Research on the Transmissivity of Some Clothing Materials at Millimeter-wave Band

Zelong Xiao, Jianzhong Xu, Taiyang Hu

MMW Applied Laboratory, Electronic Engineering Department, Nanjing University of Science and Technology Nanjing 210094, P.R. China

Abstract-The passive millimeter-wave (PMMW) imaging technology used for the detection of contraband concealed on personnel has been extensively studied, due to the fact that millimeter-wave (MMW) can readily penetrate clothing materials and the passive operation mode doesnt present a safety hazard to the persons who are under inspection. However, the research on the transmission property of clothing materials covered on contraband has received relatively little attention, although it also plays an important role in design and fabrication of the whole imaging system. This paper focuses on the problem of measuring the transmissivity of some clothing materials with different thickness. These measurements are carried out in a RF anechoic chamber, based on the analysis to radiation characteristics of contraband carried on personnel. In addition, the theoretical model of the radiometric temperature contrast between human body and contraband at MMW band is also described in detail. Ultimately, the measurement results of some clothing materials are presented and analyzed.

I. INTRODUCTION

The demand to detect contraband concealed underneath a persons clothing in public areas has urged the passive millimeter-wave (PMMW) technology to an important level of development [1], [2]. The PMMW imaging is well suited for the detection of concealed contraband carried on personnel because of the fact that millimeter-wave (MMW) can readily penetrate common clothing materials. And at the same time, the PMMW imaging system doesnt irradiate the individual and will pose no known health hazard [3], [4]. The PMMW imaging makes use of both the penetration capability of MMW and the radiation characteristics distribution of objects in the field of view, to achieve the objective of detecting concealed contraband. The quality of the acquired PMMW images has a direct relation with the material and thickness of the clothing covered on contraband. Usually, people cover themselves with a wide rage of materials [1] and the thickness of these materials is variable, which makes it difficult to image contraband accurately. Therefore, it is urgent to take research on the radiation characteristics of clothing with different materials and thickness.

This paper details the radiation characteristics of contraband concealed on personnel and carries out the measurements of clothing with common materials and different thickness, on the basis of the implementation of a planner-scanning PMMW imaging system.

II. RADIATION CHARACTERISTICS OF CONTRABAND

According to the Conservation Law of Energy, the relation between reflectivity , absorptivity and transmissivity t can be expressed as [5]:

1=++ t (1) When the PMMW imaging is employed in the application

of concealed contraband detection, the schematic diagram can be depicted in Fig. 1 [6]:

Figure 1. The schematic diagram of detecting contraband concealed on

personnel by a PMMW imaging system.

It is assumed that the emissivity and reflectivity of human body are he and h respectively. The human body is opaque at MMW band and it can be anticipated that the radiation energy absorbed by human body doesnt transfer into other kind of energy but entirely emits out, thus:

1=+ hhe (2) Assuming that the emissivity and reflectivity of concealed

contraband are we and w respectively. As to the metal and plastic items, they are opaque at MMW band and the relationship between emissivty and reflectivity can be written as:

1=+ wwe (3) Assuming that the emissivity, reflectivity and transmissivity

of clothing material are ye , y and yt respectively, and therefore:

1=++ yyy te (4)

Human Body

Concealed Contraband

1

2

3

4

5

6

Radiometer

Clothing

1: Radiometric temperature of the human body; 2: Radiometric temperature of the environment reflected by human body; 3: Radiometric temperature of the clothing; 4: Radiometric temperature of the environment reflected by clothing; 5: Radiometric temperature of the contraband; 6: Radiometric temperature of the environment reflected by contraband.

978-1-4244-1880-0/08/$25.00 2008 IEEE. ICMMT2008 Proceedings

Temperatures hT , wT and yT are the actual physical temperature of human body, concealed contraband and clothing respectively. sT is the radiometric temperature of environment.

As shown in Fig. 1, when the MMW radiation from the environment passes through the clothing, a part of them will be reflected back, another part of them will reach the skin and be reflected back through the clothing, and then the rest of them will be reflected by the contraband [6], [7]. In addition, the human body, the contraband and the clothing will emit MMW radiation, which relates to both the physical temperature and the emissivity of them.

