Definition
Daylighting is the controlled admission of natural sunlight into buildings to reduce or eliminate the need for electric lighting during daytime hours. It integrates architectural design elements (windows, skylights, clerestories, light shelves, tubular daylight devices) with automated lighting controls (photosensors that dim electric lights when sufficient daylight is available). Effective daylighting is not simply 'more windows' — it requires careful design to maximize useful daylight penetration while minimizing glare, solar heat gain (cooling load penalty), and excessive brightness contrast. A well-designed daylighting system provides 40-60% lighting energy savings in perimeter zones (within 4-6m of windows), improves occupant well-being and productivity (access to daylight and views is the #1 desired workplace amenity), and contributes to LEED, BREEAM, and WELL building certifications.
Key Data
| Parameter | Value / Explanation |
|---|---|
| Daylight Factor (DF) | Ratio of indoor to outdoor illuminance under overcast sky. DF ≥2% = 'daylit' appearance. No longer the primary metric. |
| Daylight Autonomy (DA) | % of occupied hours that daylight alone meets target illuminance. DA ≥50% = good daylighting. Modern metric. |
| Useful Daylight Illuminance (UDI) | 100-2,000 lux range. <100 = too dark, >2,000 = potential glare/overheating. UDI 50-80% target. |
| Glare control (DGP) | Daylight Glare Probability. DGP <0.35 = imperceptible, >0.45 = disturbing. Critical for workstation placement. |
| Energy saving potential | 40-60% lighting energy reduction in perimeter zones when paired with automatic dimming controls. |
| WELL v2 Feature L05 | Requires daylight simulation (sDA ≥55% or 300 lux for ≥50% of floor area) or actual measurement verification. |
Application Guide
Open-plan office (perimeter)
Light shelf at 2.1m above floor + photosensor-controlled dimming + automated shades
Light shelf bounces daylight deeper (up to 7-9m); automated shades prevent glare; dimming captures savings
Warehouse
Rooftop skylights (3-5% of roof area) + prismatic diffusers + 0-10V dimming per bay
Daylight from above is diffuse and uniform; skylights eliminate daytime lighting in large open areas
School classroom
Bilateral daylighting (windows on 2 walls) + tubular daylight devices for interior zones
Studies show 20-26% faster learning rates in daylit classrooms; bilateral lighting eliminates harsh shadows
Conclusion & Procurement Recommendation
For B2B procurement, daylighting-responsive controls are mandatory in most jurisdictions (ASHRAE 90.1, Title 24) for spaces within daylight zones. Key specifications: (1) Define daylight zones per code: primary zone = depth of window head height, secondary zone = additional depth of window head height, (2) Specify closed-loop photosensors (measure both daylight and electric light) for interior zones, open-loop (daylight only) for skylight zones, (3) Require calibration: photosensors must be commissioned to maintain target illuminance without visible oscillations (hunting) as cloud cover changes, (4) For LEED/WELL projects: the daylight simulation must be performed by a qualified lighting designer using Radiance or similar validated software — the results determine the credit achievement.