Patent Publication Number: US-7915588-B2

Title: Arrangement and method for detecting an object which is arranged on a body, in particular for carrying out a security check

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a U.S. National Stage of International Patent Application No. PCT/EP2007/009000 filed Oct. 17, 2007 and claims priority under 35 U.S.C. §119 of German Patent Application No. 10 2006 049 152.1 filed Oct. 18, 2006. Moreover, the disclosure of International Patent Application No. PCT/EP2007/009000 is expressly incorporated by reference herein in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an arrangement and a method for detecting an object arranged on a body, but possibly concealed by at least one covering material. 
     2. Discussion of Background Information 
     Arrangements of this type are generally known. For example, at airports or at large events metal detectors are used for security control, with the aid of which metal weapons can be detected. However, this method fails, for example, with ceramic knives. Furthermore, it is known that with the aid of mass spectrometers the smallest contaminations of explosives can be detected. Finally, it is also known to use specially trained dogs for detecting illegal drugs. All of these methods have the disadvantage in common that they relate only to specific classes of substances. A number of detectors would therefore be necessary to safeguard against all possible dangerous or objectionable objects or materials. In particular for reasons of time, however, the successive use of several technologies of this type in security checks is not practicable. Finally, it is also known to detect concealed objects with the aid of x-rays. However, this method cannot be applied to the human body or only to a very limited extent. 
     SUMMARY OF THE INVENTION 
     The invention discloses an arrangement and a method of the type mentioned at the outset, which is able irrespective of specific classes of materials, to detect in a contactless manner whether an object is arranged on a body, and which is also suitable for use on the human body. 
     According to a first aspect of the invention through an arrangement for detecting an object arranged on a body, the arrangement comprises a vibration device, which sets the body in a mechanical vibration of predetermined vibration frequency and predetermined vibration phase, an emission device, which emits a coherent electromagnetic detection radiation in the direction of the body, the radiation frequency of which is selected such that it is reflected at least in part by the body and the object to be detected, a receiver device, which receives the radiation reflected by the body and the object, and an evaluation device, which filters out of the radiation received the portions having the predetermined vibration frequency and evaluates them with respect to their vibration phase difference from the predetermined vibration phase. 
     Although the invention is discussed and explained below mainly on the basis of the example of the security check of people, i.e., thus in its application for a human body, it should be noted at this point that the invention is not limited thereto, and can be used just as well for animal bodies and/or inanimate bodies and/or objects. 
     The invention is based on the realization that with forced vibrations the phase difference between the motion of the object set in forced vibration relative to the exciter vibration, in addition to the mass of the object and its attenuation, among other things also depends on the strength of its coupling to the exciter vibration. 
     If a person is now set in forced vibration, for example, if he is placed on a vibration plate, the body of the person, in the case in the example starting from the feet, starts to vibrate. Whether this forced vibration of the body thereby means that the entire body vibrates relative to the predetermined vibration phase of the exciter vibration with essentially one and the same phase difference or the different parts of the body vibrate with different phase difference also depends, in addition to the respectively selected type of excitation, on its predetermined vibration frequency. However, in any case the mechanical vibration of a body part on which the object to be detected is arranged, for its part again represents an exciter vibration, which sets the object in a forced mechanical vibration. It is to the credit of the inventor to have recognized that objects carried on the body can be detected due to the phase difference of their forced vibration relative to the phase of the vibration of the body parts on which they are arranged. The invention utilizes this effect: 
     If the type of the device exciting the body to vibration and the vibration frequency thereof are selected such that the entire body oscillates relative to the predetermined vibration phase of the exciter vibration with essentially one and the same phase difference, it can always be indicated to the staff carrying out the security check when subsections are present in the radiation sections filtered out by the evaluation device and having the predetermined vibration frequency, the phase difference of which subsections from the predetermined vibration phase of the mechanical vibration lies in a predetermined value range, via a visual, acoustic or any type of signal that the person just checked is carrying objects on his body. Based on this display the person checked can then be subjected, for example, to a body search. 
     It should be emphasized in this connection that the arrangement according to the invention is even able to detect the object if it is covered by at least one covering material, as long as the radiation frequency of the detection radiation is selected such that it is transmitted at least in part by the at least one covering material. 
