Civil Engineering Reference
In-Depth Information
Fig. 13.12
Scattering
function
ʲ
in fog as a function
of angle
θ
between direction
of light incidence on the fog
and direction of scatter
(Foitzik and Zschaeck
1953
)
1
ʸ
ʲ
thick fog
10
-1
10
-2
light fog
10
-3
0°
45°
90°
135°
180°
ʸ
low
high
high
low
CF
light the total width of the road, and the spacing has to be very short, both of which
make the installation rather expensive.
13.4.3
Snowy Weather
When it is snowing, much of the light from the road-lighting luminaires is lost
through absorption in the snowflakes and some is scattered in the direction of the
driver, giving rise to glare. No special measures can be taken to overcome this
negative influence on visibility. When the road is covered with snow, the average
road-surface luminance increases by a factor of 4 to 5 while, due to the diffuse
character of snow, the uniformity does not deteriorate much (Ekrias et al.
2007
). If
the installation permits of dimming, the lighting level can be reduced as long as the
settled snow remains clean.
13.5
Maintenance
A road-lighting installation will continue to operate efficiently and deliver a sufficient
quality of lighting only so long as it is well maintained. Some deterioration in quality
is, of course, inevitable, even with a well-maintained installation. The deterioration
will be greatest just prior to maintenance being carried out. The task facing the
lighting designer is that of fixing the maintenance period and the initial lighting level
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