The daylight factor (DF) is a measure of the amount of natural light in an interior space relative to the amount of light outdoors, expressed as a percentage . It is used in architecture and construction to assess the quality of daylighting in buildings.
1. How is the daylight factor calculated?
The daylight factor is calculated as follows:
whereby:
- = Indoor illuminance (lux).
- = Outdoor illuminance (lux) on a cloudless day.
📌 Example: If the illuminance outside is 10,000 lux and inside is 500 lux, then the daylight factor is:
2. Typical daylight factor values
| Space types | Recommended DF (%) | Example |
|---|---|---|
| Dark spaces | < 2% | Storage areas, cellars |
| Normal living and working spaces | 2 - 5% | Living rooms, offices, classrooms |
| Very well lit spaces | > 5% | Greenhouses, atriums, art galleries |
📌 A higher daylight factor means that the room is better illuminated by natural light.
3. Factors that influence the daylight factor
🔹 Windows and glazing → Size, placement and type of glass determine how much light enters.
🔹 Reflection of interior surfaces → Lighter colours and materials increase the daylight factor.
🔹 Outdoor Obstacles → Buildings, trees, or awnings can block daylight.
🔹 Building orientation → North-facing windows get less direct sunlight than south-facing ones.
🔹 Shading → Sunshades or curtains can reduce daylight.
📌 Smart placement of windows and reflective materials helps to optimize the daylight factor.
4. Benefits of a good daylight factor
🌞 Energy savings → Less artificial lighting needed, lower energy costs.
🛑 Health Benefits → Daylight improves mood and productivity.
🎨 Better color retention → Natural light gives a more realistic color perception.
🌱 Sustainability → Better use of natural resources.
📌 A well-designed building with a high daylight factor can reduce the energy requirement for lighting by 30-50%.
5. How is the daylight factor calculated in designs?
For construction applications, the daylight factor is simulated with software such as:
🔹 DIALux / Relux → Lighting simulations for architecture.
🔹 Revit / AutoCAD with light analysis → For 3D models and daylight simulations.
🔹 Velux Daylight Visualizer → Specifically for roof windows and skylights.
💡 In short:
The daylight factor (DF) is the percentage of natural light in an interior space compared to the amount of light outdoors. A high DF means better daylight penetration , which leads to energy savings, greater comfort, and better health . Architects and designers use this value to make buildings more sustainable and pleasant .