DGI

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

with

L_s: source luminance in cd/m²
L_b: average background luminance in cd/m²
ω: angular size of the source in steradians as seen by the eye
Ω: solid angle subtended by the source,modified for the effect of the position of the observer in relation to the source in steradians

Glare regions and their related glare index
Zone	Region	DGI	UGR
Discomfort zone	intolerable	>28	>28
	just intolerable	28	28
	uncomfortable	26	25
	just uncomfortable	24	22
Comfort zone	acceptable	22	19
	just acceptable	20	16
	noticeable	18	13
	just perceptible	16	10

The DGI is related to the IES glare index scale as follows:

DGI = 2/3 (IES glare index + 14)