Civil Engineering Reference
In-Depth Information
Table 20.1
Examples of luminance values (kcd/m
2
) in the access zone roughly corresponding to
100,000 lx horizontal illuminance. These values are for the Northern hemisphere. For the Southern
hemisphere, North and South should be interchanged. (Partly based on CIE
2004
)
Driving
direction
Sky
Road
Rocks
Buildings
Grass
Shrubs
Snow
mountain
Snow
flat
Dark
paint
North
8
3
3
8
2
1
15
15
0.75
East and west
12
4
2
6
2
1
10
15
0.75
South
16
5
1
4
2
1
5
15
0.75
Fig. 20.3
Entrance canopy
to shield off part of the
bright sky
visual performance and the sense of confidence of the motorists, and thus on the
lighting levels required. The three systems are characterised by the longitudinal light
distribution of the luminaires employed (Fig.
20.4
):
•
symmetrical lighting systems throw their beams symmetrically in the forward and
backwards directions,
•
counter-beam lighting systems throw the main part of their beams in the direction
against (“counter to”) the direction of traffic flow,
•
pro-beam systems throw the main part of the beam in the same direction as the
traffic flow.
Symmetrical tunnel-lighting systems light both the road surface and obstacles on
the road, very much as do normal road-lighting systems. Most, but not all, obstacles
with different reflectances will be seen in negative contrast as silhouettes, just as with
normal road lighting (Fig.
20.5
top left). The counter-beam system does provide
lighting on the road surface, but very little on the surfaces of obstacles seen by
the approaching motorist. Consequently, more of the objects are seen in silhouette
with a larger (negative) contrast (Fig.
20.5
top right). As will be discussed in more
detail in further sections, this means that the lighting level in the threshold zone with
counter-beam systems can be lower, whilst still resulting in the same level of visual
performance, but in a lower level of visual comfort and sense of confidence for the
motorist (Fig.
20.5
bottom).
Search WWH ::
Custom Search