IES Glare Index

The IES glare index system is based on a glare constant which can be derived for each light fitting using the BRS glare formula.

BRS glare constant
source luminance in cd/m²
average background luminance in cd/m² against which the source is seen
angular size of the source in steradians as seen by the eye
position index which indicates the effect of teh position of the source on its capacity to produce discomfort glare

Once all glare constants for the individual fittings have been derived, the IES glare index can be calculated.

GI = 10 log10 (0.5 Σ g)
IES glare index
Σ g
sum of the glare constants produced by the individual fittings

Values for the glare index may range from below 10 for low levels of discomfort glare to 28 for very high levels. A change of 3 units represents a significant change in the level of discomfort.