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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 14 — Jul. 9, 2007
  • pp: 8838–8843
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Millimeter wave probing of the acoustic phase for concealed object detection

F. Buersgens, G. Acuna, and R. Kersting  »View Author Affiliations


Optics Express, Vol. 15, Issue 14, pp. 8838-8843 (2007)
http://dx.doi.org/10.1364/OE.15.008838


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Abstract

We developed a far infrared technique for detecting objects that are concealed behind a barrier, which is opaque for visible light. The technique makes use of the fact that mechanically coupled objects can be distinguished by their acoustic phase during a driven vibration. The pattern of the acoustic phase is imaged using 70 GHz radiation. The experimental data demonstrate that virtually all objects can be sensed, which includes metals as well as dielectric materials.

© 2007 Optical Society of America

1. Introduction

In recent years many studies exploited the potential of millimeter wave technology for security imaging, in particular for personnel screening. The technological endeavors in this field are motivated by unique advantages of the so called terahertz range: Many covering materials such as textiles are transmittive in the upper gigahertz (GHz) or lower terahertz (THz) band [1

1. J. Bjarnason, T. Chan, A. Lee, M. Celis, and E. Brown, “Millimeter-wave, terahertz, and mid-infrared transmission through common clothing,” Appl. Phys. Lett. 85, 519 (2004). [CrossRef]

, 2

2. J. F. Federici, B. Schulkin, F. Huang, D. Gray, R. Barat, F. Oliveira, and D. Zimar, “THz imaging and sensing for security applications-explosives, weapons and drugs,” Semicond. Sci. Technol. 20, S266 (2005). [CrossRef]

, 3

3. D. Zimdars, “Fiber-pigtailed terahertz time domain spectroscopy instrumentation for package inspection and security imaging,” in Terahertz for Military and Security Applications, R. Hwu and D. Woolard, eds. (SPIE, Vol. 5070, 2003). [CrossRef]

, 4

4. H. D. Griffiths and C. J. Baker, “Radar imaging for combating terrorism,” in NATO Advanced Study Institute, http://www.nato-us.org/imaging2006/lecturers.html (Springer, 2006).

]. Additionally, imaging resolutions of the order of 1 mm can be achieved, and finally, the radiation is understood to cause no genotoxic health risks [5

5. M. R. Scarfi, M. Romaneo, R. D. Pietro, O. Zeni, A. Doria, G. P. Gallerano, E. Giovenale, G. Messina, A. Lai, G. Campuria, D. Conglio, and M.D’ Arienzo, “THz exposure of whole blood for the study of biological effects on human lymphocytes,” J. Biol. Phys. 29, 171 (2003). [CrossRef]

, 6

6. E. Berry, “Risk perception and safety issues,” J. Biol. Phys. 29, 263 (2003). [CrossRef]

], which makes it suitable for instance for passenger screening. A variety of different techniques were developed, some of which intend to identify hidden materials by their spectral properties [7

7. B. Fischer, M. Hoffmann, H. Helm, G. Modjesch, and P. Jepsen, “Chemical recognition in terahertz time-domain spectroscopy and imaging,” Semicond. Sci. Technol. 20, 246 (2005). [CrossRef]

, 8

8. K. Kawase, Y. Ogawa, Y. Watanabe, and H. Inoue, “Non-destructive terahertz imaging of illicit drugs using spectral fingerprints,” Opt. Express 11, 2549 (2003). [CrossRef] [PubMed]

, 9

9. L. Ning, S. Jongling, S. Jinhai, L. Laishun, X. Xiaoyu, L. Meihong, and J. Yan, “Study of the THz spectrum of methamphetamine,” Opt. Express 5, 6750 (2005). [CrossRef]

, 10

10. Y. C. Shen, T. Lo, P. F. Taday, B. E. Cole, W. R. Tribe, and M. C. Kemp, “Detection and identification of explosives using terahertz pulsed spectroscopic imaging,” Appl. Phys. Lett. 86, 241,116 (2005). [CrossRef]

], while others measure the reflected radiation either of coherent illumination [11

11. K. J. Siebert, T. Löffler, H. Quast, M. Thomson, T. Bauer, R. Leonhardt, S. Czasch, and H. G. Roskos, “All-optoelectronic continuous wave THz imaging for biomedical applications,” Phys. Med. Biol. 47, 3743 (2002). [CrossRef] [PubMed]

