What is the UGR or Unified Glare Ratio?

The Unified Glare Rating (UGR) is a way of assessing and comparing the glare produced by different light sources.

This index is essential for designing appropriate lighting for the particular tasks to be carried out in the premises. It is easy to understand that the lighting and visual comfort needs of a hallway are not the same as those of a classroom or an operating room.

Before defining UGR in detail, it is necessary to review what we understand by glare, the causes of glare and to define its effects.

Definition of glare

Glare is the “disturbance of the sight by excessive or sudden light”. In short, it is nothing more than a decrease in our ability to distinguish objects caused by a sudden increase in light.

glare caused by a car

Types of glare

We can distinguish between two types of glare according to the effect they have on people:

  • In the first place we find the glare that causes discomfort. This causes some difficulty in continuing with the activities we were doing and produces the need to look away.
  • The second type, although it may not produce that uncomfortable feeling, disables us from continuing with the task we were carrying out.

UGR as a measure of glare

The unified glare index is the quantitative way of measuring glare in a space recommended by the International Commission on Illumination or CIE (from the French Commission internationale de l’éclairage).

Glare is measured with a luminance meter and the UGR is simply the logarithm (in base 10) of the luminance of all light sources relative to the background luminance. Mathematically it would look like this:

Calculation of the glare index


  • Lb is the background lighting.
  • Ln is the luminance of the visible light source ‘n’.
  • wn is the solid angle of the light source as seen from the observer.
  • pn is the Guth position index of each source.
Guth position index
Guth position index

The UGR takes values ranging from 10 to 30. The higher the UGR values, the greater the discomfort generated by the light. It should be noted that vision-impairing glare is not considered on this scale.

Maximum UGR values according to activity

Normally, 16, 19, 22, 25, 28 are taken as reference values, because a variation of 1 in the unified glare index value is not noticeable, whereas a variation of three points is noticeable.

16Technical drawing, watchmaking
19Typing, CAD, meeting rooms
22Receptions, shops
25Passage areas, archives

Basically, lower levels of glare are required in spaces where activities of higher precision are carried out.

The European standard UNE 12464-1 lists all the minimum parameters, depending on the indoor lighting requirements according to the activity, to guarantee visual performance, comfort and safety. These parameters include, in addition to UGR, illuminance, colour rendering index and all types of exceptions or special applications, whenever necessary.