Public defence in Radio Science and Engineering, M.Sc. Pouyan Rezapoor

The title of the thesis: Submillimeter wave imaging: system development for stratum corneum water content mapping

Thesis defender: Pouyan Rezapoor
Opponent: Prof. Withawat Withayachumnankul, University of Adelaide, Australia  
Custos: Prof. Zachary Taylor, Aalto University School of Electrical Engineering

This doctoral dissertation explores how terahertz and submillimeter-wave radiation can be used to measure the properties of human skin. These frequencies interact strongly with water, making them promising for detecting changes in hydration. However, current imaging systems struggle to produce accurate, high-resolution measurements on living skin because of optical distortions and limited axial resolution inherent to THz systems.

The purpose of this research was to develop a new imaging platform capable of overcoming these challenges and to demonstrate how it can be used to study the outermost layer of the skin, the stratum corneum. This layer is only 10–30 micrometres thick, yet its hydration level plays a crucial role in skin health, wound healing, irritation, and many dermatological conditions.

The dissertation introduces the Telecentric Offset Reflective Imaging System (TORIS), a new imaging system designed specifically for biological measurements. TORIS improves image quality by keeping the beam angle nearly normal across the entire field of view, increasing the coupling coefficient. The system provides a wide field of view, high spatial uniformity, and stable polarization—features that are essential for reliable skin measurements.

Using this system, the research demonstrates a new method for determining skin hydration and thickness by analysing small shifts in the resonance of the imaging window. This approach is sensitive enough to detect changes that are smaller than the system’s axial resolution.

The main result of the study is a complete optical, mechanical, and analytical framework that enables accurate, non-invasive mapping of skin hydration and structure in real time. The research brings new understanding to terahertz interaction with biological tissues and offers practical tools for biomedical and dermatological applications.

Key words: THz imaging, THz spectroscopy, submillimeter waves

Thesis available for public display 7 days prior to the defence at Aaltodoc.