1.7.3 Daylight factor
Daylight factor (DF) is a daylight availability metric that expresses as a percentage the amount of daylight available inside a room (on a work plane) compared to the amount of unobstructed daylight available outside under overcast sky conditions (Hopkins,1963).
The key building properties that determine the magnitude and distribution of the daylight factor in a space are (Mardaljevic, J. (2012)):
• The size, distribution, location, and transmission properties of the facade and roof windows.
• The size and configuration of the space.
• The reflective properties of the internal and external surfaces.
• The degree to which external structures obscure the view of the sky.
The higher the DF, the more daylight is available in the room. Rooms with an average DF of 2% or more can be considered daylit, but electric lighting may still be needed to perform visual tasks. A room will appear strongly daylit when the average DF is 5% or more, in which case electric lighting will most likely not be used during daytime (CIBSE, 2002).