Glare is the difficulty of seeing in the presence of a bright light such as direct or reflected light. In the lighting industry, the UGR (Unified Glare Rating) is a method of calculating glare from luminaires and bright light sources.


The UGR depends on:


  • Technical specifications and construction of the luminaire.

  • Location of the installed luminaire (height and position towards the observer).

  • Reflective characteristics of the materials of its surrounding environment (walls, floor, ceiling…).


The unified glare rating (UGR) is a measure of the glare in a given environment, proposed by Sorensen in 1987 and adopted by the International Commission on Illumination (CIE). It is basically the logarithm of the glare of all visible lamps, divided by the background illumination.



  • Lb 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 The Guth Index. Based on the likelihood of glare, also known as Visual Comfort Probability




The European norm EN 12464-1, in which all lighting projects for interiors need to be based,  defines the UGR index for discomfort glare. This standard specify the lighting requirements based on different activies and environments  and  their maximum UGR allowed. In below table we can find the maximum values of UGR and glare criterion of these values for a better understanding of the results obtained.



The UGR is a number that quantify the glare existing in a lighting set up. In order to obtain an specific UGR parameter as reference for a specific luminaire, this is calculated in two standard situations. One is defined as a small office of width 2xH and height 3xH, and the other a translucent office of width 8xH and length 12xH. The reflective surfaces are considered for both cases as 0.7 for the ceiling, 0.5 for the walls and 0.2 for the floor.  For each situation it is calculated the UGR in all the visual directions. Below you can see an example of the UGR results obtained for this test.