, 12

12. T. Kleine-Ostmann, P. Knobloch, M. Koch, S. Hoffmann, M. Breede, M. Hofmann, G. Hein, K. Pierz, M. Sperling, and K. Donhuijsen, “Continuous-wave THz imaging,” Electron. Lett. 37, 1461 (2001). [CrossRef]

, 13

13. J. Darmo, V. Tamosiunas, G. Fasching, J. Kröll, K. Unterrainer, M. Beck, M. Giovanni, J. Faist, C. Kremser, and P. Debbage, “Imaging with a terahertz quantum cascade laser,” Opt. Express 12, 1879 (2004). [CrossRef] [PubMed]

] or of ambient far infrared light [14

14. R. Appleby, “Passive millimetre-wave imaging and how it differs from terahertz imaging,” Phil. Trans. R. Soc. Lond. A 362, 379 (2004). [CrossRef]

, 15

15. D. Zimdars and J. S. White, “Terahertz reflection imaging for package and personnel inspection,” in Terahertz for Military and Security Applications, R. Hwu and D. Woolard, eds. (SPIE, Vol. 5411, 2004). [CrossRef]

]. However, many approaches have to cope with conceptual difficulties, such as the vast number of spectrally similar materials, signal corruptions by atmospheric influences, or ambiguities of the image introduced by absorption in covering materials. These metrological challenges frequently sidetrack the attention from the ultimate question in security imaging, namely whether or not an object is present behind an opaque barrier.

Far infrared imaging of the vibrational properties of objects can provide decisive information for answering this question. Recently, we have presented the fundamental physics of a measurement scheme, which detects the presence of concealed objects, by imaging the phase of their acoustic vibrations [16

16. F. Buersgens, G. Acuna, and R. Kersting, “Acoustic phase imaging with terahertz radiation,” Opt. Express 15, 4427 (2007). [CrossRef] [PubMed]

]. Similar conclusions were drawn by Chen and Kaushik [17

17. J. C. Chen and S. Kaushik, “Terahertz Interferometer That Senses Vibrations Behind Barriers,” IEEE Photon. Technol. Lett. 19, 1041 (2007). [CrossRef]

] in an independent work, published at the same time. In this Letter we present an experimental study proving the applicability of the technique.

2. Experimental method

The basic idea behind imaging the acoustic phase is that mechanically coupled objects in general respond to a driving force with different phase lags. In the context of a future application this means that an object carried underneath a textile responds with a different phase lag than the body to which the object is coupled mechanically. In consequence, imaging the acoustic phase reveals a contrast between object and body, which allows for locating the object.

We discussed the fundamental properties of imaging acoustic phases with THz radiation in Ref. [16

16. F. Buersgens, G. Acuna, and R. Kersting, “Acoustic phase imaging with terahertz radiation,” Opt. Express 15, 4427 (2007). [CrossRef] [PubMed]

]. In the experiments described here, we use a non-commercial GHz transceiver based on a voltage controlled oscillator and a frequency mixer circuit. This approach has the advantage that the same horn antenna is used for emission and detection (Fig. 1). The frequency of the transceiver can be tuned in the range of 69–78 GHz, providing typical emission powers of about 10 mW. The output voltage of the transceiver is proportional to the homodyne mixing signal.

In our experiments we fixed several metallic and dielectric items (labeled ”Object 1” in Fig. 1) with a Velcro-type hook and loop tape to the front side of a board (labeled ”Object 2”). This board serves as a reference for the acoustic phase measurements. It consists of a 1 cm thick wood panel topped with a 0.8 cm layer of neoprene. The board is wrapped in natural leather, which is kept moist in order to achieve a realistic reflectivity in the far-infrared, even though the exact amount of reflected radiation is irrelevant for acoustic phase measurements. Acoustic vibrations are launched from the backside into the board using a loudspeaker operated at frequencies between 15–80 Hz. In consequence, both the board as well as ”Object 1” performs a driven vibration. In our experiments the vibration amplitudes of the test objects are typically in the range of Dacoust = 0.07–10 μm.

Fig. 1. Schematic of the setup used for spatially resolved phase measurements employing homodyne detection.

