Glare is the difficulty seeing in the presence of bright light such as direct or reflected sunlight or artificial light such as lamps and light fittings.
Because of this, some light fixtures are fitted with various forms of diffusers that provide anti-glare functions.
Glare is caused by a significant ratio of luminance between the task (that which is being looked at) and the glare source. Factors such as the angle between the task and the glare source and eye adaptation have significant impacts on the experience of glare.
Glare can be generally divided into two types, discomfort glare and disability glare.
Discomfort glare results in an instinctive desire to look away from a bright light source or difficulty in seeing a task. Disability glare impairs the vision of objects without necessarily causing discomfort. This could arise for instance when driving westward at sunset.
UGR (Unified Glare Rating)
The UGR (Unified Glare Rating) is a measure of the glare in a given environment, proposed by Sorensen in 1987 and adopted by the International Commission on Illumination (CIE). It is basically the logarithm of the glare of all visible lamps, divided by the background illumination.
The UGR scale ranges from 5 to 40 with the lower the rating the less the glare.
A UGR rating of 5-10 means the glare is so unobtrusive it will be unnoticeable by the human eye under normal situations.