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

Dec 08, 2017

The main purpose of General tunnel illumination is to prevent accidents by providing maximum visibility of objects on the tunnel surface and to enhance safety and comfort for drivers who pass through or enter the tunnel. 

The CIE 88-"road tunnels and underground tunnel lighting Guide" provides a guideline for tunnel lighting and is the basis for many national regulations. It divides the tunnels into different areas according to the specific requirements of each. 

First, the entry section requires a very high brightness value so that the driver's eye can adapt to the darker lighting conditions in the tunnel. The length of the entry section is determined by the maximum speed limit in the tunnel, since the higher the speed limit requires a longer entry section. 

Secondly, in the transition section, the brightness of the road is lowered slowly until the illumination level is reached in the inner area of the tunnel, thus providing smooth adaptation from the entrance to the middle section. In the first half of the entry section, the maintenance factor is 0.67, the illumination is about 150-300 $literal, and the inner area is usually limited to $number $literal. 

At present, the quality parameters of the road tunnel illumination include: the uniformity of the overall and longitudinal road surface illumination (ideally, no strobe), the illumination of the tunnel wall, the avoidance of glare and the color-color index. In recent years, the development of semiconductor technology, LED gradually replace the traditional tunnel lamp. Many national standards update the requirements of these parameters. 

New LED Customization Standard 

In addition to the rules proposed by the Swiss Astra (Bundesamtfürstra?en), the An's plan manual (PLAPB 800.562) is the first one to specifically target LED tunnel lighting systems. This standard introduces clusters into different tunnel lighting categories, detailing not only the different lighting requirements for each cluster, but also the different distances from 18 meters (cluster standard) to continuous lighting. It also makes different demands on energy efficiency, illumination color and colour rendering. Although color rendering does not seem to be the most important index in tunnel lighting, the rapid distinction between yellow and red, or blue and green, is essential for drivers, given the increasing number of different signal colors in the road tunnels. 

Moreover, compared with the traditional illumination system, the LED system's advantage also manifests in the true color illumination, the high luminous efficiency and the high efficiency lens distribution. The LED system can provide analog dimming in the tunnel interior and inlet area, thus eliminating the problem of uniformity of road brightness caused by turning off the whole set of lamps. The current LED tunnel system has become an intelligent solution that can communicate the current state of the luminaire to the driver. However, there is still discussion about where the need to integrate support and control of electronic equipment needs to be integrated. Some tunnel markets require intelligent drivers to be installed inside the luminaire, while others require that all electronic equipment be installed in the tunnel maintenance or in the control building at the entrance. Both of these solutions have advantages, and the latter reduces maintenance efforts. 

Basically, in Germany, Austria and Switzerland, the general characteristics and requirements of tunnel lamps from CRI 20-30 to CRI to 70 or even 80, 4000-4500 K led, and even some current projects need to function even more than $number LM, system life of more than 80, 000-hour system with a failure rate of less than 10% during this period.

Figure 1: Adaptive curve of tunnel brightness 

Requirements outside of luminaire specifications 

But some requirements, though important, are not enough to be met by lighting alone. Especially early LED lamps are more dazzling than traditional lamps, so avoiding glare becomes more and more important. 

Some rules already require a threshold increment of 8% or even 6%, because lower threshold increments mean less glare. The threshold increment is the relative relation between the brightness of the light curtain and the brightness of the road, the lower the brightness of the light screen or the higher the brightness of the road can reduce the glare. The brightness of the light screen itself depends on the luminous flux and light distribution of the luminaire. 

Lamp manufacturers can develop lamps that maximize the brightness of the road and also provide a relatively low light screen brightness, but there is a key factor that cannot be controlled. This is the brightness coefficient q0 of the pavement, which determines the relationship between illuminance [LX] and brightness [$literal]. According to the grade of the road, the brightness coefficient usually varies between 0.05 and 0.07. Therefore, q0 can change the brightness value of about 30% on the road. In turn, the luminous flux of the luminaire must be increased by 30%, resulting in higher brightness and higher threshold increments.

Effectiveness requirements and how to achieve 

A meaningful, but not necessarily achievable, requirement is luminous efficiency, or LM. This value perfectly describes the efficiency of LEDs, and higher luminous efficiency represents higher energy efficiency. This is also true for all tunnel lamps with symmetrical light distributions, which are generally applicable to the inner area of the tunnel. The luminous efficiency is the best choice for determining the energy efficiency of the entire luminaire, as the transmission value of the different optical systems in the market is nearly 90% and the light distribution provides similar brightness. But what about the light fixtures in the entrance section and the transition area? 

Typically, these lamps are equipped with reflective elements that reflect the light at a very horizontal angle to the driver. 

Compared with the symmetrical light distribution of lamps, reflective lamps provide a better brightness value. Therefore, reflective lighting in the past has been the choice of tunnel entrance area, both economical and efficient. However, these lamps can only be used in the entry and transition areas, because in these areas, the lamp distance is small. In the middle section, the reflective luminaire will produce too much glare in the 8-15m illumination distance and also produce the uniformity of the light. 

As shown in Fig. 4, the peak intensity of the reflective luminaire is in the vertical angle above 60°, and more importantly, the light is almost entirely in one direction. This "light bending" can lead to reflection loss, thus reducing transmission, resulting in reduced luminous efficiency of lamps. The luminous efficiency value is not taken into account for luminaires in the inlet and transition sections. Assuming the same lumen input, the 60° above the peak of the reflective lamp will produce a very high brightness on the road, while the 55° will be small, although the latter may have a higher transmission value and higher luminous efficiency.

It is also important to note that a luminaire with a peak intensity at a higher vertical angle does not necessarily produce more glare. 

Therefore, the specific surface power density is more suitable for describing and requiring the energy efficiency of the tunnel entrance illumination, including the known brightness value, the length of the inlet section and the transition area, and the power consumption of all installed inlet (reflective) lamps. 

Almost all the new standards for LED lamps are moving in the right direction, making lighting quality steady, and, of course, making lamps safer, more energy efficient and more sustainable. 

At present, the most advanced High-tech lighting, made of high-grade technical glass, can provide more than 0.95 longitudinal uniformity within 8-12 meters of luminaire distance. This means that the brightness values in the middle of the line Lane will hardly change in the tunnel, thus virtually eliminating any strobe on the road. 

The latest generation of tube systems can provide up to 125 lumens/watts of power and separate the lighting in the driving area from the power supply and control electronics in the maintenance room. But there are challenges that are not just technical, but also evolving.

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