Average daylight factor -- Algorithm 2.12

The average daylight factor is the mean value of the daylight factor on a horizontal working plane under a CIE overcast sky. If no specific information is available, the height of the working plane is taken to be 0.7m above the floor for offices and 0.85m above the floor in dwellings and industrial buildings.

Two methods are given. The first is applicable to the early design stages of a building, and can be used to find the window area required in a room to provide a given average daylight factor. It should not be used where external obstructions can not be described approximately by a single angle in section. The second method can take into account obstructions projecting to the side of windows, and an uneven skyline.

The average daylight factor is a measure of the total amount of daylight in a room, and hence the room brightness in relation to the view outside. The algorithms are based on calculations of the total luminous flux entering a window, the relative areas of window opening and internal surfaces, and the overall internal inter-reflection. They give no information about the distribution of light within the room.

In a room with two or more windows which differ in transmittance or external obstruction, the average daylight factors can be summed after calculating separately for each openings. The value assumed for internal reflectance should take into account the total area of window openings in every calculation.

a. Average daylight factor with simple external obstructions

Input

Diffuse light transmittance of glazing, t
Glazed area of the window (after subtracting the area of window bars and other obstructions), Aw m2
Angle subtended of the visible sky, measured from a point in the plane of the internal wall at the centre of the window opening, q degrees
Total area of ceiling, floor and walls, including windows, A m2
Area-weighted average reflectance of ceiling, floor and walls, including windows,

Equation

Note

q may exceed 90 with rooflights.
Corrections for the effects of dirt and window bars are given in algorithm 2.13.

Source

BS 8206 Part 2(1)

b. Average daylight factor using split-flux method

Input

Diffuse light transmittance of glazing, 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
Total area of floor and wall surfaces below the centre-height of the windows, excluding window wall surfaces, Afw 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(2) , Sharples(3)
References

1. British Standards Institution BS 8206: Part 2: 1992 Code of practice for daylighting
(London: BSI) (1992)

2. 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)

3. 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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