A one-stop, concise guide on determining and measuring thin film thickness by optical methods. This practical book covers the laws of electromagnetic radiation and interaction of light with matter, as well as the theory and practice of thickness measurement, and modern applications. In so doing, it shows the capabilities and opportunities of optical thickness determination and discusses the strengths and weaknesses of measurement devices along with their evaluation methods. Following an introduction to the topic, Chapter 2 presents the basics of the propagation of light and other electromagnetic radiation in space and matter. The main topic of this book, the determination of the thickness of a layer in a layer stack by measuring the spectral reflectance or transmittance, is treated in the following three chapters. The color of thin layers is discussed in chapter 6. Finally, in chapter 7, the author discusses several industrial applications of the layer thickness measurement, including high-reflection and anti-reflection coatings, photolithographic structuring of semiconductors, silicon on insulator, transparent conductive films, oxides and polymers, thin film photovoltaics, and heavily doped silicon. Aimed at industrial and academic researchers, engineers, developers and manufacturers involved in all areas of optical layer and thin optical film measurement and metrology, process control, real-time monitoring, and applications.