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.

*Input*- 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* *Equation*

*Note*- 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.
*Source*- Tregenza(1) , Sharples(2)
**References**- 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)

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