Lighting technology: The light bulb is synonymous with invention. But, as this case history explains, it may lose out to the light-emitting diode, which is better in many ways

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HOW long does it take to change a light bulb? According to iSuppli, a market-research company that specialises in technology trends, the answer is 131 years. That is the amount of time that will have elapsed between 1879, when Thomas Edison first demonstrated his incandescent light bulb, and 2010, when semiconductor-based light-emitting diodes (LEDs) are expected to have made significant inroads into general illumination, a market worth $15 billion.

Since LEDs were first invented over four decades ago, they have mostly been used in niche applications, first as simple indicator lights on calculators or watches and then, as their brightness improved, in displays, signs and traffic signals. More recently, some companies have begun to sell LED fixtures for residential use. “We're on the brink of a new lighting revolution,” says Jerry Simmons, head of the solid-state lighting programme at America's Sandia National Laboratory.

LEDs have become popular because they have numerous advantages over conventional light bulbs. For one thing, they last much longer: they can endure up to a decade of non-stop use compared with a few months or less for incandescent bulbs. They also take up much less space (a typical LED is about the size of the rubber on the end of a pencil), are shock resistant and, perhaps most important of all, are extremely energy-efficient.

An incandescent bulb, made of a wire filament encased in glass, emits only 5% of the energy it consumes as light; the rest is wasted as heat. Fluorescent lights, which consist of tubes filled with mercury vapour, are roughly four times more efficient. LEDs, however, contain no mercury and already rival fluorescents in efficiency. Upfront costs make them too expensive for most general lighting applications, but experts expect that to change over the next five years as prices come down and efficiencies go up.

Worldwide about 20% of all electricity generated is used for lighting. Several studies reckon that LEDs could eventually cut that amount in half. That would not only save billions of dollars in electricity bills, but also significantly reduce energy demand, environmental pollution and greenhouse-gas emissions.

Besides being environmentally friendly, LEDs allow unprecedented control over lighting. Unlike incandescent or fluorescent lamps, which spew light in all directions, LEDs generate directional light, making them ideal for selectively illuminating areas. Moreover, the ability to mix and match the output of red, green and blue LEDs makes it possible to “tune” the emitted light to produce any desired colour. Lighting designers are already using LEDs to illuminate monuments, restaurants and even famous paintings, such as Leonardo da Vinci's “Mona Lisa”. Because LEDs emit monochromatic light, any potentially harmful or unwanted radiation, such as ultraviolet or infra-red light, can be eliminated.

Back in the dark ages

The first observation of a semiconductor emitting light when zapped with electricity dates back to 1907. But because the amount of light produced was tiny, no one pursued the idea in earnest. That changed in the early 1960s, when Nick Holonyak, a researcher at General Electric, first learned that semiconductors could generate infra-red light. He then set out to make a new type of semiconductor crystal that would be able to emit visible, red light. He succeeded in 1962, inventing the first practical light-emitting diode.

An LED is based on a combination of two semiconductor materials, called n-type and p-type. The n-type is so called because it contains an excess of negatively charged electrons; the p-type contains an abundance of positively charged “holes” that can accept electrons. At the junction where the two materials meet, electrons pair up with holes to form an area depleted of charge that prevents current from flowing. But applying a large enough voltage causes electrons and holes to flow into the junction from opposite sides. As electrons and holes pair up, each electron gives up energy, which is emitted in the form of light. By changing the composition of the semiconductor materials, it is possible to determine the amount of energy given up by each electron, and hence the light's colour.

Even though Dr Holonyak's original red LEDs were dim by today's standards, they were immediately commercialised and ended up being in production for decades. Leaving GE in 1963 for an engineering professorship at his alma mater, the University of Illinois, he schooled over two dozen PhD students who themselves made major contributions to the field. Among them is George Craford, who invented the first yellow LED and is now the chief technology officer at Philips Lumileds, a leading maker of high-brightness LEDs based in San Jose, California.

In 1967 Dr Craford began working at Monsanto, then a chemical company, which mass-produced LEDs for the first time. LEDs then began to appear as indicator lights in calculators and watches, including the famous digital Pulsar watch. In 1971 Monsanto published an advertisement in the Wall Street Journal proclaiming that LEDs might eventually be used in car headlights. That was regarded as preposterous at the time, Dr Craford recalls, and a fellow scientist even called the assertion “technically irresponsible”. But Dr Craford expects the prediction to be fulfilled within a year or so.

Since those early days, LEDs have been refined in many ways. Researchers developed sophisticated methods to grow high-quality semiconductor crystals, reducing the likelihood that electrons or holes will become trapped by defects. There has also been progress in scaling up the size of LEDs, so that they can be driven at higher currents and generate more light. And design tweaks have improved efficiency by ensuring that light can escape from LEDs easily.

Singing the blues

Since the early 1970s, the efficiency of red, orange and yellow LEDs has increased tenfold every ten years. In theory, LEDs should eventually be able to achieve efficiencies close to 100%. In the mid-1980s red LEDs overcame a big hurdle when they surpassed incandescent red bulbs, which are notoriously inefficient since the white light they generate is passed through a red filter, which absorbs 90% of the light. This paved the way for the use of red LEDs in traffic signals, rear lights on cars, and outdoor signs.

