Last August, the U.S. Department of Energy announced the first winner in its ongoing competition to encourage lighting that’s more efficient—the Bright Tomorrow Lighting Prize, or L Prize. The DOE awarded Philips Lighting North America US $10 million for coming up with a lamp that’s equivalent to a standard 60-watt incandescent bulb in size and brightness but lasts at least 25 times as long and runs on less than 10 W.
Although lamps that are almost as efficient have been available for more than a year, the prizewinning design is just now going on sale. Like the backlights in modern cellphones and computer monitors, these lamps use light-emitting diodes to generate white light. They offer long lifetimes, pleasing colors, and most important, phenomenal energy efficiency.
Is it now time to throw away the incandescent bulbs still lurking in your light fixtures—and even the compact fluorescent lamps (CFLs) you’ve been switching to—and replace them all with LED superlights? With costs often hovering around $25 a pop, few homeowners are rushing to take that plunge. But prices are dropping, and performance is improving fast. So it’s clear that the day when LED lamps will dominate lighting in both residences and businesses is not far off.
Why are LED-based lamps superior, and what makes them so tricky to engineer, anyway? You might imagine that the answers would hinge on the subtleties of solid-state semiconductor physics that govern high-brightness LEDs. They do, but only up to a point. The practicality of these new lights also depends on a more mundane part of the package that’s often overlooked: the circuitry required to drive them. Here I’ll explain what the requirements are for that circuitry and why designing the appropriate electronics can be a challenge, although not one that should slow the adoption of this fantastic new form of lighting.
Like it or not, incandescent bulbs are a dying breed. Australia and the European Union started phasing out traditional incandescents in 2009. The United States is haltingly moving in the same direction, and China is aiming to eliminate incandescent bulbs by 2016. The reason is simple: Old-fashioned lightbulbs squander enormous amounts of electricity.
A full 90 percent of the energy you put into an ordinary incandescent bulb goes into making heat, not light. A standard 60-W bulb generates approximately 850 lumens of light, which comes out to about 14 lumens per watt. Halogen lamps (a more sophisticated kind of incandescent with a higher temperature filament) can provide about 20 lm/W. CFLs are considerably more efficient, producing around 60 lm/W, but they have other problems.
One common complaint is that you can’t dim them. (In truth, some can be dimmed, but their range is usually limited.) Also, CFLs are slow to light up, and because their bulbs contain mercury vapor, they present an environmental hazard. Even with recycling opportunities available, millions of these bulbs end up in landfills every year.
LED-based lights have none of those drawbacks, and they are far more efficient, some offering more than 100 lm/W. These nominally white lights, in fact, contain blue LEDs, along with a phosphor coating that converts the narrow wavelength light they emit into something the human eye perceives as white. With the appropriate mix of phosphor materials, designers can set the tone of the light from cool to warm, depending on the application they have in mind.
Next to their high energy efficiency, the most attractive quality of LED lights is their longevity. Exactly how long one will last depends on how it’s designed and operated, but most will work for 25 000 hours or more while maintaining at least 70 percent of their initial light output. And many manufacturers advertise 35 000-hour lifetimes. So if you used an LED lamp for 10 hours a day, you could expect it to last from 7 to almost 10 years. That’s a far cry from a standard incandescent bulb, which on average goes dark after only about 1000 hours of use. It also beats CFLs, which typically last from 6000 to 10 000 hours.