Glare: UGR and how to reduce it
Led Lighting · Lesson 22 · Intermediate
Purpose
Assess discomfort glare as an installation property and reduce it without assuming a luminaire UGR table is a universal rating.
Learning objectives
- Define the design boundary
- Use current product/project evidence
- Calculate with stated assumptions
- Verify maintained and failure performance
Unified Glare Rating estimates discomfort glare for a defined observer, room, background and luminaire layout. Catalog UGR tables support standard arrangements; the actual installation calculation can differ.
Core theory
Glare increases with high luminance in the field of view, larger apparent source solid angle and unfavorable position; background adaptation also matters.
Reduce through shielding/cut-off, larger lower-luminance emitting area, indirect light, layout/aiming, mounting height, surface brightness balance and task/screen orientation.
Check reflected/veiling glare and disability glare where relevant. Dimming may reduce flux but can expose flicker or leave high contrast; verify the actual scene.
| Term | Meaning | Symbol | Unit |
|---|---|---|---|
| UGR | Unified Glare Rating for indoor discomfort glare | Not applicable | Not applicable |
| Luminance | Directional brightness measure | Not applicable | Not applicable |
| Cut-off | Optical shielding limiting high-angle source visibility | Not applicable | Not applicable |
Assumptions: Supplied values are fictional design inputs; Exact current standards and photometric/product data remain governing.
- Position: Its glare position index worsens.
- Contrast: Visible source luminance/solid angle can dominate discomfort.
- Mitigate: Relayout, shield, enlarge/lower luminance or rebalance surfaces; recalculate scene.
Equal lumens do not mean equal UGR; the new observer/layout condition must be assessed.
Reasonableness check: The answer distinguishes average screening from point, uniformity, glare, emergency and maintained-performance requirements.
- Treating standby lighting as identical to escape lighting
- Using initial lumens without utilization or maintenance losses
- Selecting color quality or glare from one headline number
Where this appears in practice
Complete lighting design coordinates visual task, occupants, fire strategy, photometry, emergency modes, controls, energy, maintenance and verification.
Knowledge check
Is ‘UGR<19’ an intrinsic universal luminaire property?
No. UGR depends on the complete installation and observer geometry.
Answer: No. UGR depends on the complete installation and observer geometry.
Practical exercise
Review a screen-based office layout for direct and reflected glare controls.
Summary
- Emergency category follows risk and activity
- Average lux is only one design metric
- Controls must fail to a documented safe/useful state
Sources and review
- BS 5266-1:2025: Emergency lighting code of practice: BSI; 2025; United Kingdom
- SLL Code for Lighting: CIBSE / Society of Light and Lighting; Current edition and application guidance must be confirmed; United Kingdom
- CIE position statement on colour quality metrics: CIE; Second edition, 2025; International
- DALI-2 and DALI Emergency certification: DALI Alliance; Current product database/status must be checked; International
Editorial review date: 2026-08-22. Professional electrical review is pending.