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In Depth

LED bulbs: The end of the lightbulb as we know it?

(Copyright: SPL)

(Copyright: SPL)

Efficient LED technology looks set to flick the switch on traditional incandescent lightbulbs forever, say researchers.

Look up at the ceiling above you, and chances are there is a centuries-old piece of kit swinging from a cord. The light bulb has been hanging around for more than 150 years. Yet between its invention and the present day, its design has hardly changed. 

But now the days of the traditional incandescent bulb look numbered. These electricity-sapping glass orbs have fallen out of favour with environmentally-conscious governments and consumers. And waiting in the wings is a new breed of hi-tech light based on the humble LED (light-emitting diode), the small lights found in everything from TV remote controls to bike lights. Not only do they promise to solve the bulb’s environmental woes, their backers say they will also respond intelligently to your surroundings and even influence the way we behave.

“LEDs are about to change the way we see things forever,” says Tim Holt, chief executive of Strathclyde University's Institute of Photonics in Scotland. “We are only just at the start of the LED lighting revolution.”

Already, the efficiency and long life of LEDs is making them a popular – if costly – option in places where changing bulbs is inconvenient or expensive, such as in motorway lights, traffic signals, airport runways or on large buildings and bridges. For example, the Louvre museum in Paris is currently replacing 4,500 bulbs with LED equivalents, a change that is expected to result in a 73% reduction in energy consumption. Plans are also in place to replace the 25-year-old lighting system that illuminates Tower Bridge in London with LED lighting in time for the 2012 Olympic Games

But the real hope of the LED industry is that arrays of these tiny twinklers packed into something that resembles a bulb will become the light of choice in your bedroom, bathroom or study, allowing them to grab a slice of a global lighting market that was worth an estimated €52bn in 2010. Their cause is helped by the numerous governments around the world which have chosen to phase out the sale of incandescent bulbs. An EU-wide phase-out is already underway, and in November 2011 the Chinese government announced that the import and sale of incandescent bulbs would start to be banned from October 2012. Over 1 billion incandescent bulbs were sold in China during 2011, meaning the announcement was enough to drive investors into a frenzy: the share price of Cree, a United States-based LED manufacturer, increased by almost 10%.

Of course, the death warrant for the incandescent bulb has been signed before. Compact fluorescent lamps (CFLs) – or energy efficient bulbs, as they are more commonly known – were supposed to spell the end of the light bulb in the 1970s. But despite rising to prominence in the 90s and constantly improving, they have failed to deliver on their promise. In part this is down to them costing more than regular bulbs, taking an age to warm up and often producing low quality light. And that is without even mentioning the environmental concerns over bulbs that contain mercury.

LEDs, it is claimed, will help overcome these problems. These tiny lights were invented by GE in the early 1960s and were initially only available in red, a property that defined the look of early pocket calculators and digital watches. Over the years, however, more colours have appeared.

These are all made in the same way: from wafers of semiconductor, the material used to make computer chips. As their name suggests, LEDs consist of a simple diode. In traditional electronics these are two metal posts – an anode and a cathode – placed very close to one another, through which power can only flow in one direction. In LEDs, the anode and cathode are formed by depositing two areas of semiconductor that have been “doped” – impregnated with impurities – next to one another. As power flows through the junction between these two patches, some of it is unable to make it through and is instead emitted as photons of light. The colour of the light is dictated by the properties and structure of the semiconductor used to make the LED. For example, the bluish light seen in many pocket LED torches is created using materials such as zinc selenide and indium gallium nitride.

Like a LED balloon

Although this may sound complicated, the manufacturing process to create LEDs is well understood. And – like all processes involving semiconductors – it is coming down in cost all the time, and at predictable rates. In fact, the rates are so predictable that there is even a lawcalled Haitz’s lawthat describes the fall. This states that every decade, the cost of producing useful light will fall by a factor of 10, whilst the amount of light generated increases by a factor of 20. Haitz’s law, first proposed in 2000 by now-retired scientist Roland Haitz, is considered to be the LED equivalent of Moore’s Law – a computer industry axiom which states that the number of transistors that can be squeezed on to a computer chip at a fixed cost will double every 18 or 24 months. 

This is crucial for LEDs, which are currently around ten times more expensive than incandescent bulbs. Yet decreasing costs are only one part of the excitement.

In traditional incandescent bulbs, only about 5% of the electricity used is converted into light, while the rest is wasted as heat – the bulbs are therefore little more than tiny electric heaters which give out light as a fortuitous side-effect. A CFL tends to be around four times as efficient as a 60W bulb, but LEDs can be up to ten times as efficient.

As a result, the potential energy savings in the future from LED lights are enormous. Although the United States currently has no plans to phase out all incandescent bulbs, the US Department of Energy estimates that if the whole country switched to LED lighting over the next 20 years, this could result in energy cost savings of $120bn, reduce electricity consumption for lighting by a quarter, and reduce carbon emissions by 246 million metric tons. In China it is a similar story, where its central planning agency expects to save 48 billion kilowatt hours of electricity every year, once China's phase-out is complete, and hopes to reduce annual carbon emissions by 48 million tons

And LEDs have another environmental trick up their sleeve: they last far longer than most other light sources. Incandescent bulbs last around 1,000 hours before burning out, while comparable CFLs last 8,000 to 10,000 hours. LEDs do not burn out in the same way but simply get dimmer over time, and when they fall to 70% of their initial brightness their useful life is considered over. A good LED light should have a useful life of between 30,000 and 50,000 hours, and maybe up to 70,000 in the future, meaning less are thrown away and less are made.

