In the framework of a research project on vision models, the visual performance data of different authors have been compared with respect to their relation to background luminance, contrast and size of the critical object. Rea and Ouellette presented a model based on their own data on "Reaction Time", that allows the calculation of Visual Performance (VP) according to their definition for given visual task conditions. However, their definition of VP differed from those used in all of the other studies considered here. As part of this research project a model for VP was developed, based on the very comprehensive study of H. C. Weston in 1945. He used, as performance criteria, the speed with which the form of a target could be detected and a term comprising accuracy of recognition. Investigations of Muck and Bodmann, using similar criteria, composed of search time and accuracy, have been compared with the Weston model and showed good agreement. Likewise, the data of Simonson and Brozek, McNelis, Smith and Rea, and Loe and Waters have been examined. The differences between the Rea and Weston models can be attributed to the different criteria used for measurements of Visual Performance. In Rea's case, the inverse of the time needed to see a target varying in size and contrast was used as a measure; in Weston's case it was a composite of visual acuity, time and errors made. The differences observed are mainly due to the sensitivity to contrast: Rea's model does not show significant dependency on contrast but Weston's does. It became evident that a strong relationship between visual acuity and performance exists. The influence of age on visual acuity was found to be of major importance to Visual Performance also.