When there is no concealed contraband, the received antenna apparent temperature of the PMMW imaging system can be expressed as [6]:

syyyyhsyyhhAP TTetTttTeT +++= 1 (5) When there is no concealed contraband, the received

antenna apparent temperature of the PMMW imaging system can be expressed as:

syyyywsyywwAP TTetTttTeT +++= 2 (6) Thus the contrast is the difference between the antenna

apparent temperature when the contraband is present and that when the contraband is absent, and it can be expressed as: [7]:

21 ApAp TTT = (7)

Then according to (5) and (6), the (7) can be rewritten as:

21 ApAp TTT =

ywsyywwyhsyyhh tTttTetTttTe += 2)()( yswhywwhh tTtTeTe += (8)

As to the metal target with a smooth surface and high conductivity, it is an excellent reflector at MMW frequency and its reflectivity is equivalent to 1, i.e. 1w . Then according to (3), the (8) becomes:

2)()( yswhywwhh tTtTeTeT += 2)1( yshyhh tTtTe += (9)

When incorporated with (4), the (9) reduces to: 2)1( yshyhh tTtTeT +=

2yshyhh tTetTe = (10)

From (10), it is known that the radiometric temperature contrast can be calculated if the emissivity of human body, the transmissivity of clothing and the radiometric temperature of environment are given. And then the contraband concealed underneath clothing can be detected on the basis of the characteristics of radiometric temperature contrast.

In particular, the concealed contraband will mainly reflect the radiometric temperature of the sky when the imaging is carried out outdoors, and it provides a high contrast between

the contraband and human body [8]. However, in practical application, the radiometric temperature contrast relates to many other factors, such as the barriers of buildings and the orientation of contraband with the human body. According to a series of experiments, it is summarized that when the angle between the contraband and the horizontal exceeds D120 , the contraband is considered to only reflect the radiometric temperature of the sky, whereas when the angle is less than

D120 the radiometric temperature of environment should also be taken into account, and this will result in a lower radiometric temperature contrast [6]. For the sake of convenience, it is assumed that the contraband will only reflect the sky radiation in the following discussion.

The physical temperature of human body is generally about 37, i.e. =hT 310K. The properties of skin depend principally on wetness, and typically the reflectivity is about 0.5 at Ka band and 0.14 at W band [9]. Commonly, the reflectivity of different people and different parts of the body varies remarkably, so the following discussion is carried out on the primaries of some specific condition.

The radiometric temperature of the sky has relationship with season, climate and many other factors. It is usually considered to be about 30-150K at MMW band [10].

From the above discussion, it is known that the emissivity of skin and the transmissivity of clothing covered on contraband are the most concerned parameters. As to the emissivity of skin, it is assumed to be 0.5 at Ka band and 0.8 at W band, according to the results presented by Roger Appleby [3], [9]. The reflectivity of metal contraband can be considered as 1. And the transmissivity of clothing with different material and different thickness is measured in next section.

III. MEASUREMENT OF TRANSMISSIVITY OF SOME COMMON CLOTHING MATERIALS

The measurements are carried out in a RF anechoic chamber by utilizing an antenna test system AV3635 and some related equipments. The experiment scene is illustrated in Fig. 2.

Figure 2. The experiment scene.

The equipments in the measurement, as shown in Fig. 3, include a T/R model at Ka band, a MMW sweeping signal generator AV1418C and a vector network analyzer AV3615,

etc. Both the transmitting antenna and the receiving antenna are standard horn antennas. The measured samples include cotton fabric, woollen sweater, terylene, pure cotton coat, poncho cloth and down jacket.

Figure 3. The experiment equipments.

As illustrated in Fig.2, the distance between the transmitting

antenna and the sample under measurement is equal to the distance between the receiving antenna and the sample, and both of them are about 0.5m. Moreover, a series of combinations of different samples and different layers are adopted in the measurements, with the purpose of imitating the actual circumstance in which contrabands may be concealed.

