Advancing THz-Security Imager by using Digital Image Processing-Methods at the Hardware Level

Kurzfassung

THz-Imager are a promising alternative for improving security, e.g. in public transport. This work addresses some of the major problems THz-security imager suffer from when deployed as a means for the stand-off detection of hidden threats. The text starts with analyzing the scenario of stand-off detection and threats likely encountered therein. It is then shown how THz-imager are used in this detection scenario and also why the use of THz-radiation is a good and promising choice. The main components of a THz-imager are named and examined leading to the main problems this technology suffers from. Which are shown to be; first the necessity for mechanical scanning, second poor depth of focus and resolution and third the long measurement times needed. All the aforementioned problems are demonstrated using a THz-security imager designated Integrated Security Monitor-Imager, named after the project it was developed for. The ISM-imager constitutes the THz-part of the Integrated Security Monitor. One possible solution to the problems 1-3 is presented in this thesis. The chosen approach is the use of methods common to digital image processing. The new paradigm in this is the usage of these common methods NOT on the digital level after the image acquisition, but ALREADY during image acquisition at the hardware level. This NEW PARADIGM becomes possible with the help of devices known as THz-Spatial Light Modulators. SLMs have been used in the visible region of the electromagnetic spectrum for decades but are not readily available for the THz-region. The contribution of this work to the advancement of THz-security imaging stems from reinventing/expanding established methods to the THz-region. In doing so, at least two obstacles have had to overcome. First, the THz-region of the electromagnetic spectrum is known as the Quasioptic-region because many optical methods no longer work straightforwardly in this part of the spectrum. The second obstacle is that also methods and devices from the radiowave spectral region are no longer straightforwardly applicable. Although some of the fundamentals were there, they had to be expanded -reinvented- for the THz-region and this is the innovative contribution of this work, trying to contribute to a CHANGE OF PARADIGM FOR THZ-SECURITY IMAGING by implementing the aforementioned ideas. The (reinvented) methods range from HARDWARE EDGE-DETECTION over HARDWARE PHASE-SHIFT DETECTION and HARDWARE CALIBRATED IMAGING to COMPRESSED SENSING using a THz-SLM. In this the notion behind Compressed Sensing, just to acquire the relevant information with as little overhead as possible is key in the understanding of the new imaging paradigm presented here. Therefore, the theory and application of Compressed Sensing is a major topic in this work. In order to adapt the aforementioned methods, some of their physical properties had to be examined for and with Quasioptic-techniques. The results are presented in this thesis namely chapter 6. To promote efficient reading, every sub-chapter of chapter 6 is structured in scientific paper format and can be read as a standalone text. The thesis then concludes with a brief analysis of the results presented and discusses some advantages offered by the newly implemented techniques. In this, the final outlook chapter shows possibilities for a sustainable advancement of THz-imager/THz-imaging in the future.