings (Fig. 2b, red color, no. I, II, IV, V) and one downward ring (green color, no. III). We group the smallest pipes which leave the rings into sections (Fig. 2b). Each section consists of the downward pipe (colder, green one), which is surrounded by a few upward pipes (hotter, red ones). Thanks to such arrangement, the surface is uniformly heated.

Figure 1. Thermal Phantom of human body: polystyrene manikin (a), the manikin partially covered with pipes (b), ready-to-use phantom (c).

A pump (from Bühler Motor company), which is situated in the lower part of the phantom in a metallic box, sucks the water through the outlet artery and pump it through a 300W heater, the outlet artery and the pipes to the water reservoir (Fig. 2). We applied two thermometers (Tin, Tout) to control and stabilize the temperature of the liquid. The veins (pipes) are filled with water heated up to 37 C degree to obtain the same temperature as human body. The pipes glued to the

Water reservoir

Silicone "skin"

cotton vest

Pipes Rod

a) b) c)

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manikin were next coated with a silicone resin (Fig. 1c), which was chosen experimentally taking into account good thermal conductivity and maximum emissivity. The developed phantom is light and quite fragile. To increase its stiffness, especially during start and brake of the platform, when the relatively tall phantom is susceptible to deflection, we inserted a long plastic rod into each leg of the manikin. The weight of the skeleton with pipes and silicone resin "skin" is about 5 kilos.

Figure 2. Thermal Phantom of human body: scheme (a) and detailed layout of upward and downward pipes (b).

2.2 Moving platform

The flat moving wirelessly-controlled platform (Fig. 3) with four wheels provides the required maneuverability and load up to 100 kg. Thanks to 4 independent DC motors, this robot can operate on flat surfaces and can go forward and backward with speed up to 400 mm/s as well as turn and rotate. The platform is equipped with a set of flat 12 VDC/7.2Ah accumulators to feed the platform, a controller with a radio transceiver, and a set of 4 ultrasonic sensors to localize the platform and to avoid collision with obstacles. For normal operation, the platform is placed inside an enclosure (7x5m) made of 40 cm high walls to ensure proper conditions for the ultrasonic sensors.

a) b) Figure 3. The moving platform: photo (a) and dimensions in mm (b).

A metallic box (Fig. 4) was installed on the top of the platform as a support for the phantom and a casing for the elements. The plastic rods from the manikin can be inserted into the vertical metallic pipes to ensure good mechanical stability of the phantom. Inside the box we installed the pump, the heater, the arteries, an accumulator (12VDC/100Ah)

PUMP HEATER

CONTROLLER

Waterreservoir

PUMP HEATER

CONTROLLER

Tin Tout

Inlet arteryOutlet artery

IIIIIIIVV

Leg Leg

Tranceiver

rings

Section 1 Section 2

Ultrasonic sensor

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for the phantom, and a control module with a radio transceiver. The moving phantom is 185 cm high and its weight equals to about 70 kg. The temperature range is 35-40 °C adjustable with a 1°C±0,5°C resolution.

a) b) c)

Figure 4. Layout of the metallic box (a) and photos of the ready-to-use moving platform: with a gun (b) and with a BBIED (c).

The moving phantom is independently fed from the internal accumulators and is wirelessly controlled from a PC, which is situated nearby. The LabView-based software can be used for movement and temperature control.

The phantom is equipped with a set of ordinary clothes (made mainly of cotton and other popular materials): boxer shorts, vests, T-shirts, trousers, shirts, sweaters, jackets, dresses and belts. We purchased some weapons (small and regular pistols, a revolver, knives etc.) with necessary holsters. Moreover, we prepared five imitations of Body Borne Improvised Explosive Devices (BBIED) - metallic and non-metallic items, that can be hidden under the clothes and can mimics the original bombs (Fig. 4c).

3. MEASUREMENTS IN INFRARED RANGE First tests of the thermal phantom were carried out with the thermal camera (FLIR A320) working in 7.5-13µm range. In Figure 5 we can notice nearly the same temperature of a person and human phantom both in the "skin" region and the region covered with the cotton shirt and the sweater. We also measured other configurations and obtained satisfactory results. As expected, the temperature of the human body surface is uneven due to inherent properties of human body, which is not homogeneous and is not uniformly heated. The naked image of the phantom reveals some horizontal and oblique stripes, which are connected with a black plastic tape, which was used to keep the pipes together (Fig. 1b). The stripes cannot be seen by the THz camera due to its poorer resolution in comparison to the thermal camera.

Figure 5. Thermal images taken with the thermal camera for the person (left) and the thermal phantom (right) with various clothes.