The obtained measurement results are presented in Table I. And after analyzing the measurement results, it is concluded that:

(1) The attenuation, which is caused by the clothing samples, at W band is severer than that at Ka band under the same condition. And it means that the transmissivity of a specific clothing sample at Ka band is higher than that at W band.

(2) MMW can readily penetrate different kinds of clothing material. In this experiment, the received signal is attenuated only about 9 dB when more than 50 layers of cotton cloth are puckered together.

(3) Plastic materials, such as poncho cloth, indicate an effective capability to attenuate MMW, so it is generally more difficult to distinguish the contraband covered by plastic.

(4) The measurement results are related to many factors, such as the angle between the clothing and the antenna apertures, the smoothness of the clothing and the space between layers, etc.

IV. CONCLUSION AND DISCUSSION

From the analysis to the theoretical model and a series of measurements, it can be concluded that the quality of the PMMW images used for concealed contraband detection is closely related to the radiometric temperature contrast between human body and the contraband. Additionally, the transmission property of the clothing covered on contraband greatly influences the radiometric temperature contrast. A number of transmissivity values of clothing with different materials and different thickness are obtained in a RF anechoic chamber, including cotton cloth, plastic, terylene and woollen sweater, etc. And in the process of imaging, it is found that the quality of the PMMW images is affected not only by the angle between the antenna beam and the object being imaged, but also the smoothness of the clothing covered on contraband and the nonuniform thickness. And these factors need further investigation.

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[5] Li Xingguo, Millimeter-wave Near-sensing Techniques and Its Application. Beijing: National Defence Industry Press, 1991.

[6] Xiao Zelong, Study on Millimeter-wave Radiometric Imaging for Concealed Contraband Detection, Ph.D. Thesis of Nanjing University of Science and Technology, China, 2007.

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[8] Gordon N. Sinclair, Rupert N. Adnerton, and Roger Appleby, Outdoor passive millimeter wave security screening, IEEE 35th International Carnahan Conference on Security Technology. pp. 172-179, October 2001.

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[10] Gui Liangqi, Zhang Zuyin, and Guo Wei, Calculation of sky temperature at 3mm band, Journal of Huazhong University of Science and Technology (Nature Science Edition). vol. 33, pp. 73-75, December 2005.

TABLE I MEASUREMENT RESULTS OF DIFFERENT CLOTHING MATERIAL

Attenuation at Ka band Attenuation at W band Material Layers Thickness (mm) Transmitting Power (dBm) (dB) (dB/mm) (dB) (dB/mm)

1 2 4.70 5 1.81 0.39 2.81 0.60 2 1 0.95 5 0.19 0.20 0.60 0.68 2 2 1.90 5 1.14 0.60 2.50 1.30 3 1 2.30 5 0.49 0.21 0.93 0.41 3 2 4.60 5 5.13 1.11 7.62 1.65 4 2 0.24 5 1.39 5.79 1.61 6.67

1+2 3 5.65 5 2.05 0.36 3.45 0.61

1+3 3 7.00 5 1.94 0.28 4.24 0.61 1+4 4 4.94 5 2.13 0.43 3.73 0.76 2+3 2 3.25 5 1.34 0.41 1.86 0.60 2+4 3 1.19 5 0.56 0.48 3.73 3.13

1+2+3 4 7.95 5 3.60 0.45 8.68 1.09 5 1 0.30 5 0.14 0.47 0.56 1.87 5 2 0.60 5 0.18 0.30 -- -- 5 4 1.20 5 0.55 0.47 -- -- 5 8 2.40 5 0.83 0.36 -- -- 6 1 0.30 5 0.10 0.33 0.35 1.17 6 2 0.60 5 0.21 0.35 0.76 1.27 6 4 1.20 5 0.48 0.40 1.93 1.60 6 8 2.40 5 1.14 0.47 3.20 1.33 6 16 4.80 5 1.65 0.34 -- --

1: 1-woollen sweater; 2-terylene; 3-pure cotton coat; 4-poncho cloth; 5-dark blue cloth; 6- cambridge blue cloth. 2: And -- denotes that the corresponding results arent obtained.