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

*Input*- 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,
*either* - Average daylight factor on inside face of rooflight aperture,
*or*- 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* *Equation**Note*- 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*). *Source*- BRE Digest 310(1) .
**References**- 1. Building Research Establishment
*Estimating daylight in buildings: Part 2*BRE Digest 310 Building Research Establishment (1986)

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