Since the 1990s, energy-efficient solid-state fixtures have been replacing incandescent-based traffic lights. Today’s power LEDs are now poised to tackle the challenge of street lighting. The first LED street-light installations are already being tested, and in some cases implemented, in an effort to replicate the energy and maintenance savings of LED-based traffic lights.

With a 10- to 15-year lifetime, tripling that of current technologies, the maintenance advantages alone offer a strong financial argument for transitioning to LED-based systems. Considering then the design flexibility, sustainability and power-savings enabled by LED-based systems, then the case for solid-state street lighting is compelling.

To fully examine these potential benefits, however, an example needs to be considered.

 

 

Example design

To quantify the potential performance benefits of LED-based street lighting, Philips Lumileds built a demonstration lamp using LUXEON® Rebel LEDs (Figure 1). In this design, 50 cool-white LEDs were placed in rows and installed at varying angles to achieve the required light coverage. Each LED was driven at 350mA, with a light output of roughly 90 lumens and with a secondary collimating optic. To achieve the 60,000-hour life expectancy of the LUXEON Rebel LEDs, Philips Lumileds designed their demonstration street lamp to produce an LED junction temperature of 62°C, which is well within the required range.

Incidentally, the LUXEON Rebel was chosen for demonstration purposes because of its small footprint, high operating junction temperature and lifetime. The small size and form factor of the package enabled maximum light and packing density as well as exceptionally tight coupling of the optic to the LED – both critical factors in achieving desired luminance levels while also minimising flux loss caused by diffraction of light through the optical lens.

This demonstration was designed to comply with European specifications for residential and city-centre street lighting, including average luminance levels and overall luminance uniformity. All of the European benchmarks were reached or exceeded by this demonstration design, proving the potential of today’s LUXEON LEDs for street-lighting applications.

 

 

Performance comparison

To see how the prototype LED-based street light fares against contemporary street-light technologies, a comparison of their performance and any additional benefits is required. Figure 2 shows how the solid-state street-light demonstration stacks up against the two dominant lighting technologies used for street lighting.

With power consumption of just 67W, the LED-based demonstration lamp cuts energy consumption by 52% compared to mercury-vapour lamps and 26% compared to high-pressure sodium fixtures.

The 50lm/W efficacy of the LED-based version exceeds the mercury vapour’s 31lm/W, contributing to energy savings. It is worth noting that the higher efficacy measured for high-pressure sodium lamps, however, is misleading because it includes wasted light that in turn wastes energy. In fact, the LED-based version more than doubles the lux per watt of a mercury-vapour street lamp. The lux rating for high-pressure sodium lamps is skewed by hot spots that bump up the measurement but cause undesirable glare problems as shown in Figure 3.

 

 

Cost of ownership

The longer lifetime achieved with LUXEON Rebel LEDs translates into a 10- to 15-year life expectancy, depending on the duration of darkness in the specific geographic location. In contrast, conventional street lamps burn out after three to five years.

One of the most compelling advantages of LED-based street lamps would be the reduced cost of operating and maintaining the fixtures. This comes from the longer replacement cycle as well as reduced energy costs.

Even with the higher initial cost of a solid-state luminaire, installers could recoup the costs of an LED-based street lighting installation in four to six years (Figure 4). The return on investment would be even faster if the cost of power increases. Also, as solid-state technology advances, the light output offered by LEDs will increase, making it possible to deliver more lumens per watt for additional energy savings.

 

 

Light pollution levels

The LED demonstration also showed a more even light distribution than either of the alternatives, as indicated by the ratio between the minimum and average lux produced in the target zone (see Figure 3). Because of its ability to precisely direct light, the LED-based solution was the only one of the three to meet European street-lighting guidelines on uniformity.

LED-based street lighting, therefore, can go a long way towards eliminating hot spots and wasted light. This not only improves safety and visibility, but also reduces the lumen requirements of the luminaire.

A traditional filament emits light from a single source. Shields, reflectors and lenses are used to point the beam in the desired direction with limited control. Inevitably some light spills outside of the target zone wasting energy and creating light pollution. The light is also brighter in the middle of the target zone and dimmer at the edges, and the light concentration in the centre can be a strain on the eyes of drivers and pedestrians.

In contrast, the small LUXEON Rebel package allows the use of multiple light sources with individual optics. Each LED can be targeted to a specific area or position, providing more uniform coverage as well as eliminating central hot spots and glare. Figure 3 shows the difference in light distribution between street lamps built with LUXEON Rebel LEDs and those based on mercury-vapour bulbs.

These diagrams highlight the difference between the LED demonstration and conventional lighting. The mercury-vapour example has a hot spot in the middle, corresponding to the area directly beneath the light pole, and very little illumination at the outer edges. The LED light distribution pattern avoids both extremes and instead remains relatively consistent. This is highlighted in Figure 2 where the demonstration lamp built with LUXEON Rebel LEDs surpasses both high-pressure sodium and mercury-vapour solutions for uniformity, expressed by the minimum/average lux ratio.

 

Colour rendering and colour temperature

Finally, this demonstration design also showed that LED street lights can offer the benefit of more natural colour rendering, which can help improve safety and security. With a Colour Rendering Index (CRI) of 75, the LUXEON Rebel demonstration lamp makes it easier for drivers and pedestrians to see street signs and other objects in the illuminated area. Given that this can make people feel safer and help prevent accidents, the importance of CRI cannot be under-estimated.

The availability of cool-, neutral- and warm-white LEDs adds the option to adjust the colour temperature to the specific application. A neighbourhood sidewalk might require a warmer colour temperature, while a cooler LED might be used for roadway lighting to maximise efficiency. These options are not available with conventional lighting technologies.

 

Conclusion

Taken together, the capabilities demonstrated by Philips Lumileds’ streetlight prototype create a strong case for using power LEDs in street lighting. LED street lamps can meet all regulations for luminance level and uniformity, deliver significant energy savings, extend fixture lifetime, produce more usable light, lower the total of ownership and more. LEDs have come of age for traffic signals, automotive lighting and a host of other products. They now deserve serious consideration for roadway and pedestrian lighting.

 

Philips Lumileds / Design Note