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How can the UGR of the selected luminaire be determined?

The JISO Iluminación, S.L. Technical Department often receives queries such as this. The company therefore sheds light in its technical news of the month on the questions that are raised about the concept of the UGR, which may be due to lack of knowledge, misinformation, confusing information or a mixture of all of the above.

First of all, the concept of UGR is defined as:

UGR (Unified Glare Rating): It is the unified glare index obtained by the calculation method defined in CIE Publication 117 (Commission Internationel de l’Eclairage). In other words, it is the parameter that helps to measure how much a luminaire (or group of luminaires) that is installed in an installation dazes people.

CIE 117:1995 establishes the calculation method as well as a way to tabulate the values obtained, based on different scenarios, and obtain a parametrised UGR table.

Once the UGR has been defined, without going into technical data or complex formulas, the next step is to answer the initial question, for which the following points need to be clarified:

  1. No luminaire has a specific UGR value.
  2. The UGR value is calculated based on:
    • The luminaire.
    • The dimensions of the premises.
    • The technical specifications of the premises.
    • The position of the luminaire within a room (x, y, z).
    • The position of the observer(s).
    • The height of the observer’s vision.
    • The direction of the observer’s vision.
    • The number of luminaires in the premises.
    • The proximity between luminaires.
    • Etc.
  1. The UGR tables are only used to compare between luminaires.

In other words, if the question is about how to determine what the UGR of the selected luminaire is, the answer cannot be other than: IT IS NOT POSSIBLE to determine it in advance, since the UGR value is not an intrinsic parameter of the luminaire, rather it is a value calculated based on many different factors. The UGR must therefore really be obtained using Lighting Design tools (Dialux, Relux, etc.), for specific surfaces or points and in no case should it be considered a fixed value of the luminaires.

Based on all of the above, lighting manufacturers, to respond to customers needs and the queries that are generated daily, sometimes make use of the UGR table, which is obtained from the calculations indicated in the CIE 117:1995, to provide the customer with the option of being able to compare between luminaires with similar specifications or, also, to provide a value that is representative (not exact) of those that appear in the table. For example, it may be that a luminaire is UGR < 19 when there are values below 19 in the indicated table. For example, it may be that a luminaire is UGR<< 19 when there are values below 19 in the indicated table. Also, in some cases, due to the construction specifications of the luminaires, even without having values below 19, it could be said that a luminaire is “compatible with installations in which a UGR < < 19 is required,” since it has been calculated from a situation from which this data has been obtained, although taking into account that it is about specific products in specific facilities and is never generalised.

In summary, the UGR value is not a parameter of the luminaire, but rather a parameter calculated in the room in which the luminaires are installed and, therefore, depends on different factors, including the type of luminaire, although this is not the only factor to be considered.