A: Please see the Where do I start form for a complete list of the information needed to specify a lighting scheme. Some of the most important include: Tunnel dimensions and orientation, number of bores; road and lane dimensions and number; Access Zone Luminance (L20); luminaire mounting preference; maintenance factor to be adopted; national/international lighting standards to be adopted.

A: These are luminance meters for measuring the ‘brightness’ of light in a specific field of view. Positioned outside the tunnel, the photometer interprets how bright the entrance portal appears to an approaching driver, within a 20degree conical view. This data is then sent to the control system, which in turn switches the appropriate lights on and off, illuminating the entrance portal sufficiently for the approaching driver to see any obstructions ahead.

A: In daylight, the driver approaching the tunnel is coming from a high luminance environment to a low luminance environment. For safety and to help maintain traffic flow, the driver must be able to see any other vehicles positioned a short way inside the tunnel portal (at least the safe stopping distance). As the driver enters, the eye adapts to the lower light intensity, which takes time depending on the difference between the exterior and interior levels. The safe rate at which the light intensity can reduce is defined by the Luminance Reduction Curve, and is defined in national and international standards.

A: Tunnel lighting generally requires several stages of lighting, each with a different requirement for luminance level that reduces down the length of the road. Because the system requires more than a simple on and off switch, a control system must be adopted in order to allow the tunnel lighting to work effectively.

A:

Contactor systems

Up to a few years ago, and in many older tunnels, the photometer was linked to a controller that automatically switched different groups or stages of luminaires and lamps within the tunnel on or off to create the changes in luminance level. However, the more stages required to achieve better adjustment to external conditions, the more wiring and electrical switchgear, contactors etc were required with the associated cabling. This was very costly to invest in, install and run, and the complexity of cabling etc meant more maintenance.

Addressable Lighting Control

With the introduction of computer controlled lighting systems, with individually addressed controllers situated in each luminaire, the old switching stages with heavy mains wiring requirements are redundant. Lighting changes can be made very subtly. As these systems greatly reduce the amount of electrical installation equipment and work required, and mains power, there are great cost savings to be made. Two way communication is also possible via the signal bus wire, allowing faults to be reported and maintenance requirements to be assessed from the control room.

A: Theoretically speaking, continuous dimming is the most efficient way in which to light a road tunnel as the lighting can match the required luminance reduction curve exactly at any time. But, as found in many systems, theoretical performance is often not translated perfectly in application; continuous dimming in tunnel lighting being testament to this. One major factor to consider is that any lamp when running at full power is at its most efficient. Therefore, switching lamps off opposed to dimming can have major benefits, especially when considering dimming lamps can reduce lamp life in certain circumstances. Furthermore, high wattage HPS lamps are only generally available with hybrid dimming gear which does not benefit from the same energy savings possible with full electronic gear (only readily available up to 150W). Another problem encountered is the fact that high pressure sodium lamps can not switch on into a dimmed state. These lamps require a burn up time, commonly in the region of 10 minutes, before they can then be dimmed to the desired level. Therefore, at a change of stage, the lighting is going to be too bright in contrast to the outside environment for a period of time and using more energy than necessary, undermining any possible savings when in the dimmed state. In spite of this, i-TunneL® can offer both switching and continuous dimming systems.

A: A system using single layer communications, such as the i-Tunnel® control systems, requires only one software protocol and one hardware protocol, opposed to dual layer communications. Because of this, the single layer system is simpler and therefore installation and maintenance are in turn, simpler.

More complicated dual layer systems, such as hybrid DALI forms, can over complicate the matter and due to their complexity, specialist engineers or specially trained personnel are required.

A: The most efficient way to light a tunnel is to mount the luminaires overhead, centrally above each lane. Although, in some cases, due to tunnel geometry, height restrictions can prevent any luminaires from being mounted directly over the carriageway. In this circumstance, the luminaires are then normally mounted in the cornice, which requires either an asymmetrical reflector or the luminaires to be mounted at an angle. Although less efficient in light distribution, cornice lighting is sometimes preferred for ease of maintenance reasons as only one traffic lane may need to be closed to do so.

In mounting luminaires, spacing is important to avoid flicker of passing bright points of light at speed. This can induce mild to occasionally severe discomfort in certain susceptible individuals. The frequencies to be avoided are generally between 2.5 and 15Hz, but national and international standards will give guidance.

A: The maintenance factor, or sometimes light loss factor, is the term that considers the way in which the lighting performance of the installation deteriorates over time due to lamp output reductions and dirt on both the luminaires and tunnel surfaces. It is measured by taking the ratio of illuminance/luminance after a specific operating time compared to the initial illuminance/luminance. Typical assumptions for the maintenance factor are 0.7, or a loss of 30% from the initial scheme output. In other words the design is carried out to still give the required levels just prior to re-lamping.

If a calculation has used a maintenance factor, the calculated values are then ‘maintained’ values, as the installation should never drop below these levels if it is functioning correctly.

A: All emergency situations such as electrical failure, fire, evacuation etc, are extensively detailed under the appropriate national and international standards. These would include use of uninterruptible power supplies, fire-rated cable, battery run safety and emergency lighting rated to withstand local fires for specified periods and many more. Due to recent incidents, international safety standards are being revised upwards.

A: The interior environment of a road tunnel is highly aggressive, so the materials of construction for a luminaire need to deliver a design life of up to 30 years under these conditions. Materials such as high grade aluminium have been shown to offer the resistance required. High environmental protection factor design is necessary – IP66 and above is recommended to resist dirt and moisture ingress. Other conditions to be resisted, depending on the location, include extremes of heat and cold, so thermal dissipation of the lamp and control gear heat emissions can be critical.

A: The major parameters to consider are efficacy in use (light output per watt of electricity), service life and high reliability. Although, these are not definitive, factors such as the rendering and colour appearance of the lamps can improve the ‘comfort factor’ for a driver within the tunnel.

Some details of lamp types are listed below:

Tubular fluorescent lamps (TLD)

• Colour temp in excess of 3000K (warm to cold white)
• Efficacy of approximately 90lm/W
• Average lamp life ranges from 30,000 – 70,000 hours
• Wattage ranges from 36W – 58W
• Good rendering characteristics
• Can warm re-strike
• Generally used for interior & night time lighting

High Pressure Sodium (SON, NAV)

• Colour temp approximately 2000K
• Efficacy ranging from 110-140lm/W
• Average lamp life ranges from 20,000 – 30,000 hours
• Wattage ranges from 70W – 400W
• Poor rendering characteristics
• Cold re-strike only
• Commonly used at all stages of a tunnel, especially entrance lighting

Ceramic Discharge (CDM, HCI, CMH)

• Colour temp approximately 3000K (warm white)
• Efficacy ranging from 80-100lm/W
• Average lamp life ranges from 9,000 – 12,000 hours
• Wattage ranges from 35W – 400W
• Good rendering characteristics
• Cold re-strike only
• Commonly used at all stages of a tunnel