     It should be noted that the arrangement according to the invention evaluates portions of the reflected radiation having the predetermined vibration frequency only with respect to their phase position, but not with respect to their amplitude. The percentage of the detection radiation emitted in the direction of the body that is optionally absorbed by the at least one covering material is therefore irrelevant, as long as only the signal-to-noise ratio of the radiation reflected by the objects to be detected permits a detection of this radiation. 
     And this applies regardless of what type of covering material it is. In the case of security checks of people, at least one covering material can be a typical clothing material, that is a textile material, for example, a natural textile material, such as wool, cotton, linen and the like, or a synthetic textile material, such as polyester, polyamide, nylon and the like, a natural material, such as leather and the like, or a mixture of two or more of the above-referenced materials. 
     It should be noted that no knowledge of the concealed objects or of their properties, for example, their mechanical properties, is necessary, which means that prior calibration measurements of objects potentially to be detected are unnecessary. 
     In contrast to radar methods, the transit time of an electromagnetic signal, in the present case of the detection radiation, from the emission device via the object or the body to the receiver device is not measured. Because on the one hand, information provided by transit time measurements of this type, namely the distance of the object from the detection arrangement, is not required, since it is a matter only of detecting the presence of the object to be detected. And on the other hand, the volume of data associated with this information would considerably increase the expenditure in terms of equipment. It should further be taken into consideration that a travel distance difference of 1 mm requires a resolution per time unit of the transit time signal of approx. 3 ps, which would require a correspondingly fast and thus cost-intensive electronic system. In comparison thereto the evaluation of vibration phase differences is considerably more cost-effective. 
     The terahertz radiation is particularly suitable as detection radiation. This radiation, on the one hand does not have any effects on the human body that are harmful to health, since terahertz photons have an extremely low energy and therefore cannot ionize biological tissue, although terahertz radiation is very strongly absorbed by water. On the other hand, terahertz radiation has the desired reflection or transmission properties. The transmission coefficient of a terahertz radiation of between 0.1 THz and 0.5 THz thus lies in the order of magnitude of 90% for textile materials, such as jeans, cotton socks and linen shirts, and for 0.5 THz in the order of magnitude of 30% for 2 mm thick leather, while for 0.1 THz it is again approx. 90%. Furthermore, the reflection coefficient of skin in the frequency range in question is more than 10%. Moreover, most textiles are characterized in the Thz range by a negligible reflection, whereby they contribute only insignificantly to the determination of the phase position of the reflected radiation, since this originates essentially in the body or the object. Moreover, terahertz radiation of these frequencies has a wavelength that is greater than typical texture sizes of textile materials. Hardly any Mie scattering therefore takes place on the surface of garments. 
     In this context it should be noted that the above-mentioned frequencies could likewise be referred to as 200 GHz or 500 GHz. However, they are usually still attributed to “terahertz radiation,” since they have the same transmission, reflection and absorption properties as this. 
     Typical Gunn oscillators used to generate terahertz radiation provide an output power of approx. 30 mW. Furthermore, the reflected terahertz radiation can be resolved down to powers of approx. 30 pW. To irradiate the body a power can therefore already be sufficient that lies far below the maximum radiation power permissible for mobile phones of approx. 1 W. This underlines the harmless nature of the application of the arrangement according to the invention within the scope of a security check of people. 
     It should be mentioned in this context that there are also regulations for mechanical vibrations, which must be observed to avoid harm to the health of people tested. These are set forth, for example, in ISO 2631-1. Thus with a vibration frequency of 100 Hz and a vibration amplitude of 0.1 mm, the maximum permissible vibration duration is 10 minutes. Naturally, this value lies far above the duration tolerable for a security check. It was shown that by means of terahertz technologies vibration amplitudes down to 200 nm can be detected, which lies approximately three orders of magnitude below the vibration amplitude of 0.1 mm. It is further interesting that an amplitude of approx. 1 μm is usually given as a perception threshold and an amplitude of approx. 1 mm is usually given as a pain threshold. 
     Within the scope of the present invention, for example, mechanical vibrations with a vibration frequency in the range of between approx. 1 Hz and approx. 20,000 Hz, preferably between approx. 50 Hz and approx. 200 Hz, even more preferably approx. 100 Hz, can be used. Even at these frequencies the respectively maximum tolerable duration for the action of mechanical vibration can be substantially undercut, since vibration amplitudes down to 500 nm can be resolved with the arrangement according to the invention. 
