Standards and Recommendations - Road Lighting: Fundamentals, Technology and Application - page 293

threshold transition interior exit

L th

100%

L

40% 100%

L ex

L tr

500%

L int

100%

d stop d stop 20 m

Fig. 21.1 According to CIE ( 2004 ), luminance in the threshold zone may decrease linearly from
half the stopping distance onwards to 40 %. This 40 % point is subsequently the start of the 100 %
point of the transition curve

the “perceived contrast method”, which is the method described in Sect. 20.2.6 as the
“veiling-luminance concept”. The veiling luminances due to scattering in the eye of
light from the bright tunnel surroundings, due to scattering in the atmosphere, and due
to scattering in the windscreen together determine the lighting required in threshold
zone. The formula to calculate this required threshold-zone luminance value from
these parameters has already been given in Sect. 20.2.6 under the heading “veiling
luminance concept”. In this formula, the CIE recommended value for minimum
required perceived contrast, C perc , is taken as 0.28 with an object reflectance value
(

) of 0.20. Again according to CIE, the contrast-revealing coefficient (q c ) is 0.2 for
symmetrical and 0.6 for counter-beam systems.
The required length of the threshold zone is equal to the safe-stopping distance.
Over the first half of this distance the luminance has to be constant, but from half
the stopping distance onwards, the lighting level may linearly decrease to a value, at
the end of the threshold zone, of 40 % of the value at the beginning of the threshold
zone (Fig. 21.1 ). Note that graphs of lighting level are often given on a logarithmic
scale where the linear line of the linear scale looks curved. The gradual decrease
may, in the case of a lighting system with which continuous dimming is not feasible,
be done in steps, provided that the luminance levels do not fall below the values
corresponding to the gradual linear decrease.

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