Excessive illuminance constitutes light pollution and a waste of energy. It is always preferable to produce a moderate yet uniform illuminance. This allows the eye to adapt to ambient light levels while ensuring the required visibility. The municipalities situated in the IDSR territory have thus adopted regulations that define permitted illuminance levels for the main types of exterior lighting applications.
Horizontal illuminance is the average density of luminous flux incident on a horizontal surface, generally the ground. Limiting horizontal illuminance minimizes both the light reflected to the sky and falling on neighbouring properties.
It may sometimes be necessary to estimate vertical illuminance. Ground illuminance for a road or path can be relatively weak (e.g. less than 5 lux) if there is sufficient vertical illuminance that ensures good visibility. This second parameter must also be considered for sports grounds, such as baseball and football fields. Indeed, clear vision in the vertical dimension is essential for several outdoor sports.