It is a ratio in percent, of workplane illuminance (at a given point) to the outside illuminance on a horizontal plane which is evaluated under cloudy sky conditions only (no direct solar beam).
DF= (Ei/Eo) x 100%
Ei = illuminance due to daylight at a point on the indoor’s working plane.
Eo = simultaneous outdoor illuminance on a horizontal plane from an unobstructed hemisphere of overcast sky.
Floor plans and sections indicate average daylight area with daylight factor. Where red colour indicates maximum daylight area because that area is very adjacent to windows and blue colour indicates very less amount of natural light as these area is far from window.
There are three possible paths along which light can get into a room through glazed windows:
(a) Light from the patch of sky visible at the point considered, is expressed as the sky component,
(b) Light reflected from opposing exterior surfaces and then reached the point, is expressed as the externally reflected component,
(c) Light entering through the window but reaching the point only after reflection from internal surfaces, is expressed as the internally reflected component.
The sum of all three components gives the daylight factor i.e.:
DF = SC + ERC + IRC
SC – Sky Component
ERC – Exterior Reflectance Component
IRC – Interior Reflectance Component
Daylight factor have many definitions as follows:
It is a percentage of exterior daylight available inside the building. It is a function of window size and position, glazing transmission, sky obstructions, and interior reflectances. (Brown and DeKay, 2001)
In other words, the measure of total daylight illuminance at a point on a given plane expressed as the ratio (or percentage) which the illuminance at the point on the given plane bears to the simultaneous illuminance on a horizontal plane due to clear design sky at an exterior point open to the whole sky vault, direct sunlight being excluded.
From this it can be concluded that:
Daylight factor is an average that is highly variable based on geographical location and micro climate which does not predict daily variations in daylight contribution.
Since the sun is in a particular position only for a short period each day, direct-beam light from the sun is most often ignored in architectural approaches to daylight design (Brown and DeKay, 2001). Therefore sometime direct daylight is most often the cause of problem (e.g: glare) and also wonder in architecture.