A new day is dawning for image projection. Today’s high-brightness LEDs enable display products to achieve improvements ranging from enhanced color rendering to doubling a system’s lifespan. Manufacturers of both computer and entertainment displays may pull the plug on older illumination sources, such as cold-cathode fluorescent lamps (CCFLs), and instead prepare to “let there be light” -- semiconductor-style.
The first PC monitors using LEDs as their lightsource are scheduled to enter the retail pipeline later this year. So are the first LED-illuminated, LCD TVs. The greater color capabilities of these products will make CCFL displays look anemic by comparison, expanding the reproducible color gamut by up to 45%.
By 2005, LEDs will create a new breed of “pocket imagers” that will allow PowerPoint presentations and other materials to be shown on a device the size of two decks of playing cards. New, LED-based, car-navigation systems will hit the market at roughly the same time. Rear-projection TVs will also be LED-powered within a few years.
These applications are possible due to increased LED brightness, and the development of new backlighting and other components. Here’s a brief look at how and why LEDs will revolutionize displays.
LEDs in computer displays
Flat-panel, LED-backlit monitors are coming soon to a desktop near you. NEC-Mitsubishi will sell an unspecified desktop model in the second half of 2004, using Lumileds’ Luxeon LEDs. Other LED-powered computer displays are in development.
The key technical production challenge in producing these products involved building a backlight capable of mixing red, green and blue LEDs to produce a uniform white light. This required a radically different approach than a conventional backlight comprising white CCFLs.
In response, LED suppliers and display manufacturers developed a new, edgelit backlight that comprises high-flux, LED arrays positioned along an acrylic light guide’s edges. This guide color-mixes and distributes the resulting white light evenly over the full panel, which creates uniform color and brightness.
Displays using this new system will deliver more than the greatly improved color fidelity and saturation mentioned earlier. LED benefits in PC monitors also include real-time, dynamic color and brightness control; self-adjustment to maintain color and brightness levels over time; a reduction in the blurring effect created by fast-moving images; environmentally friendly, mercury-free, low-voltage operation; shatter-resistant components capable of withstanding the bumps and bruises of heavy use; and a 50,000-hour life expectancy.
LEDs in LCD TVs
LCD televisions are another new LED frontier. Manufacturers will elease LED-based, LCD TVs in time for the 2004 holiday season, giving consumers more reasons to retire their cathode-ray-tube (CRTs) models in favor of these slim, wall-hung entertainment centers.
Until now, CRTs have outpaced LCD TVs in terms of color performance, light output, viewing angle and moving-video quality. Replacing CCFLs with LEDs will erase that gap, but edge-lit, LED backlights developed for computer displays can’t suffice for LCD TVs.
The edge-lit system’s light guide is too heavy for larger panels, for example. Also, thermal stabilizers and UV inhibitors required for larger screens magnify acrylic’s tendency to absorb light.
The development of a direct, LED backlight system, comprising several rows of red, green and blue, side-emitting LEDs, placed in a cavity directly behind the LCD panel, has solved this problem. No light guide is necessary because colors mix within the cavity as light bounces off reflective surfaces.
Side-emitting LEDs’ perpendicular beam pattern directs most light away from the screen’s front and makes it readily available for mixing within the chamber “air.” Eighty percent of the light emits from the LEDs’ sides, at an angle between 70 and 110°, and evenly distributes the full 360° around the LED package.
Diverters and diffusers capture maximum light, help spread the mixed light over the entire panel, and prevent the light emitted from the LEDs’ fronts from forming color spots on the screen. Continuing improvements will eventually spur the ability to operate with less power than CCFL backlights.
LEDs in projection applications
In the near future, LEDs will also appear in various microdisplay-based projector products because they provide small, long-lasting, colorful, optically efficient, illumination solutions that turn on and off instantaneously, without the space requirements or moving parts of a color wheel.
Now under consideration, a new category of battery-powered “pocket imagers,” driven by high-flux, colored LEDs, have been integrated into a tiny illuminator using dichroic mirrors for color mixing. This kind of handheld projector wasn’t technically feasible in the past because the only practical light source — a high intensity discharge (HID) lamp — required too much power and produced too much heat.
With LEDs expected to bring these miniature devices to life by 2005, users will be able to project 11 x 17-in. images and presentations from portable memory cards directly onto a wall or other surface with high resolution, vibrant color and light output comparable to that of a notebook computer. Because LEDs operate at such low voltage, there will be no wait for a PC to boot up, no fan to dissipate the heat, and no constant battery changes.
Over the next three to five years, the same LED illuminator concept will also be incorporated into LED-driven, rear-projection monitors for desktop publishing, as well as digital, rear-projection TVs for home or corporate media rooms. A fringe benefit will be the ability to change white points and color gamuts dynamically to produce more color or less brightness according to the material or viewing conditions.
More to come
Other LED-powered, display applications are on the drawing board. For example, Mitsubishi is scheduled to release a 12.1-in. computer display that incorporates white LEDs geared to point-of-sale and factory-automation applications in the second quarter of 2004, while car-navigation systems utilizing white LEDs, instead of CCFLs, will be available to car manufacturers in early 2005.
With Mitsubishi’s display, LEDs’ main attraction is viewability in bright environments, such as convenience stores. For car-navigation systems, the primary appeal is mercury elimination to comply with automotive mandates. In addition, the ability to handle extremely high dimming ratios will make GPS systems easily visible in direct sunlight while allowing sufficient dimming to avoid nighttime motorist distractions.
If general lighting’s rapidly increasing use of LEDs is any indication, this newer technology’s higher price tag will be outweighed by its technical advantages.
From pre-press monitors -- where the color accuracy of LEDs will minimize iterations and potentially reproduce the same colors as the printout -- to industrial displays, where LED-based light output feedback systems can help maintain critical brightness and white-point stability, new LED-lit display products will give users important new capabilities that, eventually, will become as vital to their everyday activities as Word documents or Excel spreadsheets.
Indispensable functionality will be the key to these products’ success. Remember, PDAs, and even PCs, were once new technologies, too.
