Definition
UGR (Unified Glare Rating) is the CIE standard metric for quantifying discomfort glare from artificial lighting in indoor spaces. Defined in CIE 117-1995 and adopted by EN 12464-1, UGR is calculated from a formula incorporating: luminaire luminance (brightness of the visible light-emitting surface), background luminance (brightness of the surrounding room), the solid angle subtended by the luminaire at the observer's eye, and the Guth position index (accounting for where the luminaire appears in the observer's field of view). UGR values range from 5 (imperceptible glare) to 40 (extreme discomfort). The standard observer position is seated (1.2m eye height) looking horizontally. UGR is calculated per luminaire, not averaged across a room — the worst luminaire in the field of view determines the rating.
Key Data
| Parameter | Value / Explanation |
|---|---|
| UGR ≤10 | Imperceptible glare — museum, gallery, high-end applications |
| UGR ≤13 | Just perceptible — premium office, healthcare, luxury retail |
| UGR ≤16 | Perceptible but acceptable — CAD workstations, design studios, fine assembly |
| UGR ≤19 | Standard acceptable — general offices, classrooms, retail (EN 12464-1 baseline) |
| UGR ≤22 | Moderate — industrial, warehouse, circulation spaces |
| UGR ≤25 | Just tolerable — corridors, storage, parking — not for occupied task spaces |
Application Guide
Open-plan office
UGR ≤19, 4000K, 500 lux, CRI ≥80
EN 12464-1 standard for general office tasks; ≤16 for screen-based CAD/design work
Classroom
UGR ≤19, 4000K, 500 lux, high-uniformity optics
Students spend 6-8 hours/day under these lights — glare causes eye strain and reduced concentration
Hospital ward
UGR ≤16 (patient view), ≤19 (staff), indirect/direct mix
Bedridden patients look at the ceiling — overhead glare is extremely uncomfortable
Conclusion & Procurement Recommendation
For B2B lighting procurement, UGR is not a luminaire property — it's a room-specific calculation that depends on room dimensions, surface reflectances, mounting height, and luminaire spacing. Key procurement requirements: (1) Request the luminaire's UGR table (showing UGR values at standard room sizes and reflectances per CIE 117), not a single 'UGR <19' claim, (2) Specify the exact room dimensions and surface reflectance assumptions in the lighting design brief, (3) Require photometric calculations (DIALux/AGi32/Relux) showing UGR compliance for the actual room layout, (4) For offices with computer screens: specify low-luminance optics (luminance <1,000 cd/m² at angles >65° from vertical) — this prevents veiling reflections on screens, which UGR alone doesn't address.