Visual Performance and Sense of Confidence - Road Lighting: Fundamentals, Technology and Application - page 276

Fig. 20.9 Geometry for
calculating veiling
luminances L seq

i th surround
element

ˆ

ʸ

As discussed earlier, the scattering of bright daylight by the layer of atmosphere
containing dust and dirt particles between the driver and the tunnel entrance, hampers
the visibility of objects in the threshold zone. Part of the light that scatters within the
cone of vision arrives directly at the fovea, where it overlaps with the sharp image of
the tunnel entrance (Fig. 20.10 ). This can have an important further contrast-reducing
effect. Part of the light that scatters outside the cone of vision into the direction of the
eye can still reach the fovea because of light scatter within the eye. This secondary
scattering effect of atmospheric light (in Fig. 20.10 drawn in black) is small and is
usually neglected.
If bright daylight falls onto a car's windscreen, part of it will be scattered at the
windscreen into the direction of the driver's eyes to produce a contrast-diminishing
effect. Dirty windscreens increase the seriousness of this effect. Here, too, that part
of the light that is scattered at the windscreen to enter the cone of vision is radiated
towards the fovea, where it overlaps with the sharp image of the tunnel entrance
to reduce the contrast of objects seen there (Fig. 20.11 red drawn scatters). The
secondary scattering effect of scattered light from parts of the window outside the
cone of vision (black drawn scatter) is small and is neglected.
The three different kinds of veiling luminance originating from:
• light scatter in the eye of light from the bright tunnel surrounds (L seq ),
• light scatter from the layer of air into the direction of the fovea (L atm ),
• light scatter from the windscreen into the direction of the fovea (L ws )
thus all have a contrast-reducing effect for objects in the threshold zone of a tunnel
as seen by the approaching driver from the access zone.
The threshold zone luminance needed to acquire a certain minimum value of
perceived contrast can be determined (CIE 2004 ). The formula is:

˄ ws L atm + / 1

C perc

L seq

ˁ
ˀq c −

1 1

L ws +

L th = −

˄ ws

where: