Given a balanced distribution of luminance in the Visual Field (cf. Ten Guidelines for Right Light), the human eye adapts to a mean luminance for optimum visual discrimination.

The difference in luminance needed between a visual object and its surroundings to permit the former to be perceived is then lowest. Where the distribution of luminance includes very high luminance peaks (e.g.unshielded lamps, direct sunshine) the human eye adapts at a higher level and is then less capable of perceiving visual objects in the darker zones (equivalent veiling luminance). This state is known as physiological glare. It has a significant impact on the performance of the human eye, and represents a quantifiable restriction on powers of vision.

In his standard work written on the subject in the Fifties, H. C. Weston describes the relationship between visual performance and the size of the visual object for different contrast conditions. His research shows that 500 lx is enough for optimum visual performance to be achieved with large objects (visual angle 4.5') and high levels of contrast, whereas with small objects (visual angle 1.5') and reduced contrast, visual performance continues to increase even beyond the 10,000 lx mark, although it can never reach the same level as with large objects and pronounced contrast.

Most of the provisions in the standards relating to Illuminance are based on the results of the above study.