# Daylighting Fundamentals ## Table of Contents - [[#Overview]] - [[#Benefits of Daylighting]] - [[#Photometric Quantities]] - [[#CIE Standard Sky Types]] - [[#Daylight Factor]] - [[#Components of Daylight Factor]] - [[#Daylight Factor Design Targets]] - [[#Climate-Based Daylight Metrics]] - [[#Daylight Autonomy]] - [[#Useful Daylight Illuminance]] - [[#Spatial Daylight Autonomy and Annual Sunlight Exposure]] - [[#Green Building Rating Credits]] - [[#Simulation Tools]] - [[#Integration with Building Design]] - [[#Key References and Standards]] --- ## Overview Daylighting is the controlled admission of natural light into a building to reduce artificial lighting energy, improve visual comfort, and enhance occupant well-being. As an architectural discipline, daylighting requires careful integration with building form, facade design, and environmental control. It is inseparable from solar gain management (see [[Passive Solar Design]]) and must be coordinated with glare prevention and electric lighting design. This article provides the foundational metrics and methods. Detailed treatment of specific strategies is found in [[Daylight Factor Calculation]], [[Toplighting Strategies]], and [[Sidelighting Strategies]]. --- ## Benefits of Daylighting ### Energy - Artificial lighting accounts for 20-40% of commercial building energy consumption - Effective daylighting with daylight-responsive controls can reduce lighting energy by 40-60% - Reduced internal heat gains from artificial lights also lower cooling loads ### Health and Productivity - Exposure to natural light regulates circadian rhythms (melatonin suppression, alertness) - Studies show 15-20% improvement in productivity in daylit offices - Reduced absenteeism documented in daylit workplaces - Improved patient recovery times in daylit hospital rooms (Ulrich, 1984; subsequent studies) - Student performance improvements of 7-26% in classrooms with effective daylighting (Heschong Mahone Group, 1999) ### Architectural Quality - Dynamic variation of daylight throughout the day and seasons enriches spatial experience - View and connection to the outdoors is consistently rated as the most valued workplace amenity - Daylight reveals form, texture, and colour with a quality that artificial light cannot replicate --- ## Photometric Quantities | Quantity | Symbol | Unit | Definition | |----------|--------|------|-----------| | Luminous flux | Phi | lumen (lm) | Total light output of a source | | Illuminance | E | lux (lm/m2) | Light falling on a surface | | Luminance | L | cd/m2 | Light emitted/reflected from a surface (brightness) | | Luminous intensity | I | candela (cd) | Light output in a specific direction | ### Key Illuminance Levels | Task | Recommended Illuminance (lux) | |------|------------------------------| | Circulation/corridors | 100-150 | | General office | 300-500 | | Detailed drawing/drafting | 500-750 | | Classroom | 300-500 | | Hospital ward | 100-300 | | Retail (general) | 300-500 | | Operating theatre | 10,000-100,000 | Standards: BS EN 12464-1, CIBSE LG series, IESNA Lighting Handbook. --- ## CIE Standard Sky Types The Commission Internationale de l'Eclairage (CIE) defines standard sky luminance distributions used for daylight calculations. ### CIE Overcast Sky (Type 16) - Luminance varies with altitude angle only (not azimuth) - Zenith is three times brighter than the horizon - **Used for Daylight Factor calculations** because it represents worst-case diffuse conditions - Formula: L(theta) = L_z x (1 + 2 sin(theta)) / 3, where theta = altitude angle ### CIE Clear Sky (Type 12) - Luminance varies with both altitude and azimuth - Sun position creates a bright circumsolar region - Used in climate-based simulations where clear sky conditions are significant ### CIE General Sky The 2003 CIE General Sky standard defines **15 standard sky types** ranging from overcast to clear, with intermediate conditions. Modern simulation tools use these in combination with measured climate data for annual performance assessment. --- ## Daylight Factor The Daylight Factor (DF) is the ratio of internal illuminance at a point to the