Unified Glare Rating (UGR) Explained

Glare is in the eye of the beholder! UGR measures its impact on visual comfort while helping to achieve WELL points.

What is UGR?

The Unified Glare Rating (UGR) is a metric used to predict discomfort glare in interior applications and considers the direct light component. It has gained renewed interest of late to achieve points toward WELL certification. Well v2 of the WELL Building Standard, under the L04 Electric Light Glare Control category, allots points for achieving a UGR of 16 or lower as a luminaire consideration, or as a space consideration, for regularly occupied spaces. As a glare evaluation method, UGR has been defined in CIE documents: CIE 117-1995, CIE 190-2010, CIE 232-2019.

UGR is not meant to be an attribute of the luminaire alone – but should be based on an application.
It indicates the contrast, or ratio, between luminaire luminance (perceived as brightness) to background luminance from a reference vantage point, i.e., a patient sitting up in bed, looking straight ahead, with a multi-function luminaire above their head and recessed general ambient luminaires in the adjacent guest area.

Line of sight is important – the relationship of viewers to luminaires will result in multiple UGR values, as shown in the application example at right, where:

Type A-1 is the BalancedCare Flexible Ambient 1×1 (BCFA) and
Type C is the BalancedCare Multi-Function 2×4 (BCMF24).

Application Example

Figure 1 (left): calculation results in UGR of 11.95 from patient viewpoint (Ambient mode shown).
Figure 2 (right): calculation results in UGR of 11. 77 from guest viewpoint (Ambient mode shown).

How is it calculated?

Software programs such as AGi32 and Dialux can calculate
UGR values – average, max and min – based on this formula
UGR = 8 log [0.25/Lb * ∑ Ls2*ω/ρ2] that factors in:

  • background luminance (cd/m2)
  • luminance of the apertures of each luminaire in the direction of the observer’s eye (cd/m2)
  • solid angle of the luminous parts of each luminaire at the observer’s eye (sr)
  • displacement of luminaire from line of sight

What do the numbers mean?

Values are given in the range of 10-30, lower is better; 16 is considered perceptible, 19 just acceptable; above 19 it becomes more uncomfortable. The table above compares UGR values with corresponding 7-step discomfort glare criteria (developed by R.G. Hopkinson).

While these numbers are an indication, it is important to remember that interpretations of glare are subjective, as each person’s perception differs.

Factors contributing to high UGR, in general:

  • Lumen package – increased lumens could mean higher UGR
  • Larger room sizes – more sources in field of view
  • Lower ceiling heights, which indicate lower mounting heights
  • Luminaire distributions with high angle brightness
  • Lower surface reflectances
  • Luminaire aperture size
  • Spacing – farther apart could mean higher UGR
  • Relationship of luminaire (max candela angle) to observer’s line of sight

UGR considers reflectances of surrounding room surfaces, and luminaires within a person’s line of sight that may cause them to sense glare

Single luminaire values are possible, but they do not tell the whole story

Although it is misleading to assign a UGR value to a single luminaire, for those seeking that number Photometric Toolbox calculates for uniform arrays of luminaires, based on a single IES file. It sorts results into a table of 190 individual calculations based on an assortment of 19 common room shapes and five combinations of surface reflectances for two observer positions.