Increased application of computer graphics in areas which demand high levels of realism has made it necessary to examine the manner in which images are evaluated and validated. In this paper, we explore the need for including the human observer in any process which attempts to quantify the level of realism achieved by the rendering process, from measurement to display. We introduce a framework for measuring the perceptual equivalence (from a lightness perception point of view) between a real scene and a computer simulation of the same scene. Because this framework is based on psychophysical experiments, results are produced through study of vision from a human rather than a machine vision point of view. This framework can then be used to evaluate, validate and compare rendering techniques.