     Although the use of a vibration plate for introducing the mechanical vibrations into the body is preferred, a coupling via loudspeakers or the like can fundamentally also be considered. The coupling of vibrations into the body can also be carried out via a seat or quite generally via other parts of the body. 
     In order to be able to facilitate the evaluation of the reflected radiation, the evaluation device can comprise a superimposition device, which superimposes a reference radiation on the received reflected radiation, which reference radiation is in a fixed radiation phase relationship to the detection radiation. This reference radiation can be, for example, a portion of the generated detection radiation directly fed by the emission device of the superimposition device. The superimposition of the reference radiation on the reflected radiation can be described as an interference that depends on the difference between the track of the reference radiation on the one hand and the sum of the tracks of the radiation emitted towards the body and reflected thereby on the other hand. To this end, it is necessary only that the coherence length of the detection radiation generated by the emission device is greater than the sum of the tracks of the radiation emitted towards the body and reflected therefrom. 
     The reference radiation with its predetermined radiation frequency and its predetermined radiation phase position is accordingly used as a tool for scanning the reflected radiation. Of course, however, other methods could also be used for scanning the reflected radiation, similar to those such as are known, for example, from radio frequency technology for scanning radio waves. Time-resolved methods are also possible, in which the vibration of the electromagnetic field is directly detected. Even though these methods have hitherto been available for only a part of the terahertz spectrum, it cannot be ruled out that similar methods will also be developed over the next few years for higher terahertz frequencies. 
     The actual evaluation or analysis of the reflected or superimposed radiation with respect to its vibration phase difference from the predetermined vibration phase can be carried out with the aid of known techniques. Since the objects to be detected are modulated in their spatial position, in particular modulation techniques can be used to this end, which are characterized by high contrast and low noise, for example the lock-in technique or digital data processing. 
     In particular for the field of security checks of people, it is desirable to facilitate the evaluation of the result of the examination for the security staff with the aid of an imaging technology. This can also be realized with the aid of terahertz radiation, since its wavelength is smaller than the typical size of the objects to be imaged. An imaging can be provided in a simple manner in that an imaging optical system with spatial resolution is arranged upstream of the receiver device. The term “optical system” in the present case naturally does not relate to visible electromagnetic radiation, but to the terahertz radiation used. The imaging optical system with spatial resolution can be realized, for example, by sequential scanning in two directions, for example, the vertical and a horizontal direction, or through parallel detection methods, in which entire image lines, entire image columns or the entire image are processed in a parallel manner. Methods are also possible which have become known under the catchword “synthetic aperture.” 
     The display of the result of the check can be carried out on an imaging device, for example, a screen. To this end different values can be assigned to different vibration phase differences by means of evaluation algorithms, and these values can be displayed, for example, by a different color. Further measurement data can also be taken into account in the formation of the values. If the exciter vibration is selected such that essentially the entire body oscillates relative to the predetermined vibration phase of the exciter vibration with one and the same phase difference, the predetermined vibration phase can be used as a reference phase in the formation of the vibration phase differences. If the exciter vibration, however, leads to the different parts of the body oscillating with different phase difference, in the determination of the vibration phase difference to be assigned to a specific image area, the phase position of the signal of an image area surrounding the specific image area and/or adjacent thereto can be used as a reference phase. 
     Fundamentally it would be possible not only to represent the concealed objects on the screen, but also the person examined. However, this would inevitably lead to a representation of the naked body of the person examined, which is not tolerable for ethical reasons. Therefore, in connection with the use of an imaging technology it is also preferred to display a location as a pixel only when the imaging value lies in a predetermined value range. In this manner it can be ensured that only the objects to be detected are displayed. Through this the security staff receive additional information on the object, namely on the one hand information on the location where the object is carried on the body and on the other hand information on the type of object in question from the shape of the image. 
