Basic Lighting Quantities - Road Lighting: Fundamentals, Technology and Application - page 7

Fig. 2.1 Light incident
towards the road results in the
horizontal illuminance, E hor
on the road, while the light
reflected from the road
surface results in the
road-surface luminance, L

E hor

L

pattern and, in turn, in differences in brightness. Since brightness is finally determined
not by illuminance but by luminance, the visual performance and visual comfort of
a road user are directly influenced by the complex pattern of luminances existing in
his view of the road ahead. The refection properties of cars, bicycles, pedestrians,
obstacles and other objects in the field of view vary widely. Dry road surfaces, on
the other hand, have relatively similar reflection properties for the normal viewing
directions of a motorist towards the road area some 50-150 m in front of him. The
road-surface luminance, as achieved from a particular road-lighting installation, can
therefore be accurately predicted. The road surface often forms the background to
objects on or close to the road. This is why the road-surface luminance concept is
usually better suited than the (horizontal) illuminance concept for use in defining the
visual performance and comfort of the motorized road user.

2.2 Illuminance for Pedestrians, Cyclists and Residents

The viewing directions of pedestrians and cyclists are far more varied than those of a
motorist. The surface of interest to the former is not only the surface of the road but
also that of the pavement (sidewalk in America), the facades, and the faces of other
people in the street. For these two reasons it is not possible to work with standard
reflection properties for this category of road users. For road lighting specifically
meant for non-motorized road users we therefore fall back on the illuminance as the
basic lighting parameter.