Externally reflected component of the daylight factor De -- Algorithm 2.21

The externally-reflected component will normally represent only a very small fraction of the total daylight factor value at a point in a room. Consequently a simplified procedure is usually adopted for its calculation. This procedure involves calculating an 'equivalent sky component' to the area of sky obscured by the external obstruction, assigning a value of the luminance of the obstruction as some fraction of the luminance of the obscured sky, and then multiplying the equivalent sky component by the relative luminance. The outline of the obstructions viewed from a point in the centre of the room is normally reconfigured so that it becomes a horizontal line with an area of obstruction below it similar in size to the actual obstruction outline.


Equivalent sky component of the obstructed area of sky, Dse (algorithm 2.32)
Relative luminance of external obstruction to luminance of sky obscured, Lx/Ls (algorithms 1.33 & 1.42)



When the external obstruction is not large and of average reflectance, the value of Lx/Ls may be taken to be 0.2 for a CIE standard overcast sky and 0.1 for skies which are uniform or irregularly overcast.


BRE digest 309(1)

1. Building Research Establishment Estimating daylight in buildings: Part 2 BRE Digest 309 Building Research Establishment (1986)

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