simultaneous unobstructed external horizontal illuminance under a CIE overcast sky: ### Formula **DF = (Ei / Eo) x 100%** Where: - Ei = illuminance at the internal reference point (lux) - Eo = simultaneous unobstructed external horizontal illuminance (lux) ### Characteristics - DF is a **static metric** -- it describes the potential for daylight under overcast conditions - It is independent of orientation (the CIE overcast sky is symmetrical) - It does not account for direct sunlight or clear sky conditions - It does not capture the dynamic nature of daylight over the year - Despite these limitations, DF remains the most widely used metric in building regulations and is a useful quick design check --- ## Components of Daylight Factor The total DF at any point is the sum of three components: ### Sky Component (SC) Direct light from the visible sky patch through the window. Typically the largest component. Depends on: - Window size and position - Visible sky angle from the reference point - Glazing transmittance - External obstructions ### Externally Reflected Component (ERC) Light reflected from external surfaces (neighbouring buildings, ground) into the room. Usually small (0.5-2% of total DF) unless facing a highly reflective surface. ### Internally Reflected Component (IRC) Light reflected from internal room surfaces (walls, ceiling, floor) to the reference point. Depends on: - Room surface reflectances - Room proportions - Multiple inter-reflections **Typical surface reflectances for good daylighting:** | Surface | Recommended Reflectance | |---------|------------------------| | Ceiling | 0.70-0.85 | | Walls (above desk) | 0.50-0.70 | | Floor | 0.20-0.40 | | Desk/work surface | 0.30-0.50 | | Window wall (splays) | 0.60-0.70 | --- ## Daylight Factor Design Targets | Space Type | Minimum Average DF | Minimum Point DF | |-----------|-------------------|-----------------| | Office (general) | 2.0% | 0.8% | | Office (deep plan with supplementary lighting) | 1.5% | 0.5% | | Classroom | 2.0% | 0.8% | | Hospital ward | 1.5% | 0.5% | | Residential living room | 1.5% | 0.5% | | Residential kitchen | 2.0% | 0.6% | | Art gallery | 4.0-5.0% | 2.0% | ### Uniformity Daylight uniformity ratio (minimum DF / average DF) should be at least **0.3**, ideally **0.4** or above, to avoid excessive contrast between window zone and room interior. ### Rule of Thumb: Room Depth Adequate daylight typically penetrates to a depth of approximately: **Maximum daylit depth = 2.0 to 2.5 x window head height above desk level** For a window head at 2.7 m and desk at 0.8 m: depth = 2.5 x (2.7 - 0.8) = 4.75 m Beyond this depth, supplementary electric lighting is required. --- ## Climate-Based Daylight Metrics Climate-based daylight modelling (CBDM) uses hourly climate data (TMY) to simulate annual daylight performance under realistic sky conditions, overcoming the limitations of the static Daylight Factor. --- ## Daylight Autonomy **Daylight Autonomy (DA)** is the percentage of occupied hours during which daylight alone provides the target illuminance at a point. **DA = (Hours above target illuminance / Total occupied hours) x 100%** - Target illuminance is typically 300 or 500 lux for offices - DA > 50% is generally considered acceptable - Does not penalise excessive daylight (potential glare and overheating) ### Continuous Daylight Autonomy (DAcon) A refinement that gives partial credit for hours when daylight provides a proportion of the target. For example, if the target is 500 lux and daylight provides 400 lux, DAcon credits 80% for that hour. --- ## Useful Daylight Illuminance **Useful Daylight Illuminance (UDI)** defines a useful range of illuminance (typically 100-3000 lux) and reports the percentage of occupied hours within that range. | UDI Bin | Illuminance Range | Interpretation | |---------|-------------------|----------------| | UDI-f (fell short) | < 100 lux | Insufficient daylight | | UDI-s (supplementary) | 100-300 lux | Useful but supplementary lighting may be needed | | UDI-a (autonomous) | 300-3000 lux | Adequate daylight, no supplementary lighting needed | | UDI-e (excessive) | > 3000 lux | Risk