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- Terahertz time domain spectroscopy (4) (entfernen)
A time domain spectrometer in the range of 0.5–2 THz was used for time of flight (ToF) measurements of THz pulses for imperfection detection in non metallic materials. The measurements were performed in comparison to the established ultra sound pulse echo (PE) and time of flight diffraction (ToFD) technique. No coupling agent and no contact to the surface is required for THz ToFD measurements. Plastic test specimen with notches, voids and drilled side holes were used to study reflected and diffracted response signals. No major differences were observed between the UT and THz ToFD records for the first surface of the notches, voids and holes. Additionally to the UT records also signals were obtained from its back side and a second raw of holes. The THz measurement is fast and has the potential to substitute UT applications in some industrial areas. Within the German project 'Handheld' the potential for miniaturization of the time domain equipment is explored for efficient application.
The emerging technology of generation and detection of Terahertz waves (1 THz = 1012
Hz) offers diverse potentials in non-destructive testing (NDT) regarding security as well
as safety aspects. Herein, we focus on the latter with emphasis on imaging techniques.
The THz range (0.1
10 THz, 3mm
30 µm) closes the technological gap between
ultra high frequency electronics and FIR optics in the electro-magnetic (EM) spectrum.
Electro-optical sampling provides straight access to the EM wave including its phase
(rather than intensity). THz waves are well suited to characterize non-metallic materials
since they penetrate paper, plastics, ceramics and certain composites (e.g. GFC).
We employ a commercial fibre-coupled THz time domain spectrometer (TDS) for
scanning the samples laterally through a focal spot. At each position the entire temporal
pulse train is recorded, which offers the opportunity to use various parameters derivable
for imaging.
Topographic measurements are performed as reflection set-up. The achievable spatial
resolution is diffraction limited at about 100 µm -300 µm (which allows for perception
of single defects on a sub-mm scale). In contrast to typical pulse echoed ultrasound
testing THz topography is a non-contact inspection tool without coupling agents.
Several reflections of subsequent concealed layers are detected instead of just the first
encountered one. Comparing to X-ray (radiology) the non-ionizing THz-waves generate
images of similar contrast regarding metal and plastic (organic) components, while
conventional radiography emphasizes one type of material depending on the energy preselected.
Moreover, the full temporal information is exploited to derive spectra in
certain selected time windows, i.e. separated spectral properties of each layer in
multilayered structures.
Tomographic measurements are performed in transmission mode. The sample is
mounted on a rotation stage in order to allow for lateral scanning under the different
projection angles. The reconstruction is typically performed by filtered backprojection,
which is widely used for X-ray CT. The adaption of this technique to the THz range
comes along with several experimental drawbacks such as considerable refraction and
scattering. According to these difficulties we investigate the occurring artefacts. The
THz-TDS provides the opportunity to calculate a separate backprojection of each
frequency interval as obtained by the Fourier transformation of the recorded time
resolved amplitude. This results in a spectrally resolved reconstruction of each voxel,
which is unique in computed tomography technology.
A time domain spectrometer T-ray 2000® operating in the range of 0.2-2 THz was used for Time of Flight measurements. A series of ethanol water mixtures from 10% to 90% were produced subsequently filled in polystyrene cells and analyzed in the T-ray 2000 TDS system in the reflection mode. The reflectivity of the material liquid interface is strongly influenced by the ethanol concentration in water. The reciprocal reflection coefficient of the material liquid interface was found to be a useful discriminating feature for further Probably of Detection calculations which can be used for the performance evaluation of potential THz bottle scanners. THz pulse echo (PE) measurements were performed in comparison to the established ultra sound PE technique for flaw detection of plastics. Synthetic aperture reconstructions were applied for simulated data to reconstruct the flaw shape. The results were compared with B-scan images of a high density polyethylene (HDPE) test cylinder. Existing restrictions on the quality of the back projected images and limits of the existing model are discussed in detail.
Today X-ray tomography and Ultrasound Synthetic Aperture Focusing Techniques are representative examples for defect imaging in the field of nondestructive testing. THz time domain spectroscopy (TDS) operates with short electromagnetic pulses. The sensitivity of the pulses to optical material property changes makes THz-TDS attractive for the detection of flaws in dielectric materials. Unfortunately, the X-ray computed tomography (CT) model holds only for THz-TDS reconstruction if minor refraction index differences between the inhomogeneities and the surrounding material matrix exist. A Time Domain SAFT algorithm has been developed to overcome the restrictions. THz time domain measurement on representative sample sets with inherent artefacts were performed to use them for the Image reconstructions. The results will be presented and compared with optical surface images of the used test objects to evaluate the SAFT algorithm in relation to the econstruction quality.