What UGR is
UGR stands for Unified Glare Rating: the internationally standardised method — CIE 117:1995 — for describing discomfort glare, meaning glare that is uncomfortable to work under without measurably reducing what a person can see. Disability glare is a different quantity, and the UGR method was not developed for it.
In EN 12464-1 the glare rating is a limit, not a score. Every activity row carries a glare limit value for the installation, RUGL, next to maintained illuminance (Ēm), uniformity (Uo) and colour rendering (Ra) — those three are covered at momik.ai/en-12464-1. The 2021 edition renamed UGR to RUG and RUGL without touching the formula.
Before any arithmetic: UGR is not a property of a luminaire. ZVEI puts it flatly — glare is a property of a lighting installation, and a datasheet value describes the luminaire only inside its own standard room. A UGR figure belongs to an observer, at one position, looking one way, in one room. ERCO says the same: it is “not a quality characteristic of a luminaire”.
Definition and the CIE 117:1995 reference: ZVEI / licht.de, Position paper UGR method – application and limits, October 2021, https://www.licht.de/fileadmin/Publications/ZVEI_Publications/2110_E_ZVEI_UGR_method.pdf — the CIE publication itself is sold, not published free: https://cie.co.at/publications/discomfort-glare-interior-lighting — the four task quantities and the RUG rename: IBE-BIV, Guide pratique de l’éclairage des lieux de travail intérieurs, November 2023, page 10 and footnote 8, https://ibe-biv.be/wp-content/uploads/2023/11/IBE-BIV-NBN-EN-12464-1-2021-Guide-pratique-de-leclairage-des-lieux-de-travail-interieurs-adjust.pdf — ERCO: https://www.erco.com/en_us/designing-with-light/lighting-knowledge/lighting-design/ugr-method-7488/
The UGR scale
UGR is a logarithmic scale. ERCO gives the range as 10, the lowest glare probability, to 30, a very high one, in intervals of 3 units — the smallest noticeable difference. EN 12464-1 uses part of it: its limit values form a short series, 16, 19, 22, 25, 28.
The class is the unit, not the decimal. A design lands in ≤ 19 when the calculated value falls between 16.5 and 19.4, and every class rounds the same way: ≤ 16 needs 16.4 or lower, ≤ 28 stretches to 28.4.
| Glare limit | What it means, and where it applies |
|---|---|
| ≤ 16 | Strictest in the series: offices with precise work or critical screen tasks, technical drawing. |
| ≤ 19 | The everyday office class: offices, meeting rooms, classrooms; reading, writing, computer and control work. |
| ≤ 22 | More glare accepted: reception desks, assembly work, general work in industry and craft. |
| ≤ 25 | Rough work: stairs, storage rooms, machine halls, industrial activities. |
| ≤ 28 | Loosest in the series, for spaces people pass through: corridors and circulation areas. |
Decimals are not a ranking. ZVEI is explicit that a comparison based on values including one decimal place “is not intended and alone does not allow a statement ‘better’ or ‘worse’”: 19.8 and 21.2 share a class, 19.1 and 19.6 do not.
Range and logarithmic spacing: https://www.erco.com/en_us/designing-with-light/lighting-knowledge/lighting-design/ugr-method-7488/ — limit series, class boundaries and the warning on decimals: ZVEI / licht.de, pages 5-6, https://www.licht.de/fileadmin/Publications/ZVEI_Publications/2110_E_ZVEI_UGR_method.pdf — the applications per class combine two public readings of the same series: IBE-BIV guide, table 2, page 27, and ZVEI / licht.de, table 1, page 6. Check the published standard for the row that matches your activity before you specify.
How UGR is calculated
The formula in words: the glare rating is eight times the base-ten logarithm of one quantity. That quantity is a constant of 0.25 divided by the background luminance, multiplied by a sum over every luminaire in the observer’s field of view. Each luminaire adds its luminance squared, times the solid angle it occupies at the eye, divided by the square of its position index. The four terms, one by one:
- Background luminance (LB) — what the eye is adapted to, from the vertical indirect illuminance at eye level divided by π. In the denominator, so a brighter surround lowers the rating.
- Luminance of each luminaire (Li) — how bright its light-emitting surface looks from the eye, in cd/m², not how much light it puts on the desk. Squared, so a small very bright surface outweighs a large dim one.
- Solid angle (Ωi) — how much of the field of view that luminous surface occupies, seen from the eye.
- Position index (pi) — the Guth index, scaling each contribution by its displacement from the line of sight. Squared as well, so what sits near the centre of view dominates.
The sum runs over the luminaires in the field of view, so the result belongs to an observer and a viewing direction, not to a product.
Formula and the meaning of every term: ZVEI / licht.de, page 5, https://www.licht.de/fileadmin/Publications/ZVEI_Publications/2110_E_ZVEI_UGR_method.pdf — the same quantities confirmed independently in calculation-software documentation: https://docs.agi32.com/AGi32/Content/adding_calculation_points/Calculations_UGR_Concepts.htm
Table method versus calculated UGR
Two routes lead to a UGR value, and EN 12464-1 recognises one of them as proof. The standard asks for the tabular method. Applying the formula is permissible where the table does not fit, but ZVEI records the price: “the UGR value determined in this way is not proof that the limit values are complied with”.
Where a datasheet UGR comes from
The single number on a datasheet is a standard-room result: the value the luminaire reaches in a room of 4H by 8H with reflectances of 20 % floor, 50 % walls and 70 % ceiling. H is the distance from the observer’s eye to the mounting plane, so a 2.7 m mounting height with a seated observer at 1.2 m gives H = 1.5 m. ZVEI adds the line that matters at specification time: in real applications, values can therefore be lower or higher.
