Assessing glare - The Unified Glare Rating

In recent years, the Unified Glare Rating (UGR) as recommended by the CIE has become widely accepted as a general formula for assessing glare. The formula is given below:

With:

Lu	. . .	background luminance
	. . .	solid angle
p²	. . .	position index

The background luminance can simply be derived from the illuminance at the eye of the observer:

With:

Lb	. . .	background luminance
Eobs	. . .	illuminance at eye of observer
	. . .	steradian

This formula is requires the prior knowledge of the position and brightness of each potential glare source. It is quite accurate but relatively difficult to work with. It is best used from within some computer software. For artificial light sources, such packages exist from most major producers of light fittings. They all require the modelling of the scene under investigation and produce a glare index for a defined postition within the room.

Recommended limiting glare indeces