     It should be added that the operation does not necessarily need to be carried out with a single mechanical vibration of predetermined frequency and predetermined phase position as well as predetermined amplitude and predetermined vibration direction. Instead the fact can be utilized that according to Fourier any arbitrary vibration can be shown by superimposition of a plurality of vibrations of predetermined frequency, predetermined phase position, predetermined amplitude and predetermined vibration direction. The signal analysis according to the invention for each of these Fourier components of the mechanical vibration can therefore be carried out separately. 
     The same also applies to the detection radiation used: here too instead of a monofrequent signal, a composite signal can also be used, which can be represented as a Fourier sum of monofrequent individual signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in more detail below using an exemplary embodiment based on the attached drawings. It shows: 
         FIG. 1  diagrammatically illustrates the structure of an arrangement according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
       FIG. 1  shows a detection arrangement according to the invention labeled generally by  10 . It comprises a vibration plate  12 , on which the body to be examined, in the present case the person  14  to be examined, is arranged. The person  14  carries concealed beneath his clothing  16  an object  18 , in the present case a handgun, and it is the object of the detection arrangement  10  to detect this object  18 . 
     To this end, the vibration plate  12  is set in vibration with a predetermined frequency and a known phase, wherein this vibration as a forced vibration is also transferred to the body of the person  14  together with his clothing  16  and the handgun  18  carried in a concealed manner. The resulting movement of the person  14 , the clothing  16  and the concealed object  18  is scanned by means of terahertz radiation. 
     To this end, the detection arrangement  10  comprises an emission device  20  with a radiation generation device  22  for terahertz radiation, which can contain, for example, a Gunn oscillator, and a transmitter device  24 , which emits the terahertz radiation as detection radiation  26  in the direction of the body of the person  14 . 
     The radiation  28  reflected by the body  14  is picked up by a receiver device  30  and fed to an evaluation device  32 . Namely, to be more precise, it is fed to a superimposition device  34  of the evaluation device  32 , which superimposes a reference radiation  36  on the reflected radiation  28 , which reference radiation is in a known radiation phase relationship to the detection radiation  26 . In the case shown in  FIG. 1 , the reference radiation is nothing but a part of the terahertz radiation generated by the emission device  20 , to be more exact, the radiation generation device  22  thereof. 
     Since the reference radiation  36  thus has the same radiation frequency as the reflected radiation  28 , and furthermore the coherence length of the terahertz radiation generated by the radiation generation device  22  is greater than the difference of the tracks of detection radiation  26  and reflected radiation  28  on the one hand and the reference radiation  36  on the other hand, the reference radiation  36  is ideally suitable for scanning the phase length of the reflected radiation. Through the superimposition of the reference radiation  36  on the reflected radiation  28 , a signal is obtained which depicts the track change of the reflected radiation  28  as a result of the vibration of the body of the person  14 , his clothing  16  and the object  18  carried in a concealed manner. 
     The superimposed signal  38  is then sent to an analysis device  40  of the evaluation device  32 , which filters out of the superimposed radiation  38  the portions having the predetermined vibration frequency of the vibration device  12  and analyzes them with respect to their vibration phase difference from the predetermined vibration phase of the mechanical vibration of the vibration device  12 . To this end the analysis device  40  receives from the vibration device  12  a reference signal  42 , which depicts the frequency and phase of the vibration device  12 . 
     The evaluation device  32 , to be more exact, the analysis device  40  thereof, preferably emits a display signal to a display device  44  only when this vibration phase difference lies in a predetermined value range that is typical for the reaction of objects  18  carried in a concealed manner to the vibration of the vibration device  12 . 
     As shown in  FIG. 1 , the display device  44  can be an imaging display device, for example the screen of a computer  46 . In order to be able to render possible a display with spatial resolution on the screen  44 , an optical system  48  with spatial resolution is arranged upstream of the receiver device  30 , which optical system scans the body of the person  14  in a two-dimensional manner. The spatial resolution can thereby be for example 1 mm 2 . 
     In conclusion, it should be noted that the computer  46  can also be used to control the detection arrangement  10 , which is shown in  FIG. 1  by the dotted line  50 . 
     In a concrete case, with the use of CW (continuous wave) radiation a possibility results of receiving access to the acoustic phase. If the reflected radiation is mixed in the homodyne method with a reference beam, the interference depends on the path differences between the two beams. The extension of the path (x 2 −x 1 ) through the acoustic vibration of the object then leads to a periodic interference signal, which depicts the acoustic phase (ω acoust t+Φ) of the object. On the time scale of the acoustic vibrations, after notification about the THz period an interference signal results, wherein Δ=x 2.0 −x 1.0  is the path length difference in neutral position and A is the amplitude of the vibration: 
     
