Entwicklung und Charakterisierung eines Roboter-basierten Terahertz-Zeitbereichsspektrometers für bildgebende Anwendungen auf dem Gebiet der Anthropologie und Kunstrestaurierung

In the context of this work, a robotic-based THz system was developed, which moves a THz sensor relative to an arbitrarily shaped sample. THz measurements can be carried out in transmission or reflection configuration. Frequently, however, the absorption of a sample is too high to penetrate the sample completely or a transmission arrangement is not possible for geometrical reasons. The developed system works therefore with a THz sensor in reflection geometry. For this, however, it must be ensured that the sensor is perpendicular to the sample at all times and that the sample, in turn, is located in the focal plane of the sensor. In order to be able to use the system as individually as possible, the developed measuring method is divided into four steps. In a first step, the surface of the sample to be measured is detected by a fringe projection method. Based on this surface model, the measurement path for the robotic arm is calculated in such a way that the THz sensor stands vertically and at a defined distance to the measurement point. To make the measurement path of the robotic arm as efficient as possible, it should be simulated in advance. Simultaneously during the simulation of the measurement, a collision check is performed to guarantee the non-destructive inspection of valuable objects. The simulated measurement path is subsequently passed on to a THz measurement software in the last step so that the sample can be examined point by point. However, before the tomographic examination of the actual objects can be started, it is necessary to create a material database. Chapter 3 of this thesis therefore presents an overview of the dielectric properties of all relevant materials in the fields of anthropology and art restoration. It turns out that in the case of the embalming materials of ancient human remains, there is a high identification potential based on THz measurements. To tomographically reconstruct the data acquired with the robotic-based system, a special algorithm is available which automatically detects the THz pulses in the data and assigns them to the corresponding layers. The developed system is tested on various objects that are exemplary for the fields of anthropology and art restoration. In the field of ancient human remains, a detailed analysis of a human mummy hand takes place. Here, the THz measurements can be used to reconstruct up to four layers below the skin, which show a high degree of agreement with micro-CT images. By contrast, the comparison of the THz data with conventional CT images shows that the resolution of the THz measurements clearly exceeds that of the CT. Compared with the Micro CT scanner, the THz system also has the advantage that even larger objects can be examined. There is also another advantage in the mobility of the system. Mobile X-ray devices have a very poor resolution, so that THz systems could be a useful in addition to the direct work on archaeological excavation sites in the future. In addition, the requirements for occupational safety are significantly lower than for X-ray equipment. In the field of art restoration, the system is tested on a wooden sculpture (putto). This wooden sculpture has significant damages both in the wood structure and in the overlying painting and priming layers. In a first preliminary investigation, it can be seen that near-surface damages, such as that caused by the widespread bark beetle, can be detected very well. The layer structure of the painting and primer layers per se can also be correctly reconstructed on the basis of the THz data.