The acoustic phase lag of an object’s oscillation, driven at an angular frequency ω is

tanϕac=ωγkmω2,
(1)

where m is the object’s mass, k is the elastic coupling constant between the object and the drive, and γ is the damping. Thus, even under strong damping (large γ) and a negligible elastic coupling (vanishing k) a nonzero phase lag will be observed. As outlined in Ref. [16

16. F. Buersgens, G. Acuna, and R. Kersting, “Acoustic phase imaging with terahertz radiation,” Opt. Express 15, 4427 (2007). [CrossRef] [PubMed]

] the acoustic phase of the objects can be measured by homodyne mixing of the electromagnetic radiation reflected from the objects. As it is known from interference phenomena, such mixing signals depend on the path length. Hence, if the reflecting object oscillates, this results in a modulation of the detected signal. The phase of this oscillation is extracted using a lock-in amplifier referenced to the drive signal. We distinguish between the actual acoustic phase ϕac and the measured phase Φmeas, as detected by the lock-in amplifier [16

16. F. Buersgens, G. Acuna, and R. Kersting, “Acoustic phase imaging with terahertz radiation,” Opt. Express 15, 4427 (2007). [CrossRef] [PubMed]

]:

Φmeas={ϕacif(2n1)<4ΔzλGHz<2n,withnϕacπelse,
(2)

where Δz stands for the optical path length minus an odd multiple of λGHz/4.

Most remarkable about Eq. (2) is that the measured phase relates to the acoustic phase in a two-valued manner, which depends exclusively on the distance between emitter and object. Two further properties of acoustic phase imaging are worth mentioning: The measured phase does neither depend on the oscillation amplitude nor on the intensity of the detected radiation. Both properties make the technique very robust against corruptions of the signal.

Fig. 2. Interferogram (a), the measured acoustic phase (b) depending on changes of the optical path length, and the acoustic phase (c) calculated by equating ϕcalc = modmeas + π,π). The oscillation amplitude for the phase measurement was about 750 nm.

3. Results

The two-valued behavior is verified in experiments that are performed just on the vibrating board (”Object 2”) in dependence of the optical path length. Figure 2 shows an interferogram of the detected intensity (a), the measured phase (b) as well as the calculated acoustic phase (c) depending on the distance between transceiver and object. The interferogram shows the expected oscillations with maxima when the distance equals 2n+14λGHz. Whenever the spatial slope of the interferogram (a) is positive, a small modulation of the optical path length has a phase ϕac while for negative slopes one detects ϕac - π. Accordingly, the measured phase jumps by 180° whenever the intensity of the mixing signal exhibits an extremum as discussed in more detail in [16

16. F. Buersgens, G. Acuna, and R. Kersting, “Acoustic phase imaging with terahertz radiation,” Opt. Express 15, 4427 (2007). [CrossRef] [PubMed]

].

In the following, we show that concealed objects can be located by spatially resolved detection of the acoustic phase. For this experiments we use different items as ”Object 1”: i) an aluminum plate, ii) a cell phone, iii) a chocolate bar, iv) a piece of soap, and v) a 1 cm thick slice of Bavarian spam-meat (”Leberkäse”). All objects were attached to the plate using hook-and-loop tape, thus oscillating with equal frequency but different phase, depending on their individual mass, coupling, and damping, Eq. (1). The force of the fixation was sufficient in order to hold the items on the vertical back plate. In order to mimic random covering conditions in real world applications, all objects were at least covered with one textile layer of linen or wool.

The spatial resolution was achieved by scanning the objects in y-direction through the focus of the GHz-beam (Fig. 1). Figure 3 shows the phase contrasts for different items along with gray bars indicating their spatial extension. The drastic phase changes at the edges allow for locating the objects. In general the phase contrast exceeds 25°. The precise value however, rather depends on the individual coupling conditions of each object rather than on its material properties. The data recorded on chocolate and soap illustrate that not only metallic objects but also dielectrics can be detected. Additionally, we found that even multiple textile layers have no significant impact on locating the objects correctly.

There are three effects contributing to the measured phase in Fig. 3, namely i) the physical acoustic phase lag, Eq. (1), ii) the two-valued dependence of the measured phase on the optical path length, Eq. (2), and iii) the signal processing of the lock-in, which projects all phase values to a range between -180° and +180°. While effect i) is the quantity to be measured the other effects account for the deviations of some traces from the expected behavior. For instance the additional phase flip observed at the center of the cell phone position or in the chocolate pattern can be explained by ii), while in the case of the soap, the phase contrast exceeding 180° is the cumulated contribution of i) and ii). For the spam meat, we attribute the dramatic phase change at the second edge to contribution iii), while the phase structure in between is due to acoustic modes of this rather flexible material. Even though such flabby materials show complicated phase patterns, the data clearly indicate the presence of some object.

