When performing psychovisual experiments, the averaged results from a multiplicity of trials are a more reliable measure of image quality than the results from a single trial, e.g., such as those obtained from conventional three-bar measurements. This paper describes the use of multiple targets as a technique for image evaluation of an optical instrument such as a microscope. On the basis of experiments conducted at Perkin-Elmer, related to specialized photographic test targets, the Landolt Ring target array was chosen as a basic image evaluation probe signal. The targets, organized in groups having different sizes, modulations, and magnifications, are in circular arrays with the opening in the Landolt Rings randomly placed in four orientations. An observer is asked to identify the orientation of the opening. After the identification, his responses are scored against the known orientations, and the curves of probability of correct orientation are plotted as a function of target size. This function forms a quantitative measure for evaluating the performance of optical instruments. The theoretical analysis portion of this paper is con-cerned with the development of a mathematical model by which optical instrument performance may be ranked. By means of a series of psychovisual experiments, Human Factors Research, Incorporated, has independently determined curves for probability of correct orientation for a number of observers. There is generally good agreement between empirical data and model predictions, approximately 90% for both unaided eye viewing and microscope viewing of test target arrays.