The time-dependence of photodarkening (PD) in Yb-doped fibers is commonly fitted with a stretched exponential function to determine typical parameters of the process. But, the experimental conditions to obtain consistent results from the comparison of PD for different pump powers, various concentrations of the dopants, or fibers from different manufacturers are not adequately regarded up to now. We discuss the requirements concerning the measurement method as well as the impact of the initial state of the fiber under test. Further on, the experimental results of PD characteristics are discussed in the framework of a kinetic model of the observed processes. The discussion of the measurable PD loss is expanded by introducing the concept of the "PD state" that defines the PD loss as a weighted mean and takes effect on the further evolution of the process. In this way, the basis of fiber characterization and the understanding of the PD kinetics will be improved.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.