Fig. 3. Phase contrast of items coupled to a vibrating plate by hook-and-loop tape. For clarity all phase values are shifted. The objects were scanned across the focus of the GHz beam as depicted in Fig. 1. All objects were covered by textiles. The gray bars indicate the position of the respective items.

In order to further investigate the applicability of the technique we performed an analysis of the measured noise. Figure 4 shows the noise of the phase measured on the leather wrapped back-plate in dependence on the oscillation amplitude. We deduce a nearly inverse proportionality between the noise and the vibration amplitude. As we stated above, the phase measurement is independent of the strength of the detected signal, i.e. independent of the GHz power, the reflectivity of the object, absorption in covering materials or the amplitude of the oscillation. However, the accuracy of the phase measurement is proportional to the modulation ampiltude and thus to the quantities mentioned before, which explains the scaling behavior illustrated in Fig. 4. For a realistic assessment of the noise level the measurement was performed through a linen towel, which turned out not to influence the accuracy of the measurement significantly.

4. Discussion

Several properties of acoustic phase imaging have to be analyzed for assessing the limitations of future applications. One is the spatial resolution that can be achieved. In our experiments using 70 GHz radiation we achieve a diffraction limited spatial resolution of about 1.2 cm, which roughly corresponds to 3λ. This resolution is certainly sufficient for detecting most objects of relevance. Another important property is the time required for recording a meaningful image, which can be estimated by an analysis of the measurement noise. In all cases shown in Fig. 3 the phase jumps exceed 25°. Therefore, a signal to noise ratio larger than 10 can be achieved as long as the phase noise is kept < 2.5°. For an oscillation amplitude of 3 μm such signal to noise ratios translate into an integration time of about Tint=(0.08°Hz2.5°)2 ≈ 1 ms per pixel. Thus recording an image of about 200 × 200 pixels would take 0.4 s, if a 100 fold parallelization of the signal detection is assumed.

Fig. 4. Dependence of the phase noise on the oscillation amplitude of the back plate.

5. Conclusions

In summary, we developed an experimental technique, which facilitates the detection of objects concealed under textile layers. It turns out that sensing the acoustic phase is extremely robust and insensitive to signal corruptions. We found that for realistic conditions, oscillation amplitudes of 3 μm are sufficient for detecting the acoustic phase of a concealed object. Such amplitudes are physiologically inoffensive and close to the human perception level. Experiments were performed on concealed items such as cellular phones, metal objects, and also on dielectric materials such as chocolate or soap. All items were successfully detected even when concealed by several layers of textiles. Further developments of the technique toward applications in security imaging appear reasonable.

Acknowledgments

The authors acknowledge technical help by Florian Kuchler and thank Alexander Dabek from Siemens AG for support. The work of G.A. is funded by the DFG (contract KE 516/1-1).

References and links

1.

J. Bjarnason, T. Chan, A. Lee, M. Celis, and E. Brown, “Millimeter-wave, terahertz, and mid-infrared transmission through common clothing,” Appl. Phys. Lett. 85, 519 (2004). [CrossRef]

2.

J. F. Federici, B. Schulkin, F. Huang, D. Gray, R. Barat, F. Oliveira, and D. Zimar, “THz imaging and sensing for security applications-explosives, weapons and drugs,” Semicond. Sci. Technol. 20, S266 (2005). [CrossRef]

3.

D. Zimdars, “Fiber-pigtailed terahertz time domain spectroscopy instrumentation for package inspection and security imaging,” in Terahertz for Military and Security Applications, R. Hwu and D. Woolard, eds. (SPIE, Vol. 5070, 2003). [CrossRef]

4.

H. D. Griffiths and C. J. Baker, “Radar imaging for combating terrorism,” in NATO Advanced Study Institute, http://www.nato-us.org/imaging2006/lecturers.html (Springer, 2006).

5.

M. R. Scarfi, M. Romaneo, R. D. Pietro, O. Zeni, A. Doria, G. P. Gallerano, E. Giovenale, G. Messina, A. Lai, G. Campuria, D. Conglio, and M.D’ Arienzo, “THz exposure of whole blood for the study of biological effects on human lymphocytes,” J. Biol. Phys. 29, 171 (2003). [CrossRef]

6.

E. Berry, “Risk perception and safety issues,” J. Biol. Phys. 29, 263 (2003). [CrossRef]

7.

B. Fischer, M. Hoffmann, H. Helm, G. Modjesch, and P. Jepsen, “Chemical recognition in terahertz time-domain spectroscopy and imaging,” Semicond. Sci. Technol. 20, 246 (2005). [CrossRef]

8.

