TG IPMI: Terahertz Computer Tomography for cross-section measurements in extrusion profiles
Speaker: Sandrine van Frank (RECENDT)
Date: April 18, 2018 14:00
Location: SP2 416-2
Extrusion is a very common technique to produce pieces with a constant cross-section. An evaluation of the product quality inline during production would be highly desirable, but it is also challenging. Computer Tomography based on THz radiation is a very interesting candidate for such plastic profiles. To meet the requirements, the technique must be pushed further and suitable reconstruction algorithms developed. In this presentation, ideas and suggestions for a collaboration between RICAM and RECENDT on this thematic will be presented. Terahertz (THz) is a relatively young field of research, where the first sources and detectors appeared only in the 1990s. Since then, the technology has attracted a lot of interest, mostly thanks to the high penetration depth of THz radiation in many non-conductive material (e.g. up to several cm in plastic), its relatively good resolution (mm to sub-mm) and its safety of use (contrary to X-ray, THz photons have very small energy and are thus non-ionising). It is a very interesting tool to image objects of medium size (mm-cm) in 3D, as was demonstrated several times using reflection measurements . In cases where measurements in transmission are preferable, as with plastic extrusion profiles, Computer Tomography (CT) presents an interesting alternative. Standard CT approaches, as developed for medical applications using X-ray, were already tested with THz and showed promising results . There is however a significant difference between the two technologies, which is that the THz radiation is affected by the changes in refractive indices between different media. These lead to reflection and refraction effects, which are not included in the classical CT algorithms and can compromise the reconstruction of test objects. The goal of the proposed collaboration is to develop and tailor THz-CT for a specific type of use – the inline measurement of plastic extrusion profiles. The challenges lie in the required precision (sub-mm) and speed (extrusion speed up to several m/s) for inline quality control. For this purpose, a THz measurement setup developed. Advanced reconstruction algorithms including optical properties of the object will need to be investigated. The specificity of extrusion profiles – only one type of material, known target properties – can be exploited. Furthermore, sparse reconstruction methods can be tested.  S. Katletz et al., “Efficient terahertz en-face imaging,” Opt. Express, vol. 19, no. 23, p. 23042, 2011.  X. C. Zhang, “Three-dimensional terahertz wave imaging,” Philos. Trans. R. Soc. London, Ser. a, vol. 362, no. 1815, pp. 283–298, 2004.