Consensus has been developing over the past few decades on a number of measurements required for the laboratory assessment of medical imaging modalities. Nevertheless, understanding of the connection between these measurements and human observer performance in a broad range of tasks remains far from complete. Focusing primarily on projection radiography to provide concrete examples, this overview indicates areas in which consensus on methodology for physical image-quality measurement has been established. Concepts such as "noise equivalent quanta" (NEQ) and "detective quantum efficiency" (DQE) have been found useful for normalizing physical measurements on an absolute scale and for relating those measurements to the decision performance of a hypothetical "ideal observer" that effectively performs decision tasks from the image data. The connection between ideal observer performance and human performance, as determined by receiver operating characteristic (ROC) analysis, remains to be understood for many clinically relevant tasks.