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There seems to be some confusion in the industry regarding Glare and in particular the production and application of UGR tables. BS EN 12464 pt 1 Light and lighting (Lighting of work places Part 1: Indoor work places) defines the lighting specifications for all interior commercial spaces. Included in all of the specifications is an RUGL value which defines the limits for RUG which is the new nomenclature for CIE Unified Glare Rating (UGR). As these limits appear in almost every specification and manufacturers are trying to design luminaires to meet these criteria and Lighting designers are signing off schemes as compliant, we should try to understand some more about the subject. This question arises regularly from manufacturers and designers, here is my take on the matter.
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Spacing based on 0.25 or 1.0
The current 2021 version and previous versions of BS EN 12464 pt1 are consistent in stating that the only method of determining RUG (previously called UGR) values for an installation is by reference to the standard RUG /UGR table. It is clear from all versions that the limiting values are only applicable to the tabular method and that limits cannot be applied to calculations using the UGR formula.
Previous versions of BS EN 12464 pt1 have defined the method of producing the UGR table as that defined in CIE 117-1995. CIE 117 used a spacing factor of 0.25 for the production by calculation of the standard UGR table. Producing UGR tables at 0.25 therefor became normal practice. Other standards notably ISO 8995 and CIE 190 have long used 1.0 for the spacing factor to calculate UGR tables. Producing a UGR table at a spacing of 0.25 produces higher values of UGR than a table calculated at a spacing of 1.0.
It has always been my opinion that the use of 0.25 in CIE 117 was unintentional for several reasons, most notably that the limiting values are derived from ISO 8995 which used a spacing factor of 1.0, so in effect previous versions of BS EN 12464 pt1 have used a more onerous calculation method than was applicable in the documents from which the limits were derived. A bit like comparing Glare apples to Glare bananas.
BS EN 12464 pt1 2021 in clause 5.5.3.2 now states in note 1 that the UGR tablular method is detailed in CIE 117 1995 ANDCIE 190 2010. CIE 190 defines the spacing factor as 1.0 which is now inline with the document where the limiting values are defined and is a later (superseding?) document than CIE 117. Clause 5.5.3.2 also states that:
“All assumptions made concerning luminaire, room dimensions, room surface reflectance’s and spacing to height ratio in the determination of the RUGL (formerly: UGR) shall be stated in the scheme documentation.”
It seems obvious that from a luminaire manufacturing point of view that to produce UGR tables at anything other than a spacing Factor of 1.0 would place them at a commercial disadvantage. But manufacturers should always state the factor used in producing the table.
As a specifier it would seem that they have a choice to specify compliance based on 0.25 or 1.0 spacing factor, but specifiers should make it clear on what basis they are accepting claims for RUGL compliance. Just stating in accordance with BS EN 12464 pt 1 2021 does leave this somewhat open to interpretation.
Lighting designers if the specifier has not defined the criteria should ask for clarification and if none is available state in their report the basis on which any RUGL compliance is claimed.
In my opinion the default position now should be to use a spacing factor of 1.0 and that I would only use 0.25 spacing factor tables if specifically requested by the client or specifier and then I would question why they wanted to use the older reference.
Both Relux and Dialux have features in their programs that allow RUG /UGR tables to be produced with either factor. Both include the spacing factor used when the table is printed/viewed from the program.
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Reflectance values and UGR
There is a further twist here, if you examine the standard RUG /UGR tables produced either way you will see that the RUG /UGR values decrease as room reflectances increase, at any particular room dimension RUG /UGR values increase across the table as reflectances decrease. Previous versions of BS EN 12464 pt1 stated that only values interpolated from the table could be used and that for room dimensions and reflectances greater than listed in the table the maximum values from the table were to be used.
BS EN 12464 pt1 2021 has a new Annex A which clarifies the use of the tabular method outside of the normal boundary conditions. Annex 2 clause A.2.4 headed Deviating Room reflectances states that using lower reflectances than actually present in the scheme yields the higher (worst case) RUG /UGR values. But it also states that:
“alternatively a set of transfer factors can be calculated allowing calculation of a standard UGR value for the required reflectances”.
This does have implications for many interior lighting schemes. The standard table only list reflectances up to 70/50/20 Ceiling/Walls/Floor, many designs are done at higher reflectance values which means if RUG /UGR was calculated at these higher reflectances lower RUG /UGR values would result. By implication in any area with reflectance values higher than 70/50/20 by using the standard tabular values we are overestimating artificial light discomfort glare. This is a safe default but it does limit luminaire designers and Lighting designers choices when designing for UGR compliant systems.
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Sensible application of limits
Lastly can I pose a question regarding the way that RUGL value limits are applied. There is no UGR meter, discomfort glare is only defined by design calculation, you cannot go to a site and measure discomfort glare. The specification tables in BS EN 12464 pt 1 2021 give UGR limits (Unified Glare Rating limit, RUGL) that are applicable to the situation. Is this an absolute Limit? i.e., is any value greater than the published RUGL value out of the question?
BS EN 12464 pt 1 has some relevant wording on UGR values Clause 5.5.3.2 Note 3 states:
The limiting values of the RUGL form a series whose steps indicate noticeable changes in glare. This series of RUGL is: 16, 19, 22, 25, 28 where a low value means “little likelihood of discomfort glare” and a high value means “significant possibility of discomfort glare”.
Firstly, these limits indicate the likelihood/possibility of discomfort glare, we can’t measure it we can only calculate it from a table produced under standard conditions.
If I paraphrase the wording above as: ‘users of the space can only discern a noticeable difference in discomfort glare between RUG /UGR values of 16 and 19 or 19 and 22 – or any other 2 adjacent steps in the standard table.’.
Therefore, it would be logical to say that any RUG /UGR value between 17.5 and 20.5 would be unnoticeable in terms of discomfort glare from RUG /UGR value 19. (You can make the same statement for any value in the series). I would therefor posit that a rational specifier may allow some deviation from the hard limit of RUG /UGR values and allow some variation above, may be as far as half way to the next step in the preferred series. In my world UGR 20 is not noticeably different from UGR19, and anyone believes UGR19 good UGR 19.1 bad is not living in the real world.
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Perhaps someone from LIA, SLL, IALD etc might want to comment or perhaps make some statements as to how they view these matters so that the industry can have an agreed way to treat UGR methodology. Having a mixed bag of interpretations makes life difficult for everyone.
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