The ergonomic study of VDUs demonstrates the importance of visual stress in the total work load. By means of a laboratory experiment and authors showed that the distribution of light in the visual field exerts a major influence on the operator's visual acuity. For this reason, it is of great interest to measure the visual performance in light conditions as close as possible to real working conditions. The results of this experiment can also be of practical value in formulating guidelines for the design of appropriate work places corresponding to the visual requirements of VDU operators.