Basic Lighting Quantities - Road Lighting: Fundamentals, Technology and Application - page 6

Chapter 2
Basic Lighting Quantities

Abstract The first thing to determine is which are the lighting parameters that
positively influence the visual performance and comfort of the users of road lighting.
For this it is necessary to define those basic lighting quantities that play a dominant
role in the seeing process of motorists, pedestrians and residents, namely: illuminance
or luminance. For motorized traffic it is the luminance of the road surface that forms
the background to possible objects on the road. For the normal directions of view
of motorists, the reflection properties of road surfaces, needed for the determination
of road-surface luminance, can be photometrically characterized. Therefore, road
surface luminance can and is indeed used as the basic lighting quantity for road
lighting for motorized traffic. Since the directions of view of slow-moving traffic
such as pedestrians and cyclists are more varied and the reflection properties of
surfaces of interest are widely different, we have to fall back on illuminance as
the basic lighting parameter for road lighting specifically meant for non-motorized
road users. The basic measure here for the lighting of the road and pavement is the
horizontal illuminance; for the lighting of facades it is the vertical illuminance and
for the lighting of faces of persons in the street it is the semicylindrical illuminance.

2.1 Road-Surface Luminance for Motorized Traffic

A surface is made visible by virtue of light being reflected from it and entering the
eye of the observer: the greater the amount of light entering the eye, the stronger
will be the visual sensation experienced. Thus, the illuminance on a road surface,
which refers only to the amount of light reaching that surface, gives no indication
of how strong the visual sensation will be; or in other words, how bright the surface
will appear. The brightness of the road surface will depend on the amount of light
reflected from it in the direction of the observer (Fig. 2.1 ).
The photometric measure for this is the luminance (L) of the surface. That it is
the luminance and not the illuminance that determines the brightness is illustrated
by way of the four photographs of one and the same road-lighting installation shown
in Fig. 2.2 .
The illuminance pattern on the road is the same in each photograph because the
road-lighting luminaires and their configuration are the same—it is the changes in
the reflection properties of the road surface which results in changes in the luminance