How to Calculate Daylight Factor

Daylight Factor Formula:

\[ DF = \frac{\text{Indoor Illuminance}}{\text{Outdoor Illuminance}} \times 100 \]

Daylight Factor (DF):

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1. What is Daylight Factor?

Daylight Factor (DF) is a metric used in architecture and lighting design to quantify the amount of natural light available in interior spaces. It represents the ratio of indoor illuminance to outdoor illuminance, expressed as a percentage.

2. How Does the Calculator Work?

The calculator uses the Daylight Factor formula:

\[ DF = \frac{\text{Indoor Illuminance}}{\text{Outdoor Illuminance}} \times 100 \]

Where:

\( DF \) — Daylight Factor (%)
\( Indoor Illuminance \) — Light level measured inside the building (lux)
\( Outdoor Illuminance \) — Light level measured outside on horizontal surface (lux)

Explanation: The formula calculates what percentage of available outdoor daylight reaches a specific indoor location.

3. Importance of Daylight Factor Calculation

Details: Daylight Factor is crucial for sustainable building design, energy efficiency, occupant comfort, and meeting building code requirements for natural lighting in interior spaces.

4. Using the Calculator

Tips: Enter both indoor and outdoor illuminance values in lux. Measurements should be taken simultaneously under overcast sky conditions for accurate results. Both values must be greater than zero.

5. Frequently Asked Questions (FAQ)

Q1: What are typical Daylight Factor values?
A: DF values range from 0-8% typically. 2-5% is considered good daylighting, while values below 2% may require artificial lighting.

Q2: When should measurements be taken?
A: Ideal measurements are taken under overcast sky conditions to eliminate direct sunlight variations and ensure consistent results.

Q3: Where should indoor measurements be taken?
A: Measure at the work plane height (typically 0.8m above floor) at various locations in the room to get representative values.

Q4: Are there building code requirements for DF?
A: Many building codes require minimum DF values for different room types, typically ranging from 1-5% depending on the space function.

Q5: What factors affect Daylight Factor?
A: Window size and orientation, glass transmittance, room geometry, surface reflectances, and external obstructions all impact DF values.