What are the technical challenges in the rise of LED tunnel lighting?

The main purpose of general tunnel lighting is to prevent accidents by providing maximum visibility of objects on the tunnel pavement and to enhance safety and comfort for drivers passing through or entering the tunnel.  CIE 88- "Guide to The Lighting of Road Tunnels and Underpasses" provides a guideline for tunnel lighting and is the basis of many national regulations.  It divides the tunnel into different areas according to the specific requirements of each.  

First, the entrance section needs very high brightness values so that the driver's eyes can adapt to the dark lighting conditions in the tunnel.  The length of the entrance section is determined by the maximum speed limit in the tunnel, since a higher speed limit requires a longer entrance section.  

Secondly, in the transition section, the brightness of the road surface slowly decreases until it reaches the illumination level of the tunnel interior area, thus providing a smooth adaptation from the entrance to the middle section.  In the first half of the entrance section, consider a maintenance factor of 0.67 and lighting of about 150-300 CD /m2, while the interior areas are usually limited to 2-6 CD /m2.  

Currently accepted quality parameters for road tunnel lighting include: uniformity of overall and longitudinal road surface lighting (ideally no stasmowing), tunnel wall lighting, glare avoidance and color temperature rendering index.  In recent years, with the development of semiconductor technology, LED has gradually replaced the traditional tunnel sodium lamp.  Many national standards have updated the requirements of the above parameters  

New LED customized standard

In addition to regulations such as those proposed by ASTRA of Switzerland (BundesamtfurStra ßen), the Plan manual of ASFINAG of Austria (PLaPB 800.562) is one of the first standards specifically for LED tunnel lighting systems.  This standard introduces clusters into different tunnel lighting categories, not only specifying the different lighting requirements for each cluster, but also specifying the different distances of lamps from 18 meters (cluster standard) to continuous lighting strips.  And it also has different requirements for energy efficiency, lighting color and color rendering.  Although colour rendering may not seem to be the most important index in tunnel lighting, given the increasing number of different signal colours in road tunnels, it is crucial for drivers to quickly distinguish between yellow and red, or blue and green.  

Moreover, compared with traditional lighting systems, the advantages of LED systems are also reflected in true color lighting, high light efficiency and efficient lens distribution.  The LED system provides simulated dimming inside the tunnel and in the entrance area, eliminating the problem of road brightness uniformity caused by turning off the entire set of lamps.  

The current LED tunnel system has become an intelligent solution that can communicate the current status of the lamp to the driver.  However, discussions are still under way as to where support and control electronics will need to be integrated.  Some tunnel markets require the installation of intelligent drives within the lamps, while others require all electronics to be installed in maintenance rooms or control buildings at the entrances of tunnels.  Both solutions have advantages; the latter reduces maintenance.  

Basically, in Germany, Austria and Switzerland, the general characteristics and requirements of tunnel lamps have changed from SODIUM lamps with CRI 20-30 to LED lamps with CRI 70 or even 80, 4000-4500 K,  Even some current projects require systems with efficiencies of more than 110 lm/W and system lifetimes of more than 80,000 hours (with a failure rate of less than 10% during that time).  

But some requirements, important as they are, cannot be met by lighting alone.  In particular, early LED lamps were shinier than traditional lamps, so avoiding glare became increasingly important.  Some regulations already require a threshold increment of 8% or even as little as 6% because a lower threshold increment means less glare.  The threshold increment is the relative relation between curtain brightness and pavement brightness. The lower curtain brightness or the higher pavement brightness can reduce glare.  The brightness of the curtain itself depends on the luminous flux and distribution of the luminaire.  Light manufacturers can develop lamps that maximize road brightness while still providing relatively low curtain brightness, but there is one key factor that cannot be controlled.  This is the road brightness factor Q0, which determines the relationship between illuminance (Lx) and brightness (CD /m2).  Depending on the road grade, the brightness coefficient usually varies between 0.05 and 0.07.  So, Q0 can change the brightness value on the road by about 30%.  In turn, the luminous flux of the luminaires must be increased by 30%, which results in higher curtain brightness and higher threshold increments.  

Efficacy requirements and how to achieve

One requirement that makes sense, but does not necessarily yield the desired result, is the luminous efficiency, or LM/W.  This value perfectly describes the efficiency of leds, with higher luminous efficiency representing higher energy efficiency.  The same is true for all tunnel lamps with symmetrical light distribution, which generally applies to the interior areas of tunnels.  Since the transmittance values of different optical systems on the market are similar, almost 90%, and the light distribution provides similar road brightness, luminous efficiency is the best choice for determining the overall luminance performance.  But what about the lighting in the entrance section and transition area?  

Often, these lamps are equipped with reflective elements that reflect light back to the driver at a very horizontal Angle.  

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Reflective lamps provide better brightness values than lamps with symmetrical light distribution.  Therefore, reflective lamps have been the choice in the tunnel entrance area in the past, which is both economical and efficient.  However, these lamps can only be used in the entrance section and transition areas, where the lamps are less distant.  In the middle section, the reflective lamps will produce too much glare at the lighting distance of 8-15m, and also cause problems of light uniformity.  

Conclusion  

The new standards for almost all LED lighting are a step in the right direction, leading to steady improvements in lighting quality and, of course, safer, more energy efficient and more sustainable lighting.  

The most advanced high-tech lighting fixtures, made from high-grade technical glass, provide longitudinal uniformity of more than 0.95 within the luminaries' distance of 8-12 meters.  This means that the brightness values in the middle of the carriageway barely change in the tunnel, effectively eliminating all stroboscopic activity on the road.  

The latest generation of tube systems provide up to 125 lumens/watt of power and completely separate the lamps in the driving area from the power supply and control electronics in the maintenance room.  However, there are challenges, not only technical, but also evolving regulations.

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Post time: Jun-18-2022
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