Cities of light

As solid state lighting starts to overtake fluorescent lighting for overall efficiency, two North American cities are leading the way by switching to LEDs

The capacity of certain semiconductor materials to emit light was first discovered a whole century ago, but it is only now that light emitting diodes (LEDs) are becoming efficient enough for large-scale lighting applications. LEDs have a number of clear advantages over other lighting systems used throughout the last century.

The road to solid state lighting
 The first form of electric lighting used in municipal applications was incandescent lighting, which is the same technology as the common, non-energy-efficient household bulb. It is generally described as a warm light, with the highest possible colour rendering index (CRI) of 100 – the same as natural daylight. But incandescence is also the most inefficient form of artificial light, with about 98 per cent of the energy being dissipated as heat.

Toronto, an early adopter of LED lighting, has joined a North American initiative to upgrade much more of its infrastructure to solid state lighting

In 1948, a more efficient alternative came into use: the mercury vapour streetlight. These fluorescent light bulbs had the advantage of shining much brighter than incandescent lights but their bluish-green glow, with a much lower CRI of less than 20, was deemed to be less pleasant.

Coating the bulbs with phosphors generates a warmer light, but the colour of light emitted isn’t the only problem with mercury vapour. A flickering that gets worse with age and the environmental problem of residual mercury has meant that their use has steadily declined. The sale of new mercury vapour streetlights will be illegal in the US as of 2008.

Sodium oxide technology was introduced as an alternative to mercury vapour lights in the 1970s. Emitting 200 lumens per watt (lm/W), they were – and still remain – the most efficient form of light in terms of output. However, they had an extremely low CRI and produce a very orange glow that is not widely appreciated.

It was soon realised that, by increasing the pressure of the gas in the tube, the light could be made warmer. High pressure sodium lights, with a CRI of 25, are the most widely used form of streetlight. Although the bulbs are energy efficient and low in cost, they have a relatively short lifespan. This means that sodium vapour lighting costs more in the long run than mercury vapour.

The search for an alternative form of municipal illumination in recent years has led to the uptake of metal halide lamps – the type of fluorescent bulbs that have been widely used for years in business installations, warehouses, schools, hospitals and office buildings. With a CRI of 85, metal halide casts a good white light but is less efficient than sodium oxide or mercury vapour, as well as being more expensive and less durable.

The use of solid state lighting for streetlight applications
is growing rapidly as the use of mercury vapour declines
and as LED efficiency improves

Another alternative, the compact fluorescent lamp, is occasionally used on municipal walkways and street lighting. With a CRI of 85, the light is good. However, problems include overheating, short lifespan and poor performance in cold weather.

Municipal lighting authorities are now turning to the use of solid state lighting – clusters of LEDs – instead of fluorescent bulbs. This shift is driven by a number of factors.

Solid state lighting lasts longer than any of the widely used forms of municipal lighting. This means that, over time, less material is used in the manufacture of city lighting installations and less waste is produced. This lowers their overall carbon footprint as compared to fluorescent lights.

And the energy efficiency of solid state lighting is increasing all the time. A current typical efficiency level for solid state lighting is 70 lm/W. Market-leading LED manufacturer Cree has reporting efficiency of up to 131 lm/W – approaching the efficiency of sodium oxide lamps.

Fritz Morgan, chief technology officer for Color Kinetics, a leading innovator of LED lighting systems and technologies, says:  “Technical advancements at the component level are critical to growing the emerging white LED lighting space. Cree’s results speak to the exciting developments underway that will enable new white light applications and subsequently facilitate market adoption.”

Solid state lighting also emits a pleasing and versatile range of colours. With a typical CRI of around 70, LED lighting is easy on the eye. Capable of emitting light of frequencies encompassing the entire spectrum, as well as pure white light, LEDs are ideal for large-scale municipal applications and imaginative installations that require versatility.

The LED cities
The LED City programme was launched in February 2006 by Cree and officials in the City of Raleigh, capital of the US state of North Carolina. The purpose of the programme is to foster collaboration among municipalities as they deploy energy-efficient lighting to deliver cost savings and environmental benefits.

“LED lighting enables cities such as Raleigh and Toronto to save energy, preserve the environment and save our taxpayers money,” said Mayor Charles Meeker of Raleigh. “Suppliers like Cree provide a technology that gives us access to a new generation of energy-saving lighting options for our municipal infrastructure.”

Toronto, the biggest city in Canada, joined Raleigh in the LED City programme in July 2007. On joining the initiative, Toronto converted lighting in the CN Tower, the tallest tower in the world, to more LED lighting. The tower is expected now to use 60 per cent less energy on lighting.

Replacing the lights on the tower represents a tipping point in Toronto’s gradual shift to solid state lighting. Over the last four years, the Toronto Association of Business Improvement Areas has proactively demonstrated the advantages of LEDs to Toronto residents and businesses. The use of LEDs has evolved from an installation program for retail displays to streetlight installations, such as one at Exhibition Place. Other current and planned LED projects include solar-powered LED lights in a park and LED lighting in a public parking garage. Toronto has now pledged to install light-emitting diode (LED) lighting throughout its infrastructure.

Canada’s legislative agenda is focused on energy efficiency. Toronto is a participant in the Clinton Foundation’s Climate Initiative to accelerate greenhouse-gas emissions reductions and slow global warming.

“Combating climate change is the issue of our time, possibly of all time and Torontonians are demanding that this city lead by example,” says Toronto Mayor David Miller. “Through the use of cutting-edge, energy-efficient technologies, we can and will be a leader. We expect that by deploying LEDs throughout Toronto, including on our most famous landmark, the CN Tower, we will be accomplishing the goal of reducing energy use and costs and greenhouse-gas emission.”

 “Toronto and Raleigh understand how important it is to set the pace for a new generation of energy-conscious citizens and government leaders,” said Chuck Swoboda, Cree chairman and CEO. “LED technology is clearly making progress towards widespread adoption for government, commercial and residential applications. We are pleased to be part of the revolution in lighting, making an important, positive impact on our world.”