This lecture covers various techniques for measuring thin film thickness in micro and nanofabrication processes, including optical reflectometry, transmittometry, ellipsometry, and spectroscopic ellipsometry. It explains the physical principles behind these methods, such as normal reflectance, transmittance, and changes in light properties. The lecture also discusses the process of measuring SiO2 thickness using bi-morph wafers and the importance of good spectrum fitting. Additionally, it explores the use of ellipsometers for measuring thin films and compares the advantages of reflectometers and ellipsometers in terms of accuracy and methodology, emphasizing their non-contact and non-invasive nature.