Visual Performance and Sense of Confidence - Road Lighting: Fundamentals, Technology and Application - page 270

Symmetrical and counter-beam systems can be classified according to the so-
called “contrast revealing coefficient” (q c ), which is defined as:

q c = L road / E vert

The better the limitation of the vertical illuminance (on the plane facing the oncom-
ing motorists), the higher is the contrast-revealing coefficient. In many standards and
recommendations for tunnel lighting, counter-beam systems are defined as systems
with a contrast-revealing coefficient, q c , equal to or higher than 0.6. A typical value
of q c for symmetrical systems is 0.2. Here it should be noted that these values do not
normally take into account the indirect contribution to the vertical illuminance made
by reflection of light from the road-surface area in front of an object. Lighter and
more diffuse reflecting surfaces will increase this indirect contribution to the vertical
illuminance and thus lower the contrast revealing coefficient, and indeed the effec-
tiveness of a counter-beam system. Adrian and Gibbons ( 1993 , 1999 ) introduced a
provisional calculation system for the indirect contribution, but with limited validity.
A generally-applicable system is not yet available.
As has been explained, a counter-beam system directs the maximum amount of
light towards oncoming traffic. For this direction of light incidence a relatively large
amount of light is reflected from the road surface towards the oncoming motorists.
The road surface has a large luminance coefficient for this light incidence and view-
ing direction (See Chap. 12 of part 1). This effect increases the efficiency with which
illuminance is converted into the required road-surface luminance: counter-beam in-
stallations thus have a high luminance yield. However, this efficiency advantage may
sometimes be counterbalanced by the necessity of having to take special measures
in the luminaires to prevent intolerable glare from their light directed towards the
traffic.
The high luminance yield exhibited by counter-beam systems is not shared by
pro-beam installations. This means that high horizontal illuminances and thus high
luminous-flux luminaires are needed to supply the required luminances. This is the
most important reason why, to date, pro-beam tunnel lighting installations are seldom
employed. There is also the fact that with them, the visual guidance normally obtained
by lines of bright luminaires along the tunnel ceiling or walls, would be weakened.
A certain degree of brightness of the tunnel luminaires, can indeed help in guiding
motorists into and through the tunnel (Narisada et al. 1977 ).
Recently, in Japan, thought-provoking research on pro-beam lighting installations
has been carried out. This involved the visibility of the rear of preceding cars and
of a set of small objects with different reflectances (Hirakawa et al. 2007 ; Ito et al.
2011 ). One test, in one and the same tunnel tube, compared an actual pro-beam
lighting installation employing LEDs with a symmetrical installation employing
low-pressure sodium lamps (Ito et al. 2013 ). The results showed that the subjective
appraisal of the visibility of the rear of a preceding car was better for the pro-
beam system. On the basis of revealing power calculations (see Sect. 3.2 of Part
1) for objects defined in Fig. 3.10 of this topic, the conclusion was that the pro-
beam system provides such high vertical illuminances that many objects are seen