THz imaging has turned out to be a very useful technique for applications in nondestructive testing and quality control of industrial objects. Using coherent detection methods the whole electric field can be obtained. Depending on the evaluation method of the recorded THz signals different properties of the objects under investigation can be determined. This includes thickness, shape, density or internal content.
The measured values are evaluated to get the physical properties: amplitude, time delay and spectrum.
The THz image of the transmitted amplitude is shown in fig. below. In a machined plastic plate all drilled holes (down to a diameter of 1 mm) were resolved as well as the hidden counter bores.
Terahertz radiation propagates in different media with different velocities. If a transparent media with a thickness D and a refractive index of nTHz is brought into the beam path, the THz wave is delayed by Δ T = D (nTHz-1)/c0 (c0: velocity of light in vacuum). The relative time delay can be detected very accurately. The figure shows the THz time delay of commercially available scotch tape attached to a plastic holder. Here a time accuracy of Δ T = 0,1 ps was realized which corresponds to a plastic layer thickness of approx. L = 50 μm.
terahertz (THz), time-domain-spectroscopy, nondestructive testing, final inspection, quality control, industrial application
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