Visual Performance and Sense of Confidence - Road Lighting: Fundamentals, Technology and Application - page 285

Fig. 20.19 Forbidden
luminaire spacing ( shaded
areas ) as a function of driving
speed to avoid troublesome
flicker. Effect of total
exposure time characterised
by tunnel length based on
Walthert ( 1977 )

12

2.5 Hz

10

500 m

8

250 m

6

125 m

4

2

15 Hz

0

50 75 100 125

Speed (km/h)

• the number of flickers occurring per second (flicker frequency),

• the total duration of the flicker, i.e. the total time spent in a flickering environment,

• the photometric properties of the luminaire, especially its peak luminance value

and the sharpness of its light distribution.

From laboratory tests in models with light sources moving on a belt towards ob-
servers to simulate a moving driver, it became clear that flicker is not disturbing
under normal tunnel-lighting conditions so long as the flicker frequency is less than
2.5 Hz (pulses per second) or greater than 15 Hz (Jantzen 1960 ; Schreuder 1964 ;
Walthert 1977 ). The disturbance is greatest for the frequency range 5-10 Hz. Flicker
frequency is dependent upon the speed of travel through the tunnel and the spacing
of the luminaires. The total exposure time to flicker has an influence on the range of
frequencies over which the effect will prove troublesome: the shorter the exposure
time (viz. the shorter the tunnel), the smaller the range of disturbing frequencies
becomes (Walthert 1977 ). The relation between luminaire spacing and speed needed
to avoid the forbidden range of frequencies is given in Fig. 20.19 .
In the interior zone, with its permissibly-low lighting level, such a small number
of luminaires are often needed that they are spaced some metres apart. As indicated
in Fig. 20.20 top, this may easily give frequencies resulting in disturbing flicker for
many driving speeds. By positioning the luminaires in groups of three, for example,
the flicker problem is avoided for most driving speeds (Fig. 20.20 bottom). At the
same time. the spacing-to-height ratio (s/h) is small enough to provide sufficient
luminance uniformity on the road.