Any time you look up during a baseball or football game and see a large video screen, you are seeing light-emitting diodes (LEDs) in action. They are the workhorses of large-screen indoor and, increasingly, outdoor signage. Perhaps the best-known use of LED displays outdoors are the video screens in New York City's Times Square, which almost everyone sees several times a year in the background of one television show or another. Retail stores at Minneapolis' Mall of America use LED displays to attract customers, and the city of Las Vegas is becoming a veritable shrine to LEDs. As their popularity increases, the technology behind these stalwarts of the signage world is improving as well.
LEDs are nothing new, of course; they've been with us since the 1970s. What is new in the LED world is the development of "super-bright" LEDs that substantially increase the number of ways an LED can be used, making it possible to develop LED-based computer screens, televisions and projectors, among other things. LEDs consume considerably less power per lumen than current projector lamps, which makes them a good candidate in the development of battery-powered projectors — something that doesn't exist today. They're also cheap and they last a long time, anywhere from 50,000 to 100,000 hours, or 10 to 30 years. That's five to 10 times longer than the life of an LCD or plasma display.
A projector in your pocket
One company working to expand the potential of LEDs is San Jose, Calif.-based Lumileds (www.lumileds.com). At the iSuppli/Stanford Resources display conference last December in Monterey, Calif., Lumileds' VP of business development, Menko De Roos, gave a presentation in which he outlined the possible future applications for LEDs. Among them was a tiny projector, not much bigger than a candy bar, that could be attached to a digital camera, camcorder or any other input device to project a 30-inch image on a wall. In theory, the device could be powered by two AA batteries. When asked how soon the technology to develop such a device might be available, he answered, "Now." The only hitches are funding the development and finding the market. Essentially, he conceded, the world isn't quite ready for a battery-powered pocket projector.
That may soon change, though, because as a light source, LEDs have a number of attractive characteristics. First, like lasers, LEDs provide a more organized, or "coherent," form of light than conventional short-arc projector lamps. LEDs also come in different colors, so they can provide a purer light source to start with, which in theory can provide a better, more color-saturated picture.
"In a typical LCD backlight or projector illuminator, the traditional method is to start with white [disorganized] light and then separate [organize] it into the three colors needed [red, green and blue]," explains Steve Paolini, director of business development at Lumileds, who has been in the display business for 20 years. "This separation process wastes energy, space and image performance. We can choose more appropriate color points with LEDs to significantly improve the color range and depth."
Brighter is better
LEDs also do not emit ultraviolet light, which degrades electrical components, and they emit minimal infrared light, the main component of heat. These LED properties make it possible to use plastic optics instead of glass, reducing the potential cost and weight of an LED-based projection device. LEDs also happen to have an extremely quick refresh rate (the time it takes for a pixel or LED to go on and off), as well as an exceptionally wide color palette, which makes them ideal for video.
Brighter LEDs also make it possible to create smaller, more pixel-like "clusters" of LEDs, in which each cluster would contain red, green and blue components. The smaller these clusters get, the better resolution giant LED displays will have, providing a solution for situations in which the image is too large or where there is too much ambient light for a front-screen projector.
Hybrids and other applications
Paolini predicts that, as manufacturers gain control of the light frequency and efficiency of LEDs, display devices using more than three basic colors will emerge, much the way they have in the printer market. He also expects to see hybrid projection devices that combine attributes of LEDs and lasers, as well as televisions and LCD monitors that use LEDs as their backlight.
Beyond projection, Paolini thinks it's possible that we will someday be able to program the light in our homes by taking spectrum samples of different lighting scenarios — sunlight, fire, twilight — and recreating them however we wish. "Imagine looking up at the ceiling and seeing four channels (north, south, east and west) of 'blue sky,' depending on which direction you look," he says. "The sun's spectra change vastly throughout the day, depending on what time of day it is, what the atmospheric conditions are, etc. [With programmable LEDs], all of these aspects could be sampled and played back at will, like we today might select a composer and orchestra to play our favorite piece of music."
Hiring your own orchestra is a bit pricey, though — as are the economics of many of these futuristic LED applications. According to a Lumileds white paper published by Paolini and his team, however, advancements in the efficiency and density (light organization) of LED technology have improved in the last 30 years at the rate of two to three times a year, and this rate of improvement is expected to continue for at least the next decade. This will yield LEDs that are brighter, more controllable and have improved light density — all of which will bring a new family of LED-based display devices closer to reality.