Equipment and Design Aspects - Road Lighting: Fundamentals, Technology and Application - page 306

Fig. 22.4 Perspective
drawing of a tunnel entrance
with 20 ⓦ circle projected from
the safe stopping distance

22.5.1.2 Determination of L seq

The recommended concept of CIE for the determination of the threshold-zone level
is the L seq concept. The photograph or perspective drawing of the tunnel entrance
surround as seen from the safe stopping distance, described in the previous section
for L 20 use, can also be used for the determination of L seq . A polar diagram has
been developed (Adrian 1987 ), and is published by CIE (CIE 2004 ), that can be
superimposed on the tunnel photograph or drawing. It divides the tunnel drawing
or photograph into segments and concentric circle parts (Fig. 22.5 ). The sizes of
the consecutive circle parts have been determined such that, for equal luminances,
each area produces the same veiling luminance. It is now easy to determine in each
part the percentages of the actually-occurring luminances and so form the average
luminance for that segment. The total veiling luminance L seq caused by all segments
is then obtained by summing the contributions from all segments:

5,1 . 10 − 4 L segment i L seq =

Again, the luminances of the different segments such as sky, road, grass and moun-
tain, can be taken from Table 20.1. This is a simple process which can relatively
quickly be executed with the use of a spreadsheet.

22.5.2 Lighting Arrangement

With symmetrical tunnel-lighting installations the light comes from one or more
rows of luminaires, depending on the lighting levels needed. These are usually