The sub-terahertz (THz) frequency band has proved to be a noteworthy option for nondestructive testing (NDT) of nonmetal aeronautics materials. Composite structures or laminates can be inspected for foreign objects (water or debris), delaminations, debonds, etc., using sub-THz sensors during the manufacturing process or maintenance. Given the harmless radiation to the human body of this frequency band, no special security measures are needed for operation. Moreover, the frequency-modulated continuous-wave sensor used in this study offers a very light, compact, inexpensive, and high-performing solution. An automated two-dimensional scanner carrying three sensors partially covering the 70- to 320-GHz band is operated, using two complementary measurement approaches: conventional focused imaging, where focusing lenses are used; and synthetic aperture (SA) or unfocused wide-beam imaging, for which lenses are no longer needed. Conventional focused imagery offers finer spatial resolutions but imagery is depth-limited due to the beam waist effect, whereas SA measurements allow imaging of thicker samples with depth-independent but coarser spatial resolutions. The present work is a compendium of a much larger study and describes the key technical aspects of the proposed imaging techniques and reports on results obtained from human-made samples (A-sandwich, C-sandwich, solid laminates) which include diverse defects and damages typically encountered in aeronautics multilayered structures. We conclude with a grading of the achieved results in comparison with measurements performed by other NDT techniques on the same samples.
Terahertz (THz) spectroscopy is a promising technique for the stand-off identification of hidden objects. The THz band
is particularly well suited firstly because THz radiation penetrates many dielectrics like clothing and secondly because
many potentially hazardous substances have characteristic signatures in the THz spectral region. In order to demonstrate
the full potential of THz radiation for identifying possible hazards using characteristic signatures, different disturbing
influences must be taken into account.
We have performed experiments and simulations in order to investigate the possibilities and the challenges of terahertz
stand-off identification. A special emphasis is paid on humidity in ambient air and properties of the sample like surface
roughness and orientation with respect to the incident THz beam.
Water vapor absorption strongly affects the THz spectra. Since the absorption lines are strong and narrow, the
calculation must be precise. We have checked models well-known in meteorology covering the infrared and the
microwave regions of the electromagnetic spectrum and achieved an accurate description of the measured THz spectral
absorption using the program LINEFIT.
The surface roughness of the sample strongly affects the bandwidth of the reflected spectra. Specular and diffuse
reflection measurements using samples with different roughnesses have been used for determining the influences of
different properties on the reflection spectra.
Terahertz (THz) spectroscopy is a promising technique for the stand-off detection and characterization of hidden objects. The THz band is particularly well suited firstly because THz radiation penetrates many dielectrics like clothes and secondly because many potentially hazardous substances have characteristic signatures in the THz spectral region. In order to utilize the full potential of THz radiation for detecting possible hazards and recognizing characteristic signatures, disturbing influences must be accounted for. We have performed experiments and simulations in order to investigate the limits of terahertz stand-off detection. A special emphasis is paid on humidity in ambient air and properties of the sample like surface roughness, alignment and interfaces. Water vapor absorption strongly affects the THz spectra. Since the absorption lines are strong and narrow, the calculation must be precise. We have checked various models well-known in meteorology covering the infrared and the microwave region of the electromagnetic spectrum. By extending the models into the THz region, an accurate description of the measured spectral absorption is achieved. In our studies transmission spectra for different substances were tested. In a reflection scheme metallized sandpaper of various grit sizes was used to determine the influences of different surface properties. Further measurements were performed for different tilt angles to analyze the influence of the surface roughness. We are currently creating a look-up table to show which parts of the THz spectrum can be used for THz stand-off detection.
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