Although scientists made advances with red LEDs, creating bright blue and green LEDs proved much more difficult. Research on gallium nitride, a material that showed promise for blue LEDs, began in the late 1960s at the Radio Corporation of America (RCA), which wanted to use LEDs to create flat-panel televisions. In the early 1970s RCA did succeed in coaxing some light from gallium nitride-based diodes, but its blue LEDs were inefficient and dim. Discouraged, most scientists in the field gave up on the material. Gallium nitride turned out to be difficult to grow without defects, and although n-type gallium nitride could be made easily, p-type could not.

Among the few who did not give up were two Japanese researchers and eventually their persistence paid off. In the late 1980s Isamu Akasaki and his colleagues at Nagoya University created the first p-type gallium nitride by incorporating tiny amounts of magnesium. But the ease with which holes could travel through it was still too low to be practical. A few years later Shuji Nakamura, then a scientist at a small chemical company called Nichia, discovered why. He found that producing p-type gallium nitride, which made use of ammonia at high temperatures, trapped hydrogen atoms inside the crystal. These hydrogen atoms bonded with the magnesium atoms that were supposed to promote hole mobility, preventing them from doing their jobs properly. Heating the crystal in a nitrogen atmosphere, Dr Nakamura found, released the hydrogen atoms and dramatically improved the ease with which holes could move around.

In the end it was Dr Nakamura who announced the creation of the first bright blue LED in 1993, followed by the first bright green and bright white LEDs a few years later. His success took the world by surprise. When he began working on blue LEDs in 1989, he had no PhD, had never published a paper, and Nichia was an unknown company located on the smallest and least populated of the four main Japanese islands. Managers initially provided little support for his research, though Nichia's president later authorised more funding for the project.

The payoff turned out to be huge. The availability of the whole spectrum of colours opened up new applications, from full-colour video screens to display backlighting. Between 1995 and 2005 the market for high-brightness LEDs grew from $122m to $3.9 billion, an average of 41% a year, says Robert Steele of Strategies Unlimited, a research firm based in Mountain View, California. Nichia itself made a tidy sum, with sales of about $1.7 billion in 2005. Dr Nakamura wrote several influential papers and went on to become one of the pre-eminent researchers in the field. But he initially received only a pitiful $200 bonus for his inventions. He made headlines in 1999 when he left Nichia, took a position at the University of California Santa Barbara, and in 2001 sued his former employer in a patent dispute that was recently settled for $7m.

Today there are two basic approaches to generating white light with LEDs. The simplest and most common way is to coat a blue LED with a yellow phosphor. The blue light from the LED excites the phosphor, causing it to emit cool white light. But some energy is lost in the process. Combining red, green and blue LEDs to produce white light has the potential to be more efficient, and the colour of the light could be tuned depending on mood or taste. The drawback of this approach is that it is difficult and expensive to match and maintain the output of multiple LEDs over long periods.

To tackle these and other problems, many countries have created multi-million-dollar LED research programmes. America's government-sponsored Next Generation Lighting Initiative, which has funded more than 70 different LED projects to date, aims to triple the efficiency of white LEDs by 2025. To some that is a conservative benchmark. “I think we'll be there much sooner,” reckons Neal Hunter, the former chairman of Cree, one of the world's leading LED manufacturers. Dr Hunter is now the boss of LED Lighting Fixtures, a firm based in North Carolina that is working to make LEDs more suitable for use in general illumination. It recently set an efficiency record for a white-light LED fixture.

So far, only a handful of firms are specialising in this market. To compete with the light output of a single 60-watt incandescent bulb that emits about 800 lumens (a measure of light power as perceived by the human eye), companies such as LED Lighting Fixtures and Permlight of Southern California are designing lamps based on clusters of white LEDs that achieve a similar lumen output, but consume a fraction of the power. Initial costs are still higher for such fixtures than for traditional bulbs, but lower electricity bills could make up the difference within a year or two, says Dr Hunter.

“Light bulbs are among the last devices that use vacuum tubes, an old technology that has been replaced in radios and most televisions.”

The size of LEDs allows for far greater choice in fixture design, though people disagree on how receptive consumers will be to such possibilities. “The shape of lamps will change,” believes James Brodrick, who manages America's solid-state lighting research programme for the Department of Energy. Color Kinetics, a firm based in Boston, already makes LED fixtures resembling large tiles that can be mounted on walls to create checkerboard-like effects or morphing colours.

LEDs made of organic materials, called OLEDs, promise even more revolutionary design possibilities. Since they were first commercialised a few years ago, they have mostly appeared in small portable devices, such as mobile phones and digital music-players. Based on ultra-thin, lightweight plastic sheets, OLEDs emit a softer, more distributed light than conventional LEDs and might eventually be turned into softly glowing wallpaper or curtains.

Because OLEDs are not as reliable and long-lasting as conventional LEDs, some scientists have been tempted to belittle their importance, just as LEDs themselves were originally underestimated in their early years. That could be a mistake, says Fred Schubert, an engineering professor at the Rensselaer Polytechnic Institute in Troy, New York, who heads the university's Future Chips Constellation laboratory. “As researchers”, he says, “we always have to be ready for surprises.”

Those in the field may disagree about the prospects of OLEDs, but they do seem to agree on one thing: the days of the incandescent bulb are numbered. Conventional light bulbs are among the last devices that use vacuum tubes, an old technology that has long been replaced in radios and most televisions, notes Sandia's Dr Simmons. “Ultimately, incandescent light bulbs will end up in a museum, just like vacuum tubes did for electronics,” he says.