Short note: Daylight factor

Daylight factor (DF) is the ratio of the illuminance at a point on a working plane inside a room, due to skylight (excluding direct sunlight), to the simultaneous illuminance on a horizontal plane outdoors from an unobstructed sky, expressed as a percentage.

$$DF = \frac{E_i}{E_o} \times 100\%$$

where $E_i$ is the indoor illuminance (lux) and $E_o$ is the outdoor illuminance (lux).

Components

$DF = SC + ERC + IRC$

• Sky component (SC): direct light from the sky through the window.
• Externally reflected component (ERC): light reflected from outside surfaces such as nearby buildings.
• Internally reflected component (IRC): light reflected from the room surfaces.

Points

• It is a measure of the adequacy of daylight, and for a given point it stays constant under an overcast sky.
• It falls with distance from the window; points near the window have higher values.
• Recommended values: about 2 % for houses, 3 to 5 % for classrooms and offices, and 5 to 10 % for drawing rooms and workshops with fine work.
• Factors: window size and position, type of glass, obstructions, and the colours of room finishes.