Public defence in Photonics and Nanotechnology, M.Sc. Masoud Rastgou

The title of the thesis: Advancements in optical metrology for gloss and thin-film characterization

Thesis defender: Masoud Rastgou
Opponent: Prof. Mathieu Hébert, Université Jean Monnet Saint-Etienne, France 
Custos: Prof. Erkki Ikonen, Aalto University School of Electrical Engineering

This doctoral research develops and validates optical measurement methods for two everyday but hard-to-measure quantities: how shiny a surface looks (gloss) and how thick transparent coatings are (thin films). The goal is to deliver results that are accurate, comparable between instruments and traceable to standards, so that laboratories and companies can rely on them. 

First, the work examines why conventional glossmeters can disagree. Their light sensor has a fixed opening (“aperture”) whose size and position affect the reading. Using a laboratory gonio-reflectometer aligned with ISO 2813, and comparing with commercial devices on high-, medium- and low-gloss samples, the study shows that scanning the angles around the mirror-like reflection allows accurate gloss values even with narrower apertures. The scan reveals how asymmetrical and diffuse reflections from real surfaces bias standard readings and provides a sound way to quantify the uncertainty linked to common aperture settings. 

Next, the thesis compares two leading techniques for coating thickness—spectral reflectometry and ellipsometry—on silicon-dioxide and aluminium-oxide films ranging from 10 to 2000 nanometres. The methods agree within their uncertainties; ellipsometry is particularly precise for very thin layers, while reflectometry performs best for thicker films. The analysis offers clear guidance on which method to choose for a given thickness and how to evaluate the uncertainty. 

Finally, a new analysis is introduced for microscope-based reflectometry, suited to measuring tiny areas. By modelling the spread of incoming angles, correcting for the instrument’s spectral bandwidth and accounting for local thickness variations, the approach matches the laboratory reference and markedly improves accuracy and uncertainty at the microscale. 

These advances make optical metrology more dependable. They clarify the strengths and limits of competing techniques, deliver practical procedures for uncertainty evaluation and provide traceable methods that can be applied in quality control and research across coatings, optics, electronics, semiconductors and energy technologies. The overall conclusion is a set of robust, well-tested tools for measuring gloss and thin-film thickness that help industry and science make better, evidence-based decisions.

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