K. Kawase, Y. Ogawa, Y. Watanabe, and H. Inoue, “Non-destructive terahertz imaging of illicit drugs using spectral fingerprints,” Opt. Express 11, 2549 (2003). [CrossRef] [PubMed]

9.

L. Ning, S. Jongling, S. Jinhai, L. Laishun, X. Xiaoyu, L. Meihong, and J. Yan, “Study of the THz spectrum of methamphetamine,” Opt. Express 5, 6750 (2005). [CrossRef]

10.

Y. C. Shen, T. Lo, P. F. Taday, B. E. Cole, W. R. Tribe, and M. C. Kemp, “Detection and identification of explosives using terahertz pulsed spectroscopic imaging,” Appl. Phys. Lett. 86, 241,116 (2005). [CrossRef]

11.

K. J. Siebert, T. Löffler, H. Quast, M. Thomson, T. Bauer, R. Leonhardt, S. Czasch, and H. G. Roskos, “All-optoelectronic continuous wave THz imaging for biomedical applications,” Phys. Med. Biol. 47, 3743 (2002). [CrossRef] [PubMed]

12.

T. Kleine-Ostmann, P. Knobloch, M. Koch, S. Hoffmann, M. Breede, M. Hofmann, G. Hein, K. Pierz, M. Sperling, and K. Donhuijsen, “Continuous-wave THz imaging,” Electron. Lett. 37, 1461 (2001). [CrossRef]

13.

J. Darmo, V. Tamosiunas, G. Fasching, J. Kröll, K. Unterrainer, M. Beck, M. Giovanni, J. Faist, C. Kremser, and P. Debbage, “Imaging with a terahertz quantum cascade laser,” Opt. Express 12, 1879 (2004). [CrossRef] [PubMed]

14.

R. Appleby, “Passive millimetre-wave imaging and how it differs from terahertz imaging,” Phil. Trans. R. Soc. Lond. A 362, 379 (2004). [CrossRef]

15.

D. Zimdars and J. S. White, “Terahertz reflection imaging for package and personnel inspection,” in Terahertz for Military and Security Applications, R. Hwu and D. Woolard, eds. (SPIE, Vol. 5411, 2004). [CrossRef]

16.

F. Buersgens, G. Acuna, and R. Kersting, “Acoustic phase imaging with terahertz radiation,” Opt. Express 15, 4427 (2007). [CrossRef] [PubMed]

17.

J. C. Chen and S. Kaushik, “Terahertz Interferometer That Senses Vibrations Behind Barriers,” IEEE Photon. Technol. Lett. 19, 1041 (2007). [CrossRef]

18.

F. Buersgens et al., “Enhancing the sensitivity of phase contrast measurements,” manuscript in preparation

19.

D. Dieckmann, “Einfluss vertikaler mechanischer Schwingungen auf den Menschen,” Internat. Z. angew. Physiol. einschl. Arbeitsphysiol. 16, 519 (1957).

20.

B. Harazin and J. Grzesik, “The transmission of vertical whole-body vibration to the body segments of standing subjects,” J. Sound and Vib. 215, 775 (1998). [CrossRef]

21.

International Standard, ISO 2631-1 (1997). Second edition.

OCIS Codes
(110.3080) Imaging systems : Infrared imaging
(110.5120) Imaging systems : Photoacoustic imaging
(120.0280) Instrumentation, measurement, and metrology : Remote sensing and sensors
(330.4150) Vision, color, and visual optics : Motion detection

ToC Category:
Imaging Systems

History
Original Manuscript: May 10, 2007
Revised Manuscript: June 14, 2007
Manuscript Accepted: June 26, 2007
Published: June 28, 2007

Virtual Issues
Vol. 2, Iss. 8 Virtual Journal for Biomedical Optics

Citation
F. Buersgens, G. Acuna, and R. Kersting, "Millimeter wave probing of the acoustic phase for concealed object detection," Opt. Express 15, 8838-8843 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-14-8838