of glare and overheating | **Design target:** Maximise UDI-a across the occupied area while minimising UDI-e. UDI provides a more nuanced picture than DA because it identifies both insufficient and excessive daylight conditions. --- ## Spatial Daylight Autonomy and Annual Sunlight Exposure ### Spatial Daylight Autonomy (sDA) Defined by IES LM-83-12 as the percentage of analysis area that achieves a specified DA level. **sDA300/50%** = percentage of floor area receiving at least 300 lux for at least 50% of occupied hours | Rating | sDA300/50% | |--------|-----------| | Nominally accepted | >= 55% | | Preferred | >= 75% | ### Annual Sunlight Exposure (ASE) **ASE1000,250** = percentage of floor area receiving more than 1000 lux of direct sunlight for more than 250 occupied hours per year | Rating | ASE1000,250 | |--------|------------| | Acceptable | <= 10% | | Requires investigation | > 10% | ASE serves as a check against excessive direct sun that could cause glare and overheating. --- ## Green Building Rating Credits ### LEED v4.1 (Daylight Credit) - **Option 1:** Simulation: sDA300/50% >= 55% for 2 points, >= 75% for 3 points; ASE1000,250 <= 10% - **Option 2:** Measurement: illuminance between 300-3000 lux for 50% of floor area at two measurement times (9 AM and 3 PM at equinox) ### BREEAM (Hea 01 - Visual Comfort) - Average DF of 2% in 80% of occupied floor area - Uniformity ratio >= 0.3 - View of sky from desk level - Credits for glare control and occupant-controllable blinds ### WELL Building Standard (Feature L03 - Daylight) - sDA300/50% >= 55% for at least 30% of regularly occupied area (precondition) - Higher thresholds for optimisation credits - Annual Sunlight Exposure limits --- ## Simulation Tools | Tool | Type | Strengths | |------|------|-----------| | Radiance | Physics-based raytracer | Gold standard for accuracy; used in research and practice | | DAYSIM | Annual daylight simulation | Climate-based metrics (DA, UDI); built on Radiance | | Honeybee/Ladybug (Grasshopper) | Parametric daylight modelling | Integrates with Rhino; Radiance/EnergyPlus backend | | IES VE FlucsDL | DF and lux calculations | Integrated with IES VE thermal model | | Velux Daylight Visualizer | Residential/small projects | Free, user-friendly, validated | | ClimateStudio | Annual CBDM | Rhino-integrated, fast, supports LEED/WELL/BREEAM | | DIALux | Electric + daylight | Free, widely used in lighting design | ### Validation All simulation tools should be validated against CIE 171:2006 test cases. Radiance-based tools have the most extensive validation history. --- ## Integration with Building Design ### Early Design Decisions 1. **Building orientation** -- long axis east-west for maximum south-facing daylight 2. **Floor plate depth** -- limit to 12-15 m for single-sided daylighting, or use atrium/courtyard for deeper plans 3. **Floor-to-ceiling height** -- higher ceilings allow deeper daylight penetration 4. **Window-to-wall ratio** -- typically 30-50% for good balance of daylight, thermal performance, and glare 5. **Glazing specification** -- visible light transmittance (VLT) > 0.50 for effective daylighting; coordinate with SHGC requirements ### Coordination with Other Systems - **Shading** must be designed in tandem with daylighting; see [[Solar Shading Design]] - **Electric lighting** should be daylight-responsive (dimming or switching by zone) - **Interior design** -- high-reflectance finishes, open-plan layouts, low partitions near windows - **Glare control** -- occupant-adjustable blinds as a minimum; see [[Glare Control Methods]] --- ## Key References and Standards - BS EN 17037:2018 -- Daylight in Buildings - BS 8206-2:2008 -- Lighting for Buildings: Code of Practice for Daylighting - CIBSE Lighting Guide LG10 -- Daylighting - IES LM-83-12 -- Spatial Daylight Autonomy and Annual Sunlight Exposure - CIBSE Guide A -- Environmental Design (daylight data) - Reinhart, C.F. (2014). *Daylighting Handbook I* - Tregenza, P. and Wilson, M. (2011). *Daylighting: Architecture and Lighting Design* - Baker, N. and Steemers, K. (2002). *Daylight Design of Buildings* --- #environment #daylighting