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.)

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).

The entire sphere of the sky is split into 1000 fields, of which only 500 are used to form a spherical half-sky in the horizontal lighting diagram. In the area near the horizon and at low angles of inclination, half- and quarter-fields are also drawn in. As a representative piece of the sky, each field produces the same lighting strength with respect to the horizontal surface.

It is evident from the various sizes of the fields that the loss in lighting density of the covered sky amounts to roughly 70% from the zenith to the horizon. Thus the angle of received light in all rooms with side windows ranges between 10° and 60°, that is, in the area of strongly reduced lighting density. Each 10 fields in the diagram represent a 1% amount of the daylight quotient.

By carrying over the building picture into the diagram of Figure 3/18 and counting the obscured and free fields, the changes in the daylight quotient resulting from planned construction or other measures can be evaluated.

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.