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References

  1. J. Bjarnason, T. Chan, A. Lee, M. Celis, and E. Brown, "Millimeter-wave, terahertz, and mid-infrared transmission through common clothing," Appl. Phys. Lett. 85, 519 (2004). [CrossRef]
  2. J. F. Federici, B. Schulkin, F. Huang, D. Gray, R. Barat, F. Oliveira, and D. Zimar, "THz imaging and sensing for security applications-explosives, weapons and drugs," Semicond. Sci. Technol. 20, S266 (2005). [CrossRef]
  3. D. Zimdars, "Fiber-pigtailed terahertz time domain spectroscopy instrumentation for package inspection and security imaging," in Terahertz for Military and Security Applications, R. Hwu and D. Woolard, eds., (SPIE, 2003) Vol. 5070. [CrossRef]
  4. H. D. Griffiths and C. J. Baker, "Radar imaging for combating terrorism," in NATO Advanced Study Institute, http://www.nato-us.org/imaging2006/lecturers.html> (Springer, 2006).
  5. M. R. Scarfi, M. Romaneo, R. D. Pietro, O. Zeni, A. Doria, G. P. Gallerano, E. Giovenale, G. Messina, A. Lai, G. Campuria, D. Conglio, and M. D’Arienzo, "THz exposure of whole blood for the study of biological effects on human lymphocytes," J. Biol. Phys. 29, 171 (2003). [CrossRef]
  6. E. Berry, "Risk perception and safety issues," J. Biol. Phys. 29, 263 (2003). [CrossRef]
  7. B. Fischer, M. Hoffmann, H. Helm, G. Modjesch, and P. Jepsen, "Chemical recognition in terahertz time-domain spectroscopy and imaging," Semicond. Sci. Technol. 20, 246 (2005). [CrossRef]
  8. K. Kawase, Y. Ogawa, Y. Watanabe, and H. Inoue, "Non-destructive terahertz imaging of illicit drugs using spectral fingerprints," Opt. Express 11, 2549 (2003). [CrossRef] [PubMed]
  9. L. Ning, S. Jongling, S. Jinhai, L. Laishun, X. Xiaoyu, L. Meihong, and J. Yan, "Study of the THz spectrum of methamphetamine," Opt. Express 5, 6750 (2005). [CrossRef]
  10. Y. C. Shen, T. Lo, P. F. Taday, B. E. Cole, W. R. Tribe, and M. C. Kemp, "Detection and identification of explosives using terahertz pulsed spectroscopic imaging," Appl. Phys. Lett. 86, 241,116 (2005). [CrossRef]
  11. K. J. Siebert, T. Loffler, H. Quast, M. Thomson, T. Bauer, R. Leonhardt, S. Czasch, and H. G. Roskos, "Alloptoelectronic continuous wave THz imaging for biomedical applications," Phys. Med. Biol. 47, 3743 (2002). [CrossRef] [PubMed]
  12. T. Kleine-Ostmann, P. Knobloch, M. Koch, S. Hoffmann, M. Breede, M. Hofmann, G. Hein, K. Pierz, M. Sperling, and K. Donhuijsen, "Continuous-wave THz imaging," Electron. Lett. 37, 1461 (2001). [CrossRef]
  13. J. Darmo, V. Tamosiunas, G. Fasching, J. Kroll, K. Unterrainer, M. Beck, M. Giovanni, J. Faist, C. Kremser, and P. Debbage, "Imaging with a terahertz quantum cascade laser," Opt. Express 12, 1879 (2004). [CrossRef] [PubMed]
  14. R. Appleby, "Passive millimetre-wave imaging and how it differs from terahertz imaging," Phil. Trans. R. Soc. Lond. A 362, 379 (2004). [CrossRef]
  15. D. Zimdars and J. S. White, "Terahertz reflection imaging for package and personnel inspection," in Terahertz for Military and Security Applications, R. Hwu and D. Woolard, eds., (SPIE, 2004) Vol. 5411. [CrossRef]
  16. F. Buersgens, G. Acuna, and R. Kersting, "Acoustic phase imaging with terahertz radiation," Opt. Express 15, 4427 (2007). [CrossRef] [PubMed]
  17. J. C. Chen and S. Kaushik, "Terahertz interferometer that senses vibrations behind barriers," IEEE Photon. Technol. Lett. 19, 1041 (2007). [CrossRef]
  18. F. Buersgens et al., "Enhancing the sensitivity of phase contrast measurements," manuscript in preparation.
  19. D. Dieckmann, "Einfluss vertikaler mechanischer Schwingungen auf den Menschen," Internat. Z. angew. Physiol. einschl.Arbeitsphysiol. 16, 519 (1957).
  20. B. Harazin and J. Grzesik, "The transmission of vertical whole-body vibration to the body segments of standing subjects," J. Sound and Vib. 215, 775 (1998). [CrossRef]
  21. International Standard, ISO 2631-1 (1997). Second edition.

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