Terahertz layer thickness measurements is one of the most promising fields of applications for terahertz technology. Measuring subwavelength layers in multilayer systems is most commonly achieved by applying retrieval algorithms. These algorithms are computational demanding, which makes it hard for the evaluation to keep up with the increasing speed of modern terahertz systems. ECOPS-based systems now achieve measurement rates above 1 kHz. By applying a highly efficient algorithm based on desktop-grade CPU, we achieve multilayer imaging at 1.6 kHz measurement rate. A three-layer system on a metal disk of 300 mm diameter is measured in 2.5 minutes with 240000 pixels.
Layer-thickness measurement is one of the most promising and attractive fields of application for terahertz measurement systems, as they really provide benefits in comparison to competing techniques. In contrast to ultrasound systems, terahertz measurements can be carried out without a coupling medium and is therefore a truly contactless measurement. The possibility to measure individual layers in a multilayer stack is highly advantageous in contrast to established eddy current measurement devices. Unlike X-ray devices, terahertz radiation of common measurement systems is not harmful to biological tissue. Terahertz measurement systems have undergone a remarkable development in terms of the performance as well as in the evaluation algorithms. Increase of speed and enhancement of measurement robustness make these optically complex systems ready for industrial employment. In our contribution, we will cover the development of photonic terahertz measurement systems with a focus on terahertz layer thickness determination.
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.
The usability of pulsed broadband terahertz radiation for the inspection of composite materials from the aeronautics industry is investigated, with the goal of developing a mobile time-domain spectroscopy system that operates in reflection geometry. A wide range of samples based on glass and carbon fiber reinforced plastics with various types of defects is examined using an imaging system; the results are evaluated both in time and frequency domain. The conductivity of carbon fibers prevents penetration of the respective samples but also allows analysis of coatings from the reflected THz pulses. Glass fiber composites are, in principle, transparent for THz radiation, but commonly with significant absorption for wavelengths >1 THz . Depending on depth, matrix material, and size, defects like foreign material inserts, delaminations, or moisture contamination can be visualized. If a defect is not too deep in the sample, its location can be correctly identified from the delay between partial reflections at the surface and the defect itself.
The availability of light and robust structures has led to an increased use of composite materials in the aircraft industry.
In order to verify and guarantee the high quality of the conventional and new composite elements, innovative approaches
for non-destructive testing of these parts are required. The European research project "DOTNAC" proposes to develop a
fast, high resolution, non-invasive and non-contact inspection system for assessing aeronautic composite parts during
production using terahertz waves. Conventionally two categories of systems can be discussed: pulsed and continuous
wave terahertz systems. Both will be realized and their respective potential as a non-destructive inspection tool will be
evaluated against the performance of X-ray testing, ultrasound non-destructive testing, and infra-red imaging.
The present study proposes a fully integrated, semi-automatic and near real-time mode-operated image processing
methodology developed for Frequency-Modulated Continuous-Wave (FMCW) THz images with the center frequencies
around: 100 GHz and 300 GHz. The quality control of aeronautics composite multi-layered materials and structures
using Non-Destructive Testing is the main focus of this work. Image processing is applied on the 3-D images to extract
useful information. The data is processed by extracting areas of interest. The detected areas are subjected to image
analysis for more particular investigation managed by a spatial model. Finally, the post-processing stage examines and
evaluates the spatial accuracy of the extracted information.
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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