In most outdoor lighting applications, LEDs prove their advantages. For tunnel applications, the point sources integrated in solid state lighting are specifically matched. In this article, Dale Kane, Lumileds Global Markets and Business Development Manager, talks about the specific challenges that tunnels bring and how to solve demanding lighting requirements by combining LEDs with lenses.
The words "darkness" and "gray" properly describe the illumination of many road tunnels in the world. So when you see a bright tunnel with no shadows or black spots, you might wonder why all tunnels don't have such good lighting conditions. In fact, new tunnels in many parts of the world have shown that well-designed LED tunnel lighting can help drivers quickly and easily adjust their vision, minimize maintenance, and save energy while improving light quality.
We will discuss some projects that demonstrate the value of LED-based tunnel solid-state lighting (SSL). There are two main considerations for whether tunnel lighting should be upgraded with LEDs. Municipalities often focus on life cycle costs and the quality of light.
The total cost of ownership of public lighting depends on several factors such as illuminance, power consumption, equipment cost, installation cost, maintenance cost, luminaire form factor, and expected life of the luminaire. While other quantifiable factors are also conceivable, they are primarily the decision of the competent authority.
Solid state lighting provides high quality lighting
With the maturity of solid-state lighting technology, the quality of this lighting is very beneficial for use in tunnels. Compared to commonly used outdoor lighting products, LED lighting has excellent color rendering.
Road users and pedestrians attach great importance to light quality. Some cities and public utilities have chosen LED lighting to improve the commercial area, making it more attractive, adding security and comfort.
In tunnel lighting, it can be said that total cost of ownership and visual perception are factors that need to be considered. However, given the state of public funding, lower total life cycle costs must be achieved. Fortunately, the development of LED technology has reduced the procurement and power costs of LED tunnel lighting systems. Compared with traditional high-pressure gas discharge sources (HID), LEDs have significantly reduced life cycle costs, while also having other advantages of solid-state lighting systems. .
Comparison of LED and traditional lighting
At present, traditional high-pressure gas discharge sources (HID), such as high-pressure sodium lamps (HPS) and metal halide lamps (MH), are still the main force of tunnel lighting. Tunnel lighting solutions require a higher minimum illumination to provide a safe environment for the user. The luminaire must be able to provide a high level of illumination at a considerable distance from the plane. Typical installation heights for road tunnels are 5-7 meters, providing clearance for high-side vehicles.
High-pressure sodium lamps and metal halide lamps are attractive to tunnel lighting solutions designers, offering both low unit cost and high illumination. An array of high pressure sodium lamps and metal halide lamps, typically up to 400W per luminaire, to meet illuminance requirements.
However, the disadvantages of high pressure sodium lamps (HPS) (Fig. 1) and metal halide lamps (MH) are obvious. These two types of luminaires consume a lot of energy, usually low in light quality, and do not provide uniform illumination in the tunnel, and if a failure occurs, an expensive maintenance solution is required for remediation. Now, with the advancement of LED technology, tunnel operators have a new choice, which is increasingly attractive from a public and operational perspective.
Figure 1 High-pressure sodium (HPS) tunnel illumination does not provide consistent light levels and poor light quality
The most obvious advantages of LED-based tunnel lights are their long life and high efficiency:
• High quality LED lighting systems are capable of delivering >100 lumens per watt in typical tunnel lighting applications. Since the LED is a directional point source, its light output can be accurately transmitted to the desired area, providing additional system efficiency compared to the HID system.
• LEDs are very reliable semiconductor devices with very long life and greatly reduced maintenance and service costs.
Superior lighting environment
The benefits of LED lighting in road tunnels fall into two main categories: light quality and reduced impact on traffic flow. The high quality light of LED luminaires is largely due to the excellent color rendering of LEDs. For example, Lumileds' ultra-efficient Luxeon T LED has a minimum CRI of 70 and a high-pressure sodium lamp has a common CRI of 20. When comparing these two technologies, LED-based solutions create a more natural environment.
A more realistic display of colors in the spectrum can help drivers better distinguish distant objects than orange-brown light from high-pressure sodium lamps. Even in the case of the same absolute illumination level, the environment feels safer and more comfortable.
Uniform illumination of the entire tunnel is another benefit of the LED solution. The lens design developed using advanced optical stencils provides a uniform pattern of light on the road, with neither occlusion nor interference glare. This consistency is achievable because LED luminaires consist of a small but powerful array of light sources that can be beam patterned as needed, such as using a total internal reflection (TIR) ​​lens on each LED to form the desired beam. In the entire space, a smoother, more consistent illumination is created with no black spots or bright spots on the road surface.
Entrance and exit lighting
LED tunnel luminaires also enhance light control in the transition inlet and outlet areas to make the driver's eyes more adaptable. Traditional high-pressure sodium lamps and metal halide lamps are designed to simulate the illumination level of full illumination, turning off a portion or a full segment of the fixture during the day based on external light levels (Figure 2).
Figure 2 In order to achieve the required tunnel entrance light level, high pressure sodium lamps and metal halide lamps are usually too bright, and LED lamps use dimming to accurately reach this level.
If the high-pressure gas discharge lamp is to match the light level outside the tunnel, the brightness will be too bright for most of the day, which is far from the required level of illumination. The main reason is that HID lighting cannot be accurately controlled. Instead, the LEDs are fully dimmable and precisely match the level of illumination required to eliminate over-illumination and reduce energy consumption.
Because the LED is very durable, it usually does not malfunction like the HPS or MH lamp, so it is not necessary to close part of the tunnel, set up a high-rise platform, etc., to keep the road open.
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