Average internally reflected component for rooflights -- Algorithm 2.43

The average internally reflected component for a horizontal rooflight can be found using an approach similar to that used for vertical windows.


Glazed area of the rooflight aperture (with a curved glazed surface, this is the aperture area not the area of curved surface), Aw m2
Total area of ceiling, floor and walls, including glazing, A m2
Area-weighted average reflectance of ceiling, floor and walls, including glazing,
Average daylight factor on inside face of rooflight aperture,
Angle subtended of the visible sky, measured from a point in the plane of the internal surface at the centre of the window opening, q' degrees (as Algorithm 2.12a)
Diffuse light transmittance of glazing, including effects of dirt, t



Where the rooflight recess forms a deep cavity, as with skirted dome-type lights, reflection within the cavity is significant. The computation then requires 2 stages: calculation of the mean daylight factor at the base of the cavity, the ceiling plane of the room; then calculation of the IRC for the room. Alternatively, tabulated values of may be used, as in BRE Digest 310 (given there the symbol K).


BRE Digest 310(1) .

1. Building Research Establishment Estimating daylight in buildings: Part 2 BRE Digest 310 Building Research Establishment (1986)

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