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Evaluating illuminance requirements

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.

Illuminance calculations

To design a lighting system that is efficient and respects the night environment, one needs to determine the specific illuminance pattern that will be produced. It is also important to estimate whether any lighting will spill outside the property in order to minimize light trespass. This information may be found in the detailed, point-by-point illuminance calculations, which produce a plot of illuminance onto a previously defined surface area. Engineers, architects and luminaire manufacturers perform these calculations using specialized computer software. Once the components of the system have been installed and turned on, the actual illuminance can then be measured using a light meter.

Don't forget that the performance of luminaires decreases with time due to numerous factors, such as wear, dirt deposits, leaking of the seal, type of bulb, etc. This phenomenon is accounted for through the inclusion of a maintenance or light loss factor.

Look for the following information in the point-by-point calculation summary:

  1. maintained illuminance on the ground: average, minimum and maximum (Eavg, Emin, Emax);
  2. uniformity of the lighting, as determined from Eavg/ Emin;
  3. number and input power (W) of the bulbs;
  4. the light loss factor used.
The figure below shows a typical point-by-point layout and summary.
The figure below shows a typical point-by-point layout and summary.
Example of a point-by-point layout with two roadway luminaires along a street.
Each point on the grid corresponds to an illuminance value in lux.
Height (m)
Flux (lm)
IESNA Cutoff class
Helios type 2
70 W
Average illuminance
Eavg (lux)
Maximum illuminance
Emax (lux)
Minimum illuminance
Emin (lux)
Eavg/ Emin

Where only small areas (such as parking spots and building entrances) are to be illuminated, it can sometimes be tedious to calculate the illuminance required. “Equivalents in lm/ m2” may therefore be used to simplify an illuminance calculation. Remember that strong lighting may not be necessary to provide sufficient illumination for nocturnal vision. It is always preferable to favour low-power light sources, such as high-pressure sodium bulbs of 35, 50 or 70 W! Also, feel free to be inspired our typical examples!