Daylight in Internal Areas with Light from above. Rooflighting

The illumination of internal areas with daylight from 'above' is subject to the same prerequisites and conditions that apply to rooms with windows at the side, i.e. daylight illumination with a clouded sky. Whilst light from the side produces relatively poor uniformity of light distribution (and hence increased demand for D%), this is not the case with lighting from above. The quality of daylight in the latter case is significantly influenced by zenith luminance, room proportions, quality criteria, daylight from above and diminution factors.

The best place to work in the room shown (- (59)) is at a distance from the side window which is equal to the height above the working position of the overhead light source. If the same level of illuminance that is produced by the overhead light on the reference plane (0.85m above floor level) is to be generated by light from the side window, then the window must be 5.5 times larger in area than the roof light aperture. The reason for this is that the light from above is brighter, since the zenith luminance is roughly three times the horizontal luminance. This means the light from above represents 100% of the light from the sky, whereas only 50% of the light from the sky is admitted through a side window.

The illumination of a room from above is dependent on the proportions of the room, i.e. length, width and height (see – (60)). However, the possible occurrence of the 'dungeon effect' should be avoided.

Quality criteria for overhead light. The variation of daylight (D%) in an internal area with side windows is characterised by Dmin and Dmax -> A uniformity of G > 1:2 (Dmin/Dm) and a Dmin of > 2% is required for daylight illumination with overhead light in workrooms (Dm)min > 4% - (61).

Rooflighting. Rooflights arranged at points on the ceiling area generate typical minimum and maximum brightnesses in the region where the light is required, the work plane. The mean value between these 'bright' and 'dark' areas is calculated, and this is termed the mean daylight factor Dm.

Thus, Dm is the arithmetic mean between Dmin and Dmax with respect to the reference or work plane (0.85 m above floor level). The required G > 1:2 is not based on Dmax, but on Dmin, since unevenness in the daylight from above is sensed physiologically as 'stronger than contrast'. At this uniformity (Dmin = 1 and Dm = 2), Dmin must be > 2% (compare – (61)).

Furthermore, the quality criteria striven for in controlling the overhead daylight in the room are limited by the room height and the shape of the rooflight (ke factor).

An ideal uniformity is achieved when the spacing between the rooflights (O) is equivalent to the room height (h), i.e. a ratio of approximately 1:1.

In practice the rule is that the ratio of rooflight spacing to room height should be 1:1.5-1:2 (see – (62)). This figure contains a table from which these ratios and their effects can be obtained. The figure also provides a recommendation for the light shafts which should be let into the roof.