It is the objective of this discussion to present a mechanism for the appraisal of the performance of optical systems. In order for such a mechanism to be of value, it must be based on reasonable theoretical concepts and be so composed as to permit straightforward, easily interpreted physical measurements. These requirements are necessary so that a body of theory will exist which can predict with reasonable accuracy the results of experiment and the performance of actual systems. It is not our intention in this discussion to delve deeply into the theory, to go into extensive discussions of experiments and results, or to provide numerous comparisons of prediction and performance on actual optical systems. On the other hand, we will try to cover the salient features of the theory and its experimental support, as well as some insight into the practical aspects of mechanism.