3. Night levels

3.1 Weighting of different contributors

The various contributors we have just seen do not have the same weight in ground lighting at night. They can be characterized by the fraction of time they are present, their solid angle, and their average luminance. The product of these last two terms gives the illuminance at ground level. This resulting illuminance value can be variable; for example, twilight illuminance is dependent on the Sun's height below the horizon, or lunar illuminance is dependent on its phase angle Φ and its height above the horizon. Table 2 summarizes the main contributors, their characterization and the range of their contribution to ground illuminance.

At a given moment, the illuminance...