       
         
           
             
               
                 
                   
                     
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     With the development of the cosine in a power series and subsequent restriction to terms of the first harmonic of ω acoust , that is, just as it makes a frequency filter, the following results: 
     
       
         
           
             
               
                 
                   
                     
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     The last term shows that the acoustic phase results from the interference with the use of a frequency filter. However, the amplitudes converge depending on A against a positive or against a negative value of the same size. It should be noted that this measuring principle permits only two possible results, namely either an acoustic phase of −Φ or of +Φ. If the object is not driven to oscillate directly, but via a body lying in between with the phase Ψ, the two possible measuring values result in Ψ±Φ. 
     If the acoustic phase Φ deviates from 90°, the measuring principle yields the possible results Φ or Φ−π. 
     It should also be added that the signal-to-noise ratio of the signal evaluated by the evaluation device  32  to determine the acoustic phase depends on the optical phase with which the detection radiation emitted by the emission device  20  strikes the body  14 . However, it is possible to set or optimize the signal-to-noise ratio for each pixel at a high value, namely through changing the wavelength of the detection radiation emitted by the emission device  20  or through changing the position of the emission device  20  or through another suitable measure. It should be taken into consideration thereby that with a wavelength of the detection radiation in the order of magnitude of 1 mm and a distance between emission device and body in the order of magnitude of 1 m a wavelength change in the order of magnitude of 0.1% or a position change in the order of magnitude of 1 mm is already sufficient. 
     Furthermore, it is possible to avoid undesirable jumps of the acoustic phase between Φ or Ψ−π through the suitable selection of the wavelength of the detection radiation and/or the distance between emission device and body. The signal evaluated for determining the acoustic phase by the evaluation device  32  can hereby be more clearly interpreted and thus more easily evaluated. 
     With respect to the criteria referenced above, it can be summarized
         that the discussed technology is a modulation technology and thus a good sensitivity is to be expected,   that the technology leads to a sufficiently clear measurement signal,   that the criterion of completeness is met, since all objects with a mass can be detected,   that the method can be carried out on people in a physiologically tolerable manner,   that the method is ethically unobjectionable with respect to the representation of the human body. Due to the clarity of the measurement values, it is easy to represent only those objects that are distinguishable from the background in their acoustic phase.