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.
White LEDs become SPEcial
Scientists at the Lighting Research Center (LRC) of Rensselaer Polytechnic Institute have developed a method to boost the amount of light from white LEDs without using more energy. The new technique is called scattered photon extraction or SPE.
Commercial white LEDs combine a blue to ultraviolet LED with a phosphor comprised of rare-earth compounds. The phosphor glows brilliant white when struck by high-energy photons from the LED. However, light emitted by the phosphor scatters in all directions. Half of it is diverted back into the LED where it is lost to absorption.
Dr. Nadarajah Narendan and his research group moved the phosphor coating away from the LED semiconductor material. They then reshaped the LED lens geometry to redirect more white-light photons from the phosphor out of the LED assembly. Prototypes of the new SPE LED design produced 30 to 60% more light output and luminous efficiency (light output per watt of electricity).
The lighting industry has set a target for white LEDs to reach 150 lm/W by the year 2012. Under certain operating conditions, the SPE LEDs hit more than 80 lm/W. Compact fluorescent lights reach 60 lm/W while standard incandescent lamps provide only 14 lm/W.
Your name up in LEDs
Well, it could happen if you were playing at the Bardavon Theater in Poughkeepsie, N.Y. The entertainment complex hosted many noted artists over its 130-year history including Mark Twain, the Barrymores, George M. Cohan, Frank Sinatra, and Martha Graham. Now, it's hosting a relamped marquee replacing its original 3,600 incandescent bulbs with energy-efficient LED bulbs from Ledtronics Inc. of Torrance, Calif. Results of the switch to LEDs include improved aesthetics, reduced maintenance, and energy efficiency.
The Bardavon opened in 1869 as the Colllingwood Opera House presenting live performances. As vaudeville waned, the theater converted to a premier movie theater where first-run silent and then talking pictures played. Slated for demolition in 1975, a nonprofit group rescued the historic theater from the wrecking ball and started restoring the theater. As part of the restoration, the 1940-style three-sided marquee and blade sign were replaced with replicas. The replica marquee and blade were exact copies, right down to the original incandescent lamps and chaser circuitry. The use of chasers prevents every light from operating at the same time. If they did, the sign would draw an impressive 38 kW of power.
But sign maintenance was a headache. Operationally, the sign ran up to 8 hr per show about 150 nights of the year. That's an average of 900 to 1,200 hr of operation annually. The incandescent bulbs only lasted about 1,000 hr because the on-off chaser circuits created thermal stresses on the bulb filaments. This means, on average, every bulb in the sign would be gone in a year. Someone on a ladder could handle lamps in the marquee, but the top of the 50-ft blade sign is accessible only with a boom lift.
A grant from the New York State Energy Research & Development Dept. let the Bardavon marquee and blade sign switch to LED-sourced lighting. The LED lamps came packaged in sealed polycarbonate globes resembling the original incandescent lamps. The globes protect the LED emitters and associated electronics from water and seasonal weather damage as well as from the deteriorating effects of UV rays. The LED lamps even screw into the same 25-mm Edison socket used by the original lamps.
The change in power demand was noteworthy. The new LED lamps draw only 4,680 W, about one-eighth the power of the incandescent lamps. There was maintenance relief as well. The average LED lamp lasts 50,000 to 100,000 hr. So it should be 50 years before any bulb needs replacing. Each lamp assembly contains a cluster of LEDs; so even if one emitter should fail, the others still give off light. The electronic circuitry in each bulb handles the on-off cycles of the chasers perfectly without the thermal stress on filaments associated with incandescent lamps.
Your message in LEDs
If you drive along any of the interstate routes in Cleveland, you might see a billboard change before your very eyes. No, your eyes aren't playing tricks on you. Clear Channel Outdoor, San Antonio, is testing seven LED billboards that change their message every 8 sec in a pilot project throughout the Cleveland area. The boards give drivers the chance to see a variety of messages from one sign.
The prototype billboards were built for Clear Channel by Daktronics Inc. in Brookings, S.D. Daktronics is noted for its large-format electronic displays and scoreboards. It has installed more than 1,500 ProStar and ProAd systems since 1997. Derived from its ProStar line, the electronic billboards in Cleveland are the largest single display units built by Daktronics to date.
Each of the 14 48-ft billboards function as large video screens containing-449,280 daylight-visible full-color LEDs. The control system consists of a central Daktronics V-Net controller located at Daktronics headquarters in South Dakota with remote controllers in each display. The V-Net system is used to create, upload, display, schedule, and log the content shown on the seven boards.
Photocells mounted on each sign let the V-Net controllers track ambient light levels to control both LED brightness and gamma levels. Brightness, of course, is the intensity of light emitted by the LEDs while gamma controls the contrast between the lightest and darkest portion of the display. The result provides optimum viewing in conditions ranging from direct sunlight to moonless nights.
Daktronics Keyframe services group generates the billboard content during this pilot project. Initially, the board carried ads from many of Cleveland's nonprofit institutions such as the Animal Protective League and the Cleveland Zoo. Keyframe also provides technical and creative support for the networked system of billboards. Digital images created for the boards are downloaded remotely through high-speed Internet connections.
In the past, outdoor advertisers bought advertising by the amount of space the board occupied. The electronic billboards herald a change: advertisers now purchase time rather than space, similar to the economics of radio and TV. The billboards also provide near instantaneous response to critical messaging needs such as Amber Alerts, major storms, or other emergency situations.
By the light of the flickering LED
There's nothing like enjoying a dinner for two with great food, good wine, and sensuous music, all by the soft flicker of a flameless LED candle. At least, that's what Tyreida Industrial Co. Ltd. from China says about its new LED candles.
Powered by two AA flashlight cells, the LED candle flame flickers like a real candle for 15 to 20 days of continuous operation. The yellow LED should last for 100,000 hr and won't be blown out by the wind, making the candle ideal for the outdoors. On top of that, no one need suffer the pain of hot wax burns!
Tyreida also makes a line of LED table lamps holding 60 to 72 LEDs. The 90 to 240 Vac lamps draw a measly 7 W of power.
Clear Channel Outdoor, clearchanneloutdoor.com
Lamina Ceramics, (800) 808-5822, laminaceramics.com
Ledtronics Inc., (800) 579-4875, ledtronics.com
Lighting Research Center, R.P.I., (518) 687-7100, lrc.rpi.edu
Optek Technology, (800) 341-4747, optekinc.com
Osram Opto Semiconductors, (978) 777-1900, osram.com
Tyreida Industrial Co. Ltd., 86 757 22610501, tyreida.com