But LEDs do not have it all their own way. One of the biggest challenges they face is the quality of the light that they produce. LED bulbs tend to generate a stark, cold light – a world away from the warm, yellowish light we are used to from incandescent bulbs. This can cause illuminated objects to take on unnatural colours, which the human eye can find uncomfortable or just odd.

This is the result of manufacturers building bulbs from efficient blue LEDs. To create a “white” light, a phosphor coating is applied that emits other colours in the visible spectrum. When these mix – like shining light through two prisms – they produce a white light. Crucially, however, there are “gaps” in the spectrum, and it is these missing chunks that cause the harsh effect.

To get around this, scientists and manufacturers are now working on using a combination of blue LEDs and less efficient red ones to create a warmer glow. Others are working on new phosphor recipes that can convert blue LED light to more parts of the visible spectrum, effectively filling in the gaps.

But other problems remain. For example, light from LEDs also tends to be highly directional – emitting light in a tight beam. This can be very useful in certain situations, such as illuminating a painting, but it is hardly a selling point for consumers wanting to replace the bulb in their reading lamp. Most manufacturers get around this problem by fitting reflectors and diffusers in their bulbs, but while this is effective it tends to reduce the overall efficiency of the bulb.

That has not stopped the industry moving ahead, however. Already, 50W and 60W equivalent bulbs are on the market. But going any higher may be difficult.

“100W bulbs have been demonstrated in the lab, but I don't think they will be on the market for a year or two,” says Robert Karlicek, director of the Smart Lighting Engineering Research Center at the New York-based Rensselaer Polytechnic Institute.

White light, white heat 

The problem, he says, is one that blights traditional bulbs: heat.

Specifically, LEDs do not radiate heat away like other bulbs do. As a result, they have to be attached to bulky heatsinks, to allow circulating air to draw the heat away.

This is where problems arise. Currently, packing all of the components into the space of a traditional bulb allows a heatsink able to cool an LED bulb producing the equivalent of a 50W or 60W conventional bulb. But the bigger the output, the bigger the heatsink needed. Quite simply, there is not enough space for a heatsink big enough to dissipate the heat from a 100W-equivalent LED.

But this problem would become irrelevant if consumers are prepared to rethink their approach to lighting, rather than simply focusing on updating old incandescent bulbs with newer LED technology, according to Fredric Maxik, chief scientific officer at Florida-based manufacturer Lighting Science Group.

“When the first digital head was put on to a reel-to-reel audio system, it was complicated and heavy, but people thought that that was going to be the future of digital audio. That was replacing old technology with new technology. But of course, they hadn't thought of something like the iPod,” he states. “Instead of retrofitting lights with LEDs, we now have the possibility of making completely new lights that don't look like any light that we know of. Instead of hiding the heatsink, we should be making it part of the design of the light itself.” Since LEDs last so long, it should be possible to do away with the whole concept of a light fitting with a separate disposable light bulb altogether, Dr Maxik says.

The possibility of creating innovative lighting using LEDs may be enticing to interior designers, but it is the prospect of using processors built in to LED lights to make intelligent lighting systems that is getting many of the researchers involved in the industry excited. At the most basic level, this can include a motion sensor which switches on the light when someone approaches, or a system which alters the brightness of the light automatically. “You could very easily design a light that learns your habits and tunes its brightness to save energy,” says Dr Maxik. “If you like a certain amount of light in your room, then as it begins to get dark outside the window, the light would compensate by becoming brighter automatically.”

When individual lights are given the power to talk to each other, or to lighting controls, then the potential for smart lighting becomes even greater. To do this, LED lights can use visible light communication – essentially sending signals to each other by pulsing themselves on and off too rapidly for the human eye to notice. This could be used to send signals between street or motorway lights, so that they only switch themselves on when people or cars are approaching. The same street lights could also be used to transmit internet data around a neighbourhood, and internal lighting could be used to send internet data around buildings such as hospitals, where wi-fi is not allowed. A system that can transmit high definition video using LEDs has already been demonstrated by a researcher at the University of Edinburgh.

Since white light is made up of a spectrum of different colours, the possibility of manipulating the light spectrum emitted by LED lights offers some of the most intriguing possibilities. “If we can control spectral distribution, this would give us a control knob that we have never had before,” says Dr Karlicek. For example, it should be possible to tune lights to give out a dose of near-violet light that would have antiseptic-like qualities, he adds. These could be used to keep countertops in kitchens clean, or to help maintain sanitary conditions in washrooms.

Spectral tuning could also make people more productive, Dr Karlicek believes. “There's certainly evidence that certain distributions can affect the ability of the brain to sleep or learn, for example,” he states. Although more research is needed, he suggests that it may be possible to design lamps that emit tailored light for students to use, to help them study more effectively, or ones that counter natural circadian rhythms to help night workers to stay awake.

Whilst these kinds of tricks mean the LED could have a bright future, there are other technologies hot on its heels. Recent research at the US government-owned Sandia National Laboratories showed that it is possible to generate white light that is pleasing to the human eye, by mixing the light from red, blue, green and yellow lasers, while a Illinois-based company called Eden Park Illuminations hopes to commercialise technology that involves sealing plasma behind glass in tiny aluminium foil micro-cavities, to produce ultra-thin, flexible lighting sheets.

But Dr Karlicek believes that LEDs are here to stay. “Plasmas, incandescents and fluorescents will go the way of the vacuum tube after the invention of the transistor,” he promises. “People still use vacuum tubes for some applications, and similarly incandescent bulbs may never go away completely. But it is not a question of if, but of when LED lighting will be the norm throughout the world.”

You may never look up at the ceiling in the same way again.

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