Type of Rooflight and Construction

The inclination of the rooflights determines the percentage of the light component from the sky which is available. In – (63)a, the quantity of incident light admitted through a side window is compared with the quantity of light provided by rooflights at various inclinations. The greatest quantity of light is received through a horizontal rooflight.

On the other hand, the maximum illuminance from a side window is achieved only in the vicinity of the window; for glazing which is vertically overhead, the lowest illuminance is on the reference plane.

Thus there is a diminution factor (ky) for the quantity of incident light which depends on the angle of inclination of the rooflight. The diminution factors corresponding to shed roofs of various inclinations are shown in (63)b.

The diffuse incident light which falls on the rooflight is affected by the construction and depth of the installation before it supplies the room with daylight. The various levels of incident light for shafts of different proportions beneath rooflights the are shown in – (64). Excessively high and massive shafts and built-in depths should be avoided – (65)a, while a filigree, highly reflective construction is to be recommended – (66)b.

The quality of daylight in an internal area with rooflights is not only dependent on the factors discussed above. Another significant factor is the ratio of the total area of the overhead lights to the floor area of the room (kF factor).

The diagrams in – (66) show the levels of daylight from side windows with various geometrical features and overhead illumination.

In order to increase the daylight factor Dmin by 5% for side windows or opposite-facing rooflights, the proportions of the windows must be increased significantly, typically up to a ratio of 1:1.5. By contrast, for the same demands from overhead lighting, particularly with shed roof-type lights, the area need only be increased by a relatively small amount. A ratio of rooflight area to floor area of from 1:4 to 1:5 is adequate.

Additional diminution factors for rooflights are given below.
- transmittance of the glazing, t
- scatter and constructional features, k1
- soiling of the glazing, k2
- diffuse illumination, k3.

Empirical evaluation of the quality of daylight from overhead illumination. The definitive evaluation of daylight conditions should be performed against the background of a clouded sky. However, rooflights are not only recipients of diffuse radiation, they are also subject to direct solar radiation. These varying lighting conditions should be simulated, not only under an artificial sky, but also under an artificial sun. In this process, the quality criteria for the daylight on the model should be assessed by eye – (67).

Design parameters for overhead illumination are listed below (- (68) – (72) see also – (55)).
- Rooflights should not be orientated toward the south.
- Convert solar radiation into diffuse light radiation.
- Maintain quality criteria for daylight.
- Avoid excessive contrasts in luminance levels.
- Pay attention to variation in Dm.
- Ensure illumination of all room corners and enclosing surfaces.
- Avoid glare by artificial shading.
- Treat room-enclosing surfaces according to their separate technical requirements.
- Ensure that it is possible to see outside.

Side and overhead lighting. The choice between side and overhead illumination depends on the use to which the building is to be put and also on the available external light sources, i.e. the geographical location. For example, where there are extreme light and climatic conditions, appropriate forms of construction must be developed and the shapes of buildings must be designed to match the prevailing light conditions at that latitude (i.e. to make optimum use of the diffuse and direct sunlight – (73) – (76).