Average internally reflected component for vertical windows -- Algorithm 2.41

The average internally reflected component of the daylight factor represents the daylight reaching a reference point after reflections and inter-reflections from the surfaces inside a room. The value of the inter-reflected light will vary throughout the room, but for most purposes it is sufficient to calculate an average value to assign to most of the room and a minimum for points distant from the window.

This algorithm is based on the split-flux method and is related to algorithm 2.12.


Diffuse light transmittance of glazing, including effects of dirt, t
Glazed area of the window (after subtracting the area of window bars and other obstructions), Aw m2
Total area of ceiling, floor and walls, including windows, A m2
Area-weighted average reflectance of ceiling, floor and walls, including windows,
Area-weighted average reflectance of ceiling and wall surfaces above the centre-height of the windows, excluding window wall surfaces, rcw
Area-weighted average reflectance of floor and wall surfaces below the centre-height of the windows, excluding window wall surfaces, rfw
Reflectance of the ground, rg
Reflectance of external obstructions, rb
Angles of obstruction, as in diagram, radians



If obstructions form an irregular skyline, the overall value of C can be calculated by dividing the external scene into several zones in plan angle f, finding the value of C for each, and adding these together.
Corrections for the effects of dirt and window bars are given in algorithm 2.13.


Tregenza(1) , Sharples(2)

1. Tregenza P R Modification of the split-flux formulae for mean daylight factor and internal reflected component with large external obstructions Lighting Research and Technology 21 (3) 125-128 (1989)

2. Sharples S, Page J K and Souster C G Modelling the daylight levels produced in rectangular, side-lit rooms by vertical windows containing clear or body-tinted glazing Department of Building Science, University of Sheffield (1981)

Go Back to Subtask C Contents