What the full table covers
The method extends to 19 room dimensions, each with five reflectance combinations, for two luminaire orientations, and is limited to rooms between 2H and 12H. Two observers sit in the middle of the walls, looking horizontally, crosswise and endwise. No diagonal viewing direction exists in it, and the ± variation with observer position is not taken into account when the value meets the limit.
When a real calculation is needed
A datasheet UGR is a guarantee for its standard room and nothing else. Calculate per observer when your room is not that room — other reflectances, a geometry outside 2H to 12H, a plan that is not rectangular — and when the people who matter do not sit mid-wall. One warning travels with those results: a UGR maximum calculated point by point by software is not comparable with the UGR limits.
Tabular method as the required route, the 4H/8H standard room, the 19 × 5 × 2 table, the 2H-12H range and the warning on a calculated maximum: ZVEI / licht.de, pages 6-10, https://www.licht.de/fileadmin/Publications/ZVEI_Publications/2110_E_ZVEI_UGR_method.pdf — observer positions, viewing directions, the definition of H and the ± variation: IBE-BIV guide, pages 28-29, https://ibe-biv.be/wp-content/uploads/2023/11/IBE-BIV-NBN-EN-12464-1-2021-Guide-pratique-de-leclairage-des-lieux-de-travail-interieurs-adjust.pdf
Where the UGR method fails
UGR is not a bad number; it is a 1995 method with a documented validity range. ZVEI and licht.de gave the limits of that range their own chapter: twelve numbered sections on where the method stops working.
- Uneven luminance is invisible to it: the formula uses only the mean luminance of the light-emitting surface, which is why lens-based and prism-based luminaires with the same value are judged differently — CIE 232:2019 added a correction factor for it.
- Sources can be too large or too small: above 0.1 sr the source changes the observer’s adaptation state, below 0.0003 sr the squaring of the luminance no longer holds.
- Some luminaire types fall outside it entirely: wallwashers, fully indirect luminaires, luminous ceilings, adjustable spots — and LiTG advises against UGR above a 65 % indirect component.
- The real layout is not what is calculated: the method averages over viewing directions and uses a virtual luminaire arrangement, so positions exist where individual luminaires glare more than the table value suggests — and for narrow luminaire groupings it cannot be used at all.
- Rectangular rooms only: an L-shaped room can be split into parts, polygonal and round geometries cannot be evaluated.
- Reflectances are fixed — floor up to 20 %, walls 30 or 50 %, ceiling 30, 50 or 70 % — so a luminaire reading ≤ 19 on office reflectances looks equally safe in an industrial hall, where the real value is higher.
- The observer is static and seated at 1.2 m: standing and walking people — CIE 117 names 1.7 m — viewing directions above the horizon and overhead work are not considered.
- Two glare problems are out of scope: disability glare and reflected glare on screens; AGi32 adds that UGR covers electric lighting only, not daylight.
Every point above comes from ZVEI / licht.de, Position paper UGR method – application and limits, chapter 3 (the limits of the method) and chapter 4, https://www.licht.de/fileadmin/Publications/ZVEI_Publications/2110_E_ZVEI_UGR_method.pdf — the solid-angle limits, the daylight exclusion and the note on indirect lighting are confirmed independently at https://docs.agi32.com/AGi32/Content/adding_calculation_points/Calculations_UGR_Concepts.htm
UGR calculation in DIALux evo
DIALux evo is where most lighting designers in Europe meet UGR. DIAL’s knowledge base documents two routes, and they answer different questions.
- A UGR table is generated automatically for luminaires with direct light components, in the product data sheet of the documentation — the standard-room table described above, not a result for your room.
- For a project value you place UGR calculation points or surfaces yourself, in the Calculation objects tool, and choose the number of viewing directions through the step size and the viewing angle range.
The assumptions behind the table are fixed: the observer is always in the middle of the wall at a height of 1.2 m, and H is always 2 m for UGR tables.
The limitation most people trip over is quieter than an error message. DIAL states it plainly: “Luminaires with a position index outside the validity range of the table are not considered in the UGR calculation. This is the case, for example, in H/R > 2.” Those luminaires are not flagged and not approximated — they are left out of the sum, and the value that comes back was computed from fewer luminaires than the room contains.
Practically: treat the automatic table as the standard-room figure it is, place calculation objects where people actually sit and look, and ask whether every luminaire that matters was inside the validity range.
One caveat belongs to that second route. DIAL notes that calculation points and surfaces are determined with the UGR formula, which EN 12464-1 permits, but adds that too few studies are available to confirm the applicability of the limit values for that procedure. It is the same point ZVEI makes: a formula result answers your room, not the compliance question.
Automatic table, calculation objects, and the caveat on the limit values for calculation points and surfaces: https://evo.support-en.dial.de/support/solutions/articles/9000116115-ugr — observer position, H = 2 m and the position-index validity range: https://evo.support-en.dial.de/support/solutions/articles/9000073310-ugr-method-unified-glare-rating — both checked 17 September 2026.
How Momik reports UGR
Momik reports UGR per zone. Maintained illuminance, uniformity (Uo) and UGR are each checked against the standard values for the room type of that zone, and the report lists the result zone by zone, so the compliance is readable instead of assumed.
UGR is also one of the filters on the catalog — lux, UGR, IP, CCT, CRI, mounting — and one click produces illuminance, uniformity, UGR and isolux per zone. Every luminaire is calculated from its real IES or LDT photometric file.
Everything above applies to any tool that reports a glare rating, Momik included: a UGR figure is worth what the assumptions under it are worth.