The majority of images produced by optical and electro-optical equipment are destined to be viewed eventually by human observers. The human sense of sight itself exhibits a variety of imperfections of imagery (due to the dioptrics), internal noise (due to discrete image sampling at the retina) and a variety of image transformations in the neural networks preceding the perceptual levels of the cortex. In addition all these characteristics are progressive functions of retinal image position, whilst perceptual processes themselves are subject to both statistical fluctuations and influence from other factors. There are therefore many facets of imagery which interact with the human sense of sight, the importance of the individual interactions being largely dependent on the application to which the presented images are put. An approximate representation of preperceptual image formation and transformation by human vision will be presented. A unified approach to the modelling of visual performance will then be summarised which takes account of the various display/observer interface factors and attempts to allow for variability in perceptual processes. Finally it will be shown how apparently widely disparate approaches to visual performance modelling may be reconciled in terms of this unified approach.