This part of ISO 21227 gives definitions for and provides guidance in the use of optical imaging systems for the quantitative characterization of defects on coated surfaces that occur after exposure in various test methods, e.g. stone chipping, weathering or cross-cut testing. One aim of ISO 21227 is to use optical imaging to reproduce the results of already existing methods for visual assessment. Additionally, optical imaging provides further information which can be used for a more detailed evaluation of coating defects. This part of ISO 21227 contains a general introduction in optical-imaging methods and definitions. The performance of individual test methods and requirements for precision are described in other parts of the standard.