Calculations UGR Concepts
The Unified Glare Rating calculations available in AGi32 are calculated in accordance with the CIE document CIE 1171995  Discomfort Glare in Interior Lighting. The shielding effect of the observer's eyebrows and forehead are considered in this calculation. The CIE definition of Discomfort Glare is "glare that causes discomfort without necessarily impairing the vision of objects". Glare which impairs the vision of objects is termed Disability Glare and is not covered in this metric.
UGR values generally range from 10 to 30 where a high value indicates significant discomfort glare, and a low value indicates little discomfort glare. Electric lighting systems producing UGR values of 10 or less are assumed to produce no discomfort. In AGi32, all calculated UGR values of less than 10 are rounded to 10.
UGR Formula
where Lb
is the background luminance (cd/m2); 

UGR Limitations
UGR and Discomfort Glare Criteria
One UGR unit represents the least detectable step in discomfort glare evaluation, and three UGR units represent an acceptability step in glare criteria. Average UGR value range from 10131619222528. The relationship between calculated UGR value and Hopkinson's discomfort glare criteria is as follows:
UGR 
Discomfort Glare Criterion 

10 
Imperceptible 
13 
Just perceptible 
16 
Perceptible 
19 
Just acceptable 
22 
Unacceptable 
25 
Just uncomfortable 
28 
Uncomfortable 
Correlated UGR and VCP Values
Visual Comfort Probability (VCP) is a measure of discomfort glare for interior lighting applications. According to the IES Handbook, 9th Edition, page 926: "The visual comfort probability (VCP) is the probability that a normal observer does not experience discomfort when viewing a lighting system under defined conditions. ... This system was tested and validated using lensed direct fluorescent systems only. VCP should not be applied to very small sources such as incandescent and highintensity discharge luminaires, to very large sources such as ceiling and indirect systems, or to nonuniform sources such as parabolic reflectors." With these caveats in mind, UGR can be correlated with VCP for lighting systems that consist of luminaires that fall within the scope of the definition of VCP. The following table shows this correlation.
UGR 
VCP Equivalent 

11.6 
90% 
16 
80% 
19 
70% 
21.6 
60% 
24 
50% 