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Appendix F - Unified Glare Rating (UGR)

The Unified Glare Rating calculations available in ElumTools are calculated in accordance with the CIE document CIE 117-1995 - 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 ElumTools, all calculated UGR values of less than 10 are rounded to 10.

UGR Formula

where:

L_b is the background luminance (cd/m²)

L is the luminance of the luminous parts of each luminaire in the direction of the observer's eye (cd/m²)

ω is the solid angle of the luminous parts of each luminaire at the observer's eye (sr)

ρ is the Guth position index for each luminaire (displacement from the line of sight)

UGR Limitations

• There is insufficient research to indicate whether the UGR calculations can be applied for indirect lighting or luminous ceilings.
• Sources used within the UGR system should be limited between 0.0003 steradians and 0.1 steradian. This corresponds to luminaires ranging in size from an incandescent downlight seen at 10 meters away to a 1 meter square luminaire seen from 3 meters away.**
• UGR is not valid (cannot be calculated) for point sources (luminous dimensions = 0 ,0, 0).
• UGR is only calculated for electric lighting sources, not daylighting.
• ElumTools will calculate UGR for any electric lighting application. It is up to the user to understand the UGR limitations and applicability of UGR.

** NOTE: Very small sources may have very small projected areas leading to artificially low UGR values (reported as 10). It is believed that glare from these sources is generally due to Intensity rather than luminance.

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 values range from 10-30. The relationship between calculated UGR value and Hopkinson's discomfort glare criteria is as follows:

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