An F-Center's a type of crystallographic defect in which an anionic vacancy in a crystal's filled by one or more electrons, depending on the charge of the missing ion in the crystal. Electrons in such a vacancy tend to absorb light in the visible spectrum such that a material that's usually transparent becomes colored. Thus the origin of the name, F-center, which originates from the German Farbzentrum. The translation of this term also provides the synonym color center, which can also refer to s… (More on F-center)