This Volume 2 of Principles of Biophotonics continues to pour the foundation on which the next five volumes of optics and three volumes of methods will be built. While Volume 1 covered the mathematical apparatus to be used throughout the book, Volume 2 describes the emission, detection, and statistical representation of optical fields. The book starts by placing the visible spectrum in the context of the electromagnetic frequency range. This presentation stresses how thin of a sliver one normally calls the ‘optical’ spectrum. And, yet, so much can be accomplished within this narrow range of frequencies. To be able to describe properties of light with technical accuracy, the most common radiometric quantities that the reader is bound to encounter in subsequent volumes are introduced. Although the conversion to photon-based quantities is straightforward, it is presented explicitly, to avoid any confusion. For completeness, an analogy to the photometric quantities of light is drawn as well. Each chapter also contains a set of practice problems and additional references. Part of Series in Physics and Engineering in Medicine and Biology.