Unified Glare Rating (UGR) basic explanation.

Although the formula looks very complex, scarily complex basically it is actually quite simple, taken a step at a time. First a diagram:

Very roughly it is the luminance from the lamps divided by the background luminance from the room's walls and ceiling. 

(For an explanation of what luminance means, this book...

Buy Candelas Lumens and Lux as a paperback

...will help you)

Have a look at this detailed explanation of the formula for UGR:

Log is log10 by the way.

Lb, the background luminance or cd/m2rd, L is the luminance of the luminaire. 

Looking at L and Lb, glare increases with stronger lamps and lower background lighting, whereas it decreases with weaker lamps and more background illumination. 

p is the Guth index, which increases with distance from direct line of sight. So as the lamp moves further from the line of site, p increases and so the glare, as measured by the UGR, decreases.

Some useful to know limits:

  • UGR < 10 : Glare is so insignifigant it can be ignored.
  • UGR > 30 : Lots and lots of glare!
When you have that big table of UGR values “S” is the interdistance spacing of the luminaire centers in a grid in the room. You can see it has been set to 0.25 in the example below.

Note that there are many UGR ratings for the same luminaire but in different rooms. The table above gives UGRs for rooms with different wall, ceiling and worksurface reflectancies, as well as different room sizes (described as dimensions in MH (mounting height) units.)

The luminaire whose UGR is shown above is quite a low glare device, most of the room dimensions sizes give a value of around 10, which is considered a good value.

Notice also that since the luminaire is rotosymmetrical whether it is viewed endwise or crosswise does not change the UGR much. It's a different story with an assymetrical luminaire:


  1. Thanks for the write-up Owen. Esp disseminating the equation. Couple of questions:
    How can SUM take into account all luminaires in a room when L is one just one luminaire's luminance?
    The interdistance for europe is 0.25, but for UK (EN 12464) it is 1.00 I believe? What are the implications of this varying interdistance? Is the 0.25 interdistance where the 0.25 in the UGR calculation comes from?

    1. The Guth index is supposed tpo do that. It will be different for every luminaire in a differnt position. Note that the Guth index is inside the sum

  2. Thank you for the great overview!
    What software are you using for the above UGR calculation examples? We I am looking for an easy way to evaluate UGR on several luminaires and have been using AGi32 which is not the greatest for multiple configurations. The above output you pictured, provides a much better overview of URG values when trying to establish thresholds for various luminaires within different room configurations. Recommendations on what software to use for this type of analysis is much appreciated!

    1. I use PhotoView from OxyTech. There is a free version for you to try.

  3. thanks mate. this is the only place explain UGR. long life.


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