Visual performance is defined in terms of the speed and accuracy of processing visual information. To evaluate illuminated tasks in terms of visual performance, it is necessary to have a valid computational model that relates measurable, salient aspects of the visual environment (e.g. target contrast) to measurable human responses (e.g. visual response time). A model of visual performance should be independent of the influence of non-visual factors as they influence speed and accuracy. It is also important to have practical application tools that can measure the salient aspects of the visual environment and compute (predict) visual performance for real tasks. This paper describes the basis for a visual performance model and how it can be applied using a computer imaging device.