To what extent can photographic readability be formalized for use as a criterion in the evaluation of instructional materials? The purpose of this study was to develop and pilot test a readability index for evaluating photographic illustrations in instructional materials. The index uses an interdisciplinary battery of methods adapted from the fields of linguistics, reading, and cognitive psychology to assess the level of visual information processing for viewers in both initial perception and prolonged encoding phases. The Picture Readability Index investigates both stages in the perception of photographs according to the comparison of affective and cognitive domain classifications on a nomograph format. Visual semiotic analysis is also used to investigate ways in which pictorial elements group to form functional ensembles or gestalts in the first eye centrations. An expert or adoption panel was used to improve the instrument during development. The present format is considered too extensive to use with large populations and in applications with limited time, even though the expert panel suggests that most of the elements included are important. Length could be made more manageable if all components were shortened. (Contains 20 references.) (Author/SLD)