Public defence in Measurement Science and Technology, M.Sc.(Tech.) Robin Aschan

The title of the thesis: Investigations and Applications of Angle-Resolved Measurements of Spectral Reflectance and Transmittance 

Doctoral student: Robin Aschan
Opponent: Dr. Georgi T. Georgiev, NASA Langley Research Center, Unite States
Custos: Prof. Erkki Ikonen, Aalto University School of Electrical Engineering, Department of Electrical Engineering and Automation

The appearance of a material is crucial across many fields, from manufacturing and entertainment to scientific research. How a material looks can significantly influence the perceived quality and functionality of products. For instance, in the automotive industry, the finish of a car's paint can affect consumer satisfaction. In entertainment, the realistic rendering of surfaces in computer graphics can enhance visual experiences. Therefore, having precise and reliable ways to measure and characterize material appearance is essential. These measurements need to be consistent with international standards to ensure uniformity and accuracy across different applications. 

This doctoral thesis addresses the need for accurate material appearance measurements by developing advanced techniques for quantifying how materials reflect and transmit light. The research primarily focuses on two key measurement functions: Bidirectional Reflectance/Transmittance Distribution Function (BRDF/BTDF). BRDF measures how light is reflected off a material at various angles, while BTDF measures how light passes through a material at varying angles. 

The research outcomes include several significant advancements. Firstly, a measurement instrument was utilized for precise BRDF measurements. This instrument can systematically measure reflections in a hemisphere above a material from different angles. The instrument BRDF data was used to verify the accuracy of a computer-based model for simulating light reflections of a materials surface. By comparing simulated and actual BRDF data, the reliability of these computer models was enhanced. 

Secondly, a new facility was established for accurate BTDF measurements, specializing in materials that scatter light diffusely. This facility was validated through a rigorous comparison with a commercial spectrophotometer and through a detailed uncertainty evaluation. The research also adapted the 3D gonioreflectometer for measuring light transmission through materials at various angles, especially for those with uneven scattering properties. 

Furthermore, the thesis addressed the challenge of measuring thick samples. A method was developed to correct BTDF measurements based on the setup's geometry and validated using instruments with different sensitivities to sample thickness. This innovation provides a reliable way to measure and interpret the appearance of thicker materials, which is vital for various practical applications.

Keywords: Appearance, BRDF, BTDF, near-infrared light, visible light, angle-resolved reflectance, angle-resolved transmittance

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/

Contact:

Doctoral theses in the School of Electrical Engineering: https://aaltodoc.aalto.fi/handle/123456789/53