ENERGY-CONSCIOUS PLANNING AND ZONING
3.2.4 Daytime Lighting

Lighting Strength

The adequate illumination by daylight of habitable rooms has a lasting positive effect on our well-being. This applies both to rooms we live and work in. Natural daylight has complex physical and psychological implications on our organism, raises our productivity at work and can help to significantly reduce energy costs.

But the appropriate surveillance and realisation of the requirements laid down by a standard can still be improved. Deficiencies may be a reduced brightness in habitable rooms, a limited line of sight to the outside and thus a reduced quality of living for the occupants.

The most common reasons for not fulfilling the requirements are the ignorance of the particular standards and the complexity of the requested verification methods (State Office for Health and Social Affairs of Mecklenburg-Western Pomerania, 2007).

Daytime lighting in the sense of urban development means the provision of daylight for buildings and undeveloped surfaces, also in the presence of cloudy skies. In contrast to sunlight, the issue here is indirect, diffuse daylight. For this purpose a completely clouded sky is considered to be a light source. The higher the level of the cloud cover, the more it is lighted by the sun. Diffuse daylight is independent of direction and thus independent from the orientation of windows.

Lighting strength is the measure of the amount of light falling on a surface. The unit of measure is the Lux (= 1 Lumen/m²).

Out in the open, lighting strength varies according to time of day and year between 0 and 100,000 Lux. (When the sun stands at a 20° elevation, approximately 11,000 Lux are to be expected as horizontal lighting strength under cloudy skies and with an unobstructed horizon.)
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Daylight Quotient

At a singular point in a room, the same percentage of total horizontal lighting under cloudy skies always prevails depending on the size and position of windows, construction of the room, color of the walls, and furnishing. This constant percentage is termed the daylight quotient (in percent).

At roughly 10%, the proportion of external reflection makes only a minor contribution to the lighting strength in the room. An exception to this comes from white exterior surfaces directly in front of a window. The proportion of interior reflection illuminates in particular the back half of the room, as long as the room is framed with light-colored surfaces. The proportion of light from the sky, however, makes by far the greatest contribution to the brightness of the room.

DIN standard 5034-1 "Daylight in interiors – Part 1: General requirements" describes the objectives and exact requirements for the illumination by daylight. The underlying measure is the daylight factor.

The brightness in e.g. habitable rooms (and working rooms) from daylight which enters the windows is sufficient if the daylight factor on a horizontal working plane (measured 0.85 m above the floor at half room depth and 1 m from the side walls) is at least 0.9 % on average and at least 0.75 % at the most unfavourable place.

DIN standard 5034-3 "Daylight in interiors – Part 3: Calculation" contains the extensive geometrical correlations and calculation algorithms for determining the daylight factor.

High-reaching windows, skylights, roof lanterns, and shed roofs offer optimal daytime lighting and corresponding opportunities for electricity savings.

The minimum distances between neighboring buildings as established by building law give consideration evidently only to the criterion of sufficient daytime lighting. For residential uses, however, it can be assumed that not merely a sufficient but rather an ideal daytime lighting should be pursued. The practice and considerations of planning are not only concerned with the avoidance of unreasonable conditions.

The interchange of bright and shaded streets and plazas produced by the arrangement of buildings is a significant characteristic of urban design. In addition, buildings are for their part dependent in different measure upon daytime brightness, which under certain circumstances requires greater distances between buildings and correspondingly configured building height.