As icons of technology go, they don’t come much bigger or more enduring than the classic incandescent tungsten filament bulb. It is one of the few technologies still in use more than a century after it was pioneered by inventors including Thomas Edison and Joseph Swan.
Now western governments are gunning for the humble light bulb because it wastes huge amounts of energy. First to propose calling time was the state of California: on 31 January it unveiled the “How Many Legislators Does it Take to Change a Light Bulb Act”, which, if passed, will ban the bulbs by 2012. Three weeks later, Australia announced a plan to do likewise. This month the UK government promised to phase them out by 2011.
No wonder. Only 5 per cent of the electrical energy fed into the bulbs generates light. Because the rest is wasted as heat, a switch to energy-efficient alternatives offers huge scope for cutting carbon dioxide emissions from power stations burning fossil fuels, by reducing the amount of electricity they produce.
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Reliable replacements are already available. First in line are compact fluorescent lamps (CFLs), coiled-up versions of the fluorescent “tube” lights that are a familiar fixture in offices and factories. Waiting in the wings are bulbs and lights based on light-emitting diodes (LEDs), microchips which pump out huge amounts of light for their size, and use a fraction of the energy of conventional bulbs. Although LEDs are commonplace in car headlights and display screens, technical deficiencies mean they cannot yet replace light bulbs to give warm, ambient lighting.
In the meantime though, CFLs are a good stopgap, producing light for just 20 per cent of the energy incandescent bulbs use. They also last 10,000 hours compared to just 1000. The potential savings, in energy and money, are considerable, as lighting soaks up around a fifth of the electricity industrialised countries consume.
In the US, there are around 4 billion incandescent light bulb sockets, says Dutch company Philips, which launched a campaign in Washington DC on 14 March to scrap all inefficient lighting in North America by 2016. Replacing incandescent bulbs with energy-efficient ones would cut the US’s annual electricity bill by $18 billion, and cut CO2 emissions by 158 million tonnes, the company says.
“Replacing incandescent bulbs would cut US electricity bills by $18 billion a year”
Dump the bulb
Europe uses almost as many incandescent bulbs – 3.9 billion – and so could make similar savings. “If everyone changed over from incandescent bulbs, we could save the energy equivalent to 10 million households,” says a spokeswoman for the European Commission, which in May will consider legislation to dump the bulb.
The CFL tube contains a gas that produces ultraviolet light in response to an electric current. When the UV light strikes the phosphor coating on the inside of the tube it generates visible light. Until now, CFLs have been too pricey, ugly and “flickery” to displace their energy-hogging rivals. “When we first tried to launch these 27 years ago, they were about $25 each,” says Philips’s spokesman, Steve Goldmacher. “Now, they’re less than $2 each,” he says. They also come with their own built-in transformers to fit most ordinary light sockets. Around 85 per cent of all CFLs are now replaceable, says the American Lighting Association. As a result, last November, supermarket giant Wal-Mart launched a campaign to sell 100 million CFLs by the end of 2007. One of the few limitations of CFLs is that they cannot be dimmed like conventional bulbs.
The most significant changes to the way we light our homes are likely to come when LEDs become cheap and reliable enough to provide ordinary diffuse white light. This is because CFLs, while much more efficient than incandescent bulbs, still only emit around 15 per cent of the electrical energy fed into them as light, or up to 30 per cent in “tube” form. This compares with 30 per cent for existing white LEDs, with a target of up to 70 per cent. “It will be CFLs first, but LEDs may eventually bypass them,” says Colin Humphreys, a pioneer of LEDs at the University of Cambridge.
LEDs are semiconductor devices that emit light when a voltage is applied across them. Each LED is typically a stack of five very thin layers of the semiconductor indium-gallium-nitride, separated by gallium nitride layers, and measures just 1 millimetre square. By varying the amounts of indium, engineers can alter the colours produced. For example, 10 per cent indium gives blue light, and 20 per cent gives green. To produce white light, blue LEDs are coated with phosphor, which generates yellow light. This merges with the blue light from the LED to create a somewhat harsh white light.
Already, some LED-based domestic light sources are appearing. Last month Philips unveiled a globe-like lamp based on four LEDs – two red, one blue and one green. By varying the intensity of the LEDs it’s possible to create mood lighting in up to 16 million different colours. Launched in the Netherlands, the lamp, called LivingColors, is operated with a simple remote control. Philips stresses that this is a long way from the LED-based “bulb” that people can simply screw into existing sockets. But it’s a start.
Most white LEDs for the home are likely to appear first in sharp, functional lighting such as desk lamps. Earlier this month Siemens’s subsidiary Osram unveiled an LED spotlight called Ostar, which the company says can easily illuminate desks from a height of 2 metres. The lights should also last 50 times longer than incandescent lamps, and five times as long as CFLs.
To produce LEDs that can replace incandescent bulbs, the challenge is to develop devices that create a warmer white light. Humphreys’s team and others around the world are tackling this by coating individual LEDs with red, blue and green phosphors. “In principle, we can mimic the quality of sunlight,” says Humphreys. “We’re not there yet, but we’re getting close,” he says.
If white LEDs are ever going to be used as light bulbs, they will also have to get much cheaper. Nowadays, a single LED lamp costs up to $60, mostly because indium-gallium-nitride wafers have to be grown on expensive sapphire crystals. Humphreys is confident LEDs can be grown on silicon instead. This would cut the cost drastically, as a 5-centimetre sapphire substrate costs $40, compared to just $5 for silicon. His team has already grown blue LED structures on 5-centimetre silicon wafers in the laboratory.
Humphreys and his collaborators have also banished a gremlin that was causing LEDs to fail early. The units stopped working after just 400 to 500 hours of use because of heat trapped by the transparent epoxy resin dome that caps and protects the chip. By exchanging epoxy for a type of silicone, Humphreys stopped the LEDs overheating, vastly prolonging their working lives. Once they’re cheap enough, LEDs could last the entire life of a lighting unit, he says.
Whenever LEDs are ready to take over, one thing is certain: the incandescent light bulb is finally on its way out. “It’s amazing it’s lasted this long,” says Humphreys.