Accumulator

Metallic pipe

Control unit

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4. MEASUREMENTS IN TERAHERTZ RANGE For our studies, we chose TS4 camera from ThruVision Systems Ltd.4, which seems to have quite good performance and image quality. The camera detects natural human radiation at the frequency 0.25 THz thanks to the fact that clothing transmits well in this range. A heterodyne detection method bases on a GaAs Schottky mixer combined with a local oscillator. The number of pixels in THz image is 80 x 150; frame rate is 6 Hz. The distance camera-target can be in the range 3-15 meters. TS4 can detect objects hidden under clothes (bombs, guns, knives, wallets, belts, etc.) due to their different temperature and emissivity than human body. In the middle of the THz aperture, a small visible camera is installed to observe a scene. The images can be recorded independently (Fig. 6a,6) or THz image can be superimposed on the visible image (Fig. 7b).

Firstly, we compared the THz images taken for phantom and a man (height: 188cm, weight: 95 kg). Both targets were clothed in cotton boxer shorts. Distance to the target was 4 meters. One can notice, that we obtained satisfactory similarities between the man and the phantom. The phantom image seems to be more homogeneous. We can even notice the shadow casted by the boxer shorts' waist band in Fig. 6b.

a) b) c) Figure 6. Photo of the phantom clothed in the boxer shorts (a). Passive THz images: the thermal phantom (b) and the man (c) Due to privacy issues we do not publish photo of the man.

Figure 7 presents THz images taken for the phantom and the man clothed in trousers and a cotton shirt with a 30 cm long steel knife (Fig. 7 - inset). Distance to the target was about 4 meters. Also in this case the similarities are satisfactory.

Figure 7. The man with the hidden metallic knife: photo (a) and THz image (b). The phantom with the hidden metallic knife: photo (d) and THz image (c). One can notice (in b and c) the knife hidden under the cotton shirt.

Next, we presents the visible and THz images of the phantom with the weapon and BBIED hidden under the cotton shirt. Distance to the target was about 4 meters. Moreover, we show the initial results of image processing, which can improve

a) b) c) d)

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contrast of the images and determine the location of the hidden object in more precise way. The images were analyzed by different techniques including image deblurring, various segmentation techniques (single-threshold gradient method, local thresholding methods), median filtration, and edge detection (Sobel and Canny methods).

Figure 8 presents the results of signal processing of the THz image of the phantom with the hidden knife described above. Photo, THz image and processed images for the phantom with a 21x13 cm metallic pistol are shown in Fig. 9. Figure 10 presents the photo, THz and processed images of the BBIED, which is also depicted in Fig. 4c. The BBIED consisted of 4 metallic pipes (3 cm diameter, 20 cm length) with cables mounted in a belt. It is clearly seen, that application of the developed algorithms can improve contrast of the images and determine the location of the hidden object in more precise way.

Figure 8. The phantom with the hidden metallic knife: photo (a), original THz image (b) and processed images (c-d).

Figure 9. The phantom with the hidden metallic pistol: photo (a), original THz image (b) and processed images (c-d). Inset shows the pistol.

a)

a)

b)

b) c) d)

c) d)

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Figure 10. The phantom with the hidden BBIED: photo (a), original THz image (b) and processed images (c-d).

5. SUMMARY We think that screening cameras working in millimetre (or low THz) band require some testing targets to avoid the human-related problems. We proved both in IR and THz range, that the proposed solution based on the manikin covered with water heated pipes can mimic the human body accurately in various considered scenarios. One can conclude from this report that computer processing of images from the THz camera is a really promising and cost-effective way for demanding security and defense applications. Exploitation of the exploratory data analysis tools and preparation of predictive models is the next step in the future research. In future we are going to focus on the principal component analysis (PCA) to enhance visibility of hidden objects.

ACKNOWLEDGEMENT The Project is co-financed by the European Regional Development Fund within the framework of the 2. priority axis of the Innovative Economy Operational Programme, 2007-2013, submeasure 2.1. "The development of centres with high research potential". Contract no. POIG.02.01.00-14-095/09.

REFERENCES [1] Kemp, M. C., "Millimetre Wave and Terahertz Technology for the Detection of Concealed Threats – A Review",

Proc. SPIE 6402, 64020D (2006). [2] Palka, N., "THz Reflection Spectroscopy of Explosives Measured by Time Domain Spectroscopy", Acta Physica

Polonica A 120, 713-715 (2011). [3] Yun-Shik, L., [Principles of Terahertz Science and Technology], Springer, New York, 1-19 (2008). [4] ThruVision System Ltd., website: www.truvision.com. [5] Brijot Imaging Systems Inc., website: www.brijot.com. [6] May, T. et al., "Passive stand-off Terahertz imaging with 1 Hertz frame rate", Proc. SPIE 6949, 69490C (2008). [7] Luukanen A. et al., "Real-time passive terahertz imaging system for standoff concealed weapons imaging", Proc.

SPIE 7670, 767004 (2010).

a) b) c) d)

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