While most other lighting applications are completing their transition to LED light sources, manufacturers of floodlights illuminating soccer fields, tennis courts and other large outdoor sports fields have thus far maintained a broad preference for conventional high intensity discharge (HID) lamps. HID lamps offer high flux and high quality light output, and electrical and optical considerations are easy for projector designers to manage.
ArenaVision MVF404 metal halide lamp from Philips Lighting, part of its range of high flux HID lamps for sports lighting. (Image credit: Philips Lighting, a Signify brand).
However, there are signs that the tide is changing in favor of new and improved LED solutions and drivers.So what are the factors encouraging the switch to LED light sources in sports field lighting? And what is the difference between designing an LED sports field lighting system and a HID projector?
High cost of replacing a HID lamp
There are two good reasons why stadium and sports arena managers will be glad to see the end of HID lamp technology. The first is the limited operating life of the lamps: a typical B50 life (mean time before failure) of only 5,000 hours. This means that HID projectors require frequent lamp replacements. And because the lamp being replaced is very likely to be atop a narrow 25m-high pole, special access and safety devices and skilled technicians are required – all at a considerable cost.
The other major drawback of HID lamps is their behavior in the event of an unexpected shutdown, such as when there is a stadium power circuit failure. HID lamps require a 10-15 minute cool down period before restarting and then another 5 minutes to reach full brightness. This period of darkness and partial lighting is potentially catastrophic if a power failure occurs during a match with a large crowd: keeping a crowd of tens of thousands in a darkened stadium, with the match suspended, could have extremely serious consequences for safety and public order.
An LED light source eliminates both problems: the life span of an LED light source in a projector depends on how the design is implemented, but will typically be between 50,000 and 100,000 hours; furthermore, after a mains power failure, when the power is switched on or restored, the LEDs can be turned on to full brightness immediately.
There are other important advantages of using LED light sources for sports field lighting:
- Efficacy: An LED system can achieve a light source efficacy of over 120 lm/W, while the nominal efficacy of a HID lamp is around 90 lm/W.
- Low Voltage Starting:An HID lamp requires a high starting voltage of approximately 5kV. This requires careful implementation of safety and circuit protection functions. Also, starting HID lamps at temperatures below 0°C can be difficult. Such a high trigger voltage is not required for LEDs .
- Dimmability:HID lamps are not dimmable. Conversely, the emission of LEDs can be easily dimmed down to very low levels. This is useful for saving energy when taking advantage of daylight; or when, at junior training sessions or matches, with fewer spectators than competitive senior matches, a lower level of lighting can be provided.
- Dynamic lighting effects:Because LEDs can be switched on and off instantly, stadiums can provide dynamic lighting effects to cause excitement in the crowd, such as when a goal is scored or at the start or end of a power play period .
LED floodlight system design considerations
The advantages deriving from the use of LEDs in headlamps are therefore considerable. However, the nature of the application places particular constraints on the design of high-mast LED lighting; until recently, these have hindered attempts by OEMs to use LEDs in sports field lighting. Over the course of 2023, however, LED , driver and optics suppliers have introduced a number of new products that address the particular needs of high-mast stadium lighting and, as this article shows, it is now easier than ever to create an effective floodlight design based on an LED light source .
The difficulty of making LED projectors comes from a stringent set of requirements. The playing area requiring uniform shadow-free lighting is large: for tennis, the court itself is 260 m2 and the total area, which includes space for line judges, seating for players and so on, it is about 680 m 2 . For football, the space is even bigger: the pitch is around 7,000m2 and the total illuminated area is around 10,000m2. The lighting requirement depends on whether the pitch is for amateur use, requiring less bright light, or for a top-level professional spectator sport: in the latter case, a light intensity may be required on the play area of over 1,000 lux. Depending on the height of the pole and the size of the playing area, a light output that can range from hundreds to thousands of kilolumen from the light source is required.
Furthermore, this light emission must be provided by the smallest possible light emitting surface. This is to avoid obstructing the view of spectators: for pole-mounted lights, the pole should be as narrow as possible and this results in a small light source which will impose the least possible wind load on the pole; when mounted on the roof of a stadium, the lamp must be small and light, as the roof structure of a stadium is not normally designed to support heavy objects.
