Equipment and Design Aspects - Road Lighting: Fundamentals, Technology and Application - page 308

Fig. 22.6 Lighting
arrangements and
corresponding light
distributions

of some three or four luminaires can often solve potential flicker problems in the
interior zone.

22.5.3 Daylight Variation and Lighting Control

As has already been discussed, a tunnel-lighting installation should be laid out to
satisfy the lighting requirements relevant to the highest daylight levels regularly
prevailing. This means, of course, that when daylight levels are lower, the tunnel
lighting has to be switched or dimmed to a lower level. In many older tunnels, this
is done in response to signals received from photocells, which monitor the daylight
illuminance level at the entrance. However, this can sometimes lead to tunnel lighting
that does not satisfy the visibility and sense of confidence requirements.
The luminance in the threshold zone of a tunnel must at all times be an appropriate
specified percentage of the value of L 20 or L seq , or at least of some quantity closely
related to it. For this purpose luminance meters have to be employed, one at each
tunnel entrance, mounted as close as possible to the stop-decision point and facing
in the direction of the tunnel entrance. From the continuously-measured values that
these meters send back, the luminances required in the threshold zone at each moment
are determined. A control system then provides the required lighting steps in the
various tunnel zones. If the installation allows for dimming, the luminance levels
determined in this way are obtained by continuous dimming. It is then possible, with
a more advanced control system, to keep the actual required ratio of L th /L 20 or L th / Lseq
constant. For this to be possible, an additional luminance meter in the threshold zone
(and sometimes in other zones as well) measures the actual luminance obtained from