Daylight factor

One of the simplest ways of beginning to develop an understanding of daylight in buildings is to become familiar with the daylight factor (DF). It defines a constant relationship of the daylight available at an unobstructed place outside, which is received at a point inside a space. For most spaces, the optimum DFave is in the range 2-5%.

Average daylight can be found from summing the daylight available:

T= diffuse transmittance of glazing M = maintenance factor for glazing Aw = window area, excluding area of frame q = angle of visible sky above the horizon from centre of window

A = combined area of walls, floor and ceiling, including windows

R = average reflectivity of room surfaces The concept is best understood by practical experience. It is worthwhile taking time to calculate the DFave in a number of rooms, and then to compare your calculations with measurements using illuminance meters. This will provide an opportunity to get a sense of the numbers involved.

Daylight factors vary throughout a space. They tend to be high near the windows or directly below roof openings and rapidly decrease further away. The average daylight factor (DFave) is often used to give an approximation of the available daylight in a space. Approximate measurement of the daylight factor involves taking simultaneous readings - on a horizontal plane -of light levels in a space and at an unobstructed place outside. Taking readings on a grid enables a map to be drawn of the daylight available in a space and this can be averaged to provide the average daylight factor (DFave).

A quick calculation can give an approximation of available daylight in a space. It involves estimating DFave across the horizontal plane based on the window areas, their light transmission properties and factors such as room size. This gives a useful prediction of the brightness of a daylit interior and guidance on whether the window area is excessive or insufficient. It should be noted that successful daylighting requires qualitative as well as quantitative assessment. neither will be adequate alone.

Diffuse transmittance of clean glazing -T (approx)

Clear single glazing 0.8

Clear double glazing 0.7

Low-e double glazing 0.65

This should be multiplied by the appropriate maintenance factor. Values of T for other glazing systems and M can be found in the CIBSE Daylighting and

Window Design Guide Code and from manufacturers.


Reflectivity of surfaces affects distribution of light within a room. Low reflectivities and dark colours reduce the amount of available daylight. A perfect black surface, R = 0, absorbs all light. If all incident light is reflected, R = 1.

It is found by weighting the reflectance of each surface. An estimate of 0.5 is often used for light surfaces.

In reverse, the equation can be used to estimate the area of glazing for a required DFave. That is, by assuming a value of, say, 2-5%, the equation can be used to provide a first estimate of total window size for a particular space.

It is not adequate as an indicator of light quality when used alone, because it is only quantitative. However, it is a useful measure when used together with other methods that deal with variability, brightness, glare, uniformity, gloom and solar gain.

There is also a need to consider optimum glazing ratios in relation to solar gains or heat loss. The lighting and thermal (LT) method, described below, provides a way of estimating this optimum.

Estimating DFave - In the early stage of designing a three-court badminton hall (18m x 27m x 7.6m), a double-glazed clerestory to the north is proposed of 2m x 27m where the angle of visible sky is 70°. The place is to have light internal finishes (R = 0.5) and is in a clean environment. The first approximation gives:

T x M x I Window Area x sky angle

1656 x 0.75

This value is on the low side and further glazing would be recommended.

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