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As it is uneconomic to provide interior tunnel lighting levels that match exterior lighting levels in daytime, the tunnel lighting must provide a safe transition between exterior and interior lighting levels. The amount of light required within a tunnel is dependent on the level of light outside, and the point inside the tunnel at which visual adaptation of the user must occur (CIE 88-1990).
In practice, the lighting of a tunnel is divided into five principal zones and these are described when entering the tunnel from normal daylight.
This is actually exterior to the tunnel entrance, defined by the stopping distance of vehicles at the traffic design speed, and the amount of luminance which occurs in a 20° cone of vision extending from the driver’s eye at that point (L20). The human eye adapts more slowly from light to dark than the other way around so, in daylight hours, a driver will experience a ‘black hole’ effect on approaching the tunnel threshold, unless the correct amount of luminance is provided within the threshold of the tunnel.
The length of the threshold zone from the tunnel portal is again normally determined by the stopping distance of vehicles at traffic design speed. The luminance within it (Lth) can be reduced to a percentage of the exterior daylight value by applying a factor (k) determined by the class of tunnel and can continue to decrease from halfway along the threshold zone. Control of the level of this luminance, and of the luminance levels in the rest of the tunnel, will depend on the exterior (L20) luminance and be a ratio of this value. In modern lighting schemes, this level is controlled by a photometer(s) sited at the tunnel entrance portal.
Further reductions in luminance levels are achieved in multiple stages within the transition zone(s) until the interior zone is reached, where the lighting level is at it’s lowest economical and practical level. Stages in transition must not lower the levels by more than 3:1, as this is the maximum ratio to which the human eye can safely adapt. The transition zone ends at the point where the luminance is no more than 3 times the level of the interior zone.
Frequently the longest zone in the tunnel, the luminance here (Lin) is maintained at a safe level linked to the class of the tunnel. The whole of the tunnel is in fact an interior zone during the night.
As the human eye can adapt from low light levels to high light levels quickly, the exit zone provides a minimum transition zone, at a length of kph = m, with a luminance level of 5 times that of the interior zone.
There are several options in lighting distribution control, determined by the luminaire optics and requirements of the application, such as the road surface and tunnel wall reflectance, multiple lanes and the safety requirement to provide contrast between the surface and objects in the driver’s field of view. i-TunneL® is able to provide luminaires with optics designed to meet the distribution requirements of any tunnel scheme requirement or standard.
One safety aspect to be noted is the spacing of luminaires to avoid any distracting flicker effect as they enter and leave a driver’s field of view. Frequency of between 2 and 15Hz are particularly dangerous, where a driver might suffer sensitivity to the frequency that could affect concentration or are prone to epilepsy.
The most widely used lighting technique employs luminaires mounted centrally over the carriageway with a symmetrical distribution, with the main beam plane either parallel to the road axis or at 90° to the axis. This style is used for both threshold/transition zones and in the interior zone of a tunnel. For improved maintenance access, the luminaires are sometimes mounted in the junction of wall and ceiling; in this way only one lane of the carriageway need be closed.
The luminaire uses an asymmetrical beam that is directed against the flow of traffic. This results in a high object contrast so showing up as a dark object against a light background.
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i-TunneL® deliver complete tunnel lighting and control solutions. When you partner with i-TunneL®, you are choosing the best designed luminaires in the industry.