One of the standout attractions at Chicago's Millennium Park is the Crown Fountain. On either side of a reflecting pool are two 50-foot glass block towers. Underneath those glass bricks are LED video screens that, when illuminated, showcase videos of the faces of nearly 1000 Chicago residents, in random rotation, all smiling out at the world while a stream of water cascades over their visages. Every so often, the person in the video will open his or her mouth, and carefully placed nozzles will spray water into the center of the pond -- as if the faces were spewing the water directly onto the populace, much like the gargoyles found in older historical fountains. Needless to say, kids love it.

The most central technology that makes the Crown Fountain possible is the light-emitting diode (LED). During this morning's session on energy efficiency, Shuji Nakamura of the University of California, Santa Barbara, outlined the current status of LED-based solid state lighting, and some of the existing and emerging applications for these ingenious little devices.

LEDS are solid state devices mounted on a substrate of clear plastic, glass or foil. Typically, there is a transparent anode layer that injects "holes," and a cathode layer that injects electrons when a current is applied across the device. Sandwiched between those two layers are hole- and electron-transporting layers, separated by an emissive layer. It's the emissive layer that emits light when a voltage is applied. The color of the light depends on the type of semiconducting material is used.

So at heart, LEDS are tiny light bulbs that fit easily into an electrical circuit, except the illumination comes solely from the movement of electrons in the semiconductor material. There is no need for a filament, so they don't burn out, nor do they get especially hot. Less heat means less energy is wasted, leaving more electrical energy devoted to illumination. LEDs used to be quite pricey, but as semiconducting materials costs have lowered, the technology has become more affordable. LEDs are efficient, cost-effective and long-lasting.

Red LEDs were first created in the 1960s, but it took 30 years for blue and green LEDs to come about (in 1993 and 1995, respectively), and now there's even yellow LEDs. This means that we can make any color of light we want by mixing the primary colors, or we can add them all together to make white LEDs. The best white LEDs now available use far less power than a 60W bulb. Since lighting is the single largest sector of the electricity consumption in the US, that could translate into substantial energy savings. That's why scientists like Nakamura believe that solid state lighting is the wave of the future.

Nakamura envisions a world entirely lit by white LEDs, replacing all conventional lighting, from incandescent bulbs and fluorescent lamps to halogen bulbs. The current white LED technology generates light inside a crystal of gallium nitride (GaN), powered by a 3-volt battery. Energy efficiencies are now about 60% (about 150 lm/W) in the lab, and between 25% and 52% in real-world usage. That's still pretty good: incandescent bulbs are only about 5% energy efficient, and fluorescent lights are only marginally better with 15%-25% efficiency. Whereas conventional lighting has pretty much remained stagnant over the last few decades, LEDs continue to improve, so much so that Nakamura thinks it's entirely feasible to reach 90% efficiency, or 280 lm/W, by 2011.

This sort of advance is particularly vital to developing countries, where people have no electricity and no light at night. Nakamura briefly mentioned the Light Up The World Foundation, specifically, its efforts to supply a tiny Mubarak village in Pakistan with LED lighting in 2004. Among other things, it's been a boon to local education, making it possible to hold classes by night.

The biggest limiting factor is cost competitiveness: LED prices are currently ten times higher than regular incandescent light bulbs, and even though they last much longer and are more efficient, consumers still opt for cheaper lighting. But there are useful niche markets, and chances are, you see LED-based lighting around you every day. For instance, the technology is perfect for outdoor garden lights (powered by solar cells), street lights, large video displays like that used in the Crown Fountain (or by NASDAQ, for that matter), and the small screens on cell phones, PDAs, digital cameras, and iPods. Philadelphia recently replaced more than 14,000 of its red traffic light signals with LEDs, for a projected five-year savings of $4.8 million, and now has an LED installation on the Benjamin Franklin Bridge.

Nakamura foresees even more applications in the future. Camera flashes in cell phones, portable desk task lamps, display case lighting in stores, and using LED light to grow plants (on the space station, or on earth) are just a few of the emerging applications. Ultraviolet LED light is effective in killing bacteria, so it's perfect for air and water purification applications. It could also be used as refrigerator lights to extend the storage life of fruits and vegetables by a week or so -- Mitsubishi already has a product along those lines.

Something Nakamura didn't mention (perhaps in the interests of time) is a subclass known as organic light-emitting diodes (OLEDs) are now blazing their own path to commercial glory. It's already big business: the market for OLEDs is estimated at around $1.4 billion, expected to increase to $10.9 billion by 2012. OLEDs are organic, so they degrade over time and are easily damaged by exposure to water. But polymer OLEDs bring the advantage of a thin, flexible substrate that can be printed with just standard inkjet printer technology instead of expensive lithographic etching, for example. They're more easily integrated with other electronic components, too, and that flexibility could one day lead to roll-up displays or even displays embedded in clothing. OLEDs also show a great deal of promise as eventual cheap, disposable microarrays of chemical and biological sensors which light up in the presence of toxic compounds or gases.

In short, it's a promising technology that still has enormous untapped potential. For those skeptical curmudgeonly sorts who don't want to just take Nakamura's word for it, check out these online reports: here's a DOE study on the promise of solid state lighting, and an OIDA report from 2001 for good measure. Or you can just look around you and see all the places where LEDs are already being used, and realize that the technology is pretty much here to stay. LEDs could soon be the Light of the World.