Daylight factor definition D -- Algorithm 2.11

Daylight factor is the ratio of illuminance at a point on a given plane due to light received directly or indirectly from a sky of known or assumed luminance distribution, to illuminance on a horizontal plane due to an unobstructed hemisphere of sky(1) . The ratio is usually multiplied by 100 to allow the daylight factor to be expressed as a percentage.

Unless stated otherwise the sky luminance distribution is assumed to be that of the CIE standard overcast sky (see Algorithm 1.33).

Daylight factor consists of three components :

Sky component Ds: ratio of that part of illuminance at a point on a given plane that is received directly from a sky of known or assumed luminance distribution, to illuminance on a horizontal plane due to an unobstructed hemisphere of this sky.

Externally reflected component De: ratio of that part of illuminance at a point on a given plane that is received directly after reflection from external obstructions under a sky of assumed or known luminance distribution, to illuminance on a horizontal plane due to an unobstucted hemisphere of this sky.

Internally reflected component Di: ratio of that part of illuminance at a point on a given plane that is received after reflection from interior surfaces under a sky of assumed or known luminance distribution, to illuminance on a horizontal plane due to an unobstructed hemisphere of this sky.

Input
Sky component, Ds
Externally reflected component, De
Internally reflected component, Di
Equation
D = Ds + De + Di %
Source
BS 8206 Part 2(1)
Reference
1. British Standards Institution. Code of practice for daylighting.British Standard BS 8206:Part 2(1992)

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