LEDs are rapidly taking over areas traditionally served by incandescent and fluorescent lights. And there's even brighter technology in the wings.
The incandescent light bulb that resulted from Thomas Edison's thousands of Menlo Park experiments might soon be a thing of the past. Light-emitting diodes, or LEDs, are starting to move into traditional lighting areas now dominated by incandescent and fluorescent lights. These solid-state lamps promise high efficiency, low heat, and a major boost in life expectancy over their traditional cousins.
The reason LEDs can be efficient light sources comes out of their principle of operation. They typically are composed of two types of silicon material similar to an ordinary solid-state diode. As with a common diode, the material is classified as N-type and P-type. When placed in direct contact, the two different materials create a region called the PN junction. When an electric current of sufficient voltage and polarity is applied to this junction, electrons in the N-type material combine with holes in the P-type material. The electric current supplies additional electrons to the N-type material to replace those combined with holes, while positive voltage pulls electrons away from the P-type material creating new holes. The process continues as long as electric current flow is present.
Light emission comes when electrons combine with holes. The process generates photons, actually electromagnetic wave particles. Light creation is very efficient, producing little heat or energy loss unlike incandescent lamps. It contrasts with that of incandescent lamps where electric current heats a filament till it is white hot. Most of the energy used by incandescent lamps goes into creating heat, not light.
Typical incandescent bulbs exhibit a luminous efficiency of only 12 to 14 lumens/Watt (lm/W), while LEDs now typically run 25 to 30 lm/W. LEDs still have a way to go to catch up with compact fluorescent-lights that reach levels of 60 lm/W. However, the solid-state lighting industry feels compact fluorescent lights are as efficient as they can get, while LED lighting is just getting started. LED manufacturers have set a goal of 150 lm/W for LED lighting by the end of 2012.
Light from an LED has a characteristic frequency determined by the LED semiconductor material. Early LEDs generated light in the infrared range, invisible to human eyes. Because solidstate devices react well to IR energy, many of these early LED's found use as IR light sources in industrial sensors.
LED technology matured with the discovery of new materials that produced other wavelengths of light. The first visible-light LED was red and was introduced in the latter half of the '60's. It gave off little light, typically measuring in the millicandela (mcd) region. While not suitable for general illumination, they worked well as low-power, long-life indicator lights.
Slowly, LEDs worked their way up the color spectrum, moving into red-orange, then green. Manufacturers combined red and green LEDs on a common die to devise the bicolor LED. Bicolor LEDs displayed red, green, or an orangish yellow if both red and green were lit at the same time. Finally, the creation of blue LEDs completed the primary color triumvirate.
Today full-color LED's actually hold three LED junctions, each producing one of light's three primary colors: red, green, and blue. When combined in proper proportions, the three individual colors appear to the human eye as white.
Full-color LEDs excel at applications where the ability to control color gradients is important. Mixing and matching of the three primaries creates any hue. So far, typical applications for full-color LEDs are in video and other display devices, rather than in general illumination. Fullcolor LEDs do find use in accent lighting where their ability to adjust color to suit mood or style is eminently useful. An entirely new architectural lighting industry flourishes around the use of LED accent lighting for buildings and bridges.
While full-color LEDs dominate accent lighting, the white light they produce does not work well in close quarters. The reason is colored shadows and halo effects form around objects they illuminate. Because of this, white LEDs in general illumination create their light using a different technique. It comes from a blue LED coated with a special phosphor made from rare-earth compounds. When the high-energy photons emitted from the blue LED strike the phosphor, the phosphor glows with a brilliant white light. White LEDs made this way create the brightest LED light to date.
Because the phosphor glow is omnidirectional, some of the white light heads back toward the junction material where it is absorbed. Researchers are exploring ways to direct this backscattered white light outward where it is usable.
Use of multiple LEDs is another method for creating brighter lights. A device called a light engine combines the light output from individual LEDs. One such engine from Lamina Ceramics uses over 1,100 LEDs to generate 28,000 lm across its 5-in.-diameter surface. That's about the same amount of light that would come from a 400-W mercury vapor (24,000 lm) or a 250-W high-pressure sodium (27,500 lm) lamp.
One of the most intriguing aspects of LED technology doesn't involve semiconductors at all, but rather organic polymers. Scientists have known organic chemicals can generate light for some time. Certain animals such as fireflies and some deep-sea fish carry their own flashlight with them. A plant given firefly DNA glows with an eerie light at night. Research into these organic compounds discovered that certain polymers, or plastics, also generate light when stimulated properly.
The area of organic LEDs, or oLEDs, is still in its infancy; but some devices are beginning to find their way to market. One item is a video screen that consists of a plastic sheet only 1-mm thick used as a viewscreen on the back of digital cameras. Besides being thinner than standard liquid-crystal displays (LCDs), oLED displays are much more visible in daylight and consume less power than LCD backlighting. Batteries in laptop computers equipped with oLED displays should last much longer than conventional LCD laptops.
Looking ahead, oLED fibers could even be woven into fabric, creating clothes that change colors to suit the wearer. Who knows? One day you might "wear" your portable HDTV and computer screen on your jacket.