Civil Engineering Reference
In-Depth Information
Chapter 8
Lighting Quality Parameters
Abstract
The quality of a road-lighting installation is expressed in terms of photo-
metric criteria that influence both visual performance and visual comfort. There are
two different categories of quality parameters: parameters using solely photometric
units and parameters using performance metrics. Examples of the latter category are
revealing power, visibility level, small-target visibility and facial identification. A
short discussion regarding which category is most suitable for use in road-lighting
specification and design is given in this chapter. The exact photometric parame-
ters used in specifying, designing and measuring road-lighting installations will be
defined. For lighting installations designed mainly to suit the needs of motorized
traffic, these parameters are: average road-surface luminance, overall and longitudi-
nal uniformity, surround ratio, and threshold increment. For the needs of pedestrians,
cyclists and residents the parameters are: average horizontal and minimum illumi-
nance together with a maximum value of horizontal illuminance, semi-cylindrical
and average facade illuminance. Lighting parameters for the specification of the
spectrum of light sources, so far as they are relevant to road lighting, will be defined
in the last section of this chapter.
For specification and design purposes, the quality of a lighting installation needs to
be expressed in terms of values for quality parameters. There are two fundamentally
different categories of quality parameters:
•
parameters based solely on photometric units, such as lighting level, uniformity
and glare restriction,
•
parameters based on task-performance metrics, such as revealing power, visibility
index, small-target visibility and facial identification.
The first category depends only on lighting-installation aspects. The road surface is
here considered as part of the lighting installation. The second category, however,
depends on a combination of lighting-installation and non-lighting-installation as-
pects such as type of target (size, shape, reflectance etc.), type of maneuver to be
supported (keeping distance, reacting to position changes of other cars, cycles and
pedestrians etc.). A performance metric can only serve one or a limited number of
visual-task aspects and therefore ignores the others. Performance metrics also ignore
the importance of visual-comfort aspects. This may lead to wrong conclusions as far
as the overall quality of the road-lighting installation is concerned. Road lighting
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