In professional stadium lighting, the quality of light is also strictly defined by the TV broadcasters. The Television Lighting Consistency Index (TLCI) was developed by the European Broadcasting Union (EBU) to measure the ability of a light source to render colors adequately when viewed from a video camera. There is a correlation between the TLCI and the color rendering index (CRI), which measures the accuracy of color rendering as perceived by the human eye rather than a camera. LED specifiers , however, should not assume that a high CRI LED also meets TLCI requirements.
The HID lamp is a particularly effective solution to the problems of size and weight: a single 2k HID lamp produces between 150,000 and 200,000n, so only a few lamps need to be mounted on each pole.
When designing LEDs , the challenge is to keep the light emitting surface area (LES) as small as possible, which is difficult when using general purpose LED chips, which typically produce around 300 lm each. With such devices, a high-mast floodlight would require too many units and take up too much area.
Placement of high power drivers
The design challenge is not limited to the selection and integration of LEDs : the choice of LED power supply also has serious consequences.
Two important factors determine the choice of LED drivers . First, the driver delivers a high load: potentially more than 500W in professional football stadium lighting systems, for example. No driver will be 100% efficient and power loss is dissipated by the driver as waste heat. A 500W driver operating at a high efficiency of 95% will therefore generate 26W of waste heat. If not properly managed, this could cause the power system to exceed its safe thermal shutdown threshold.
LED luminaire designers tend to think of an integrated driver unit as a self-managing black box that does not require external terminal management. In this case, however, the designer would do well to consider how large amounts of heat can be safely drawn from the driver enclosure to the ambient air without superheating the air contained in the system enclosure.
An Inventronics EFD-1K2SxxxDV 1.2k LED driver weighs 6.6 kg
The other concern in driver selection is weight: the lighter the luminaire, the narrower the pole can be, and the less structural support is needed for canopy lighting. Inventronics ‘ EFD-1K2SxxxDV series of 1.2k LED drivers, for example, are an excellent choice for high-flux lighting installations. Weighing 6.6 kg (see figure 3), it is lighter than an equivalent power system consisting of multiple drivers, each with a lower power rating.
An alternative to mounting the driver at the top of the mast, close to the LEDs , is remote mounting at the foot of the mast. This is a new approach to LED power system design , for which there is no requirement in other applications. Due to the long distance control signals have to travel between a remote driver and the LEDsremote mounting may affect the feedback loop operation, causing instability in the output power and, in turn, a marked reduction in light output. In addition, the designer will need to account for the voltage drop along the cable and manage the electromagnetic compatibility (EMC) requirements of the system. Driver manufacturers, however, are starting to consider remote mounting requirements when developing new specialty driver products.
In their choice of LED drivers, high-mast fixture designers will also consider the specification for output ripple, which causes light to flicker and is therefore vitally important for broadcast events: it should be limited to less than 2%. It is also important to consider all operating conditions, especially cold weather starting. The forward voltage of the LEDs is higher in cold weather than in a normal warm operating temperature. Designers should leave significant headroom in the driver output voltage specification above the level expected in normal operation.
The choice of dimmable interface is also important for enabling dynamic lighting features, such as flashing or dimmable patterns. Fixture manufacturers tend to look for DMX control due to its faster response time than DALI. Few high power drivers offer a DMX interface making 0-10V DMX converters such as the Inventronics CNV-DMXR invaluable.
Optical design considerations
AIKO sports lighting lense
The last important system element to consider in sports field lighting is beam control. In a typical four-pole topology, with one pole at each corner of a rectangular playing area, the light must accurately illuminate one quadrant of the floor area from a great height.
Traditional LED collimators provide excellent beam control from a small point on the light source. Small optical losses in a collimator are dissipated as waste heat. In very high flux systems, this generates a lot of heat, even too much for the polycarbonate (PC) or PMMA materials normally used in LED lenses , which tend to warp at high temperatures.
Here too, component suppliers have developed specialized products suitable for sports field lighting. For example, AIKO’s special lense for LED sports lighting.
A different approach is to use floodlights, but the need to leave sufficient air and creepage distance between the floodlight and the high voltage LED arra may result in some optical losses such as light leaks behind the leading edge of the floodlight.
Improved total cost of ownership and much more
As is often the case with LED technology , the initial additional cost of LED- based sports field lighting will be recouped over the life of the system through reduced maintenance and replacement costs and lower energy consumption. The quality and flexibility of the operating controls of an LED lighting system provide important additional benefits.