Inner Workings: Inside an LED-based Light Engine

Lost in the rush of attention being lavished on sleeker and sexier flat-panel HDTVs is the fact that there's still a lot of innovation going on in rear-projection microdisplay sets. In the past year, we've seen developments ranging from the prosaic - bigger screens, more models with 1080p resolution, and slimmed-down cabinets - to the exotic, such as 3-D video.

But there's also been considerable effort expended to improve the performance of rear-projection TVs (RPTVs). One area of innovation is the development of solid-state light sources, such as Light Emitting Diodes (LEDs) and lasers, which will likely one day replace conventional ultra-high-performance (UHP), or arc, lamps as backlights in rear-pro and LCD displays. While some companies - most notably Mitsubishi - have set their sights on laser, that technology has been more problematic (and expensive) to harness. As a result, no laser-lit sets are currently available, although Mitsubishi has promised to deliver the first before the end of this year. By contrast, LED is already here: TV manufacturers, including Samsung and NuVision, have been marketing LED-powered DLP rear-pros since 2006.

Samsung, in particular, has been a strong proponent of LED backlights. Its second-generation lineup of LED DLPs, introduced this past spring, consists of six "Slim LED DLP" models with 50-, 56-, and 61-inch screens. And the company plans to expand its LED-based offerings in 2008. Both Samsung and NuVision use special PhlatLight (Photonic Lattice Light) LED chip sets, made by Luminus Devices. Luminus was founded by a group of M.I.T. scientists who'd been experimenting with photonic lattices as a way of coaxing greater brightness - that is, more photons - out of typical LEDs.

There are many advantages to using LEDs instead of conventional lamps in DLP rear-pro sets. For one, LEDs are more reliable and last longer. Where the bulbs in conventional lamps cost several hundred dollars and need to be replaced every 3,000 to 5,000 hours, LEDs last about 120,000 hours - which means you can expect them to last for the life of the TV. LEDs also don't wind up in landfills like spent conventional bulbs, and they don't contain the hazardous substances (such as mercury, xenon, or metal halide) found in arc lamps.

Also, while bulbs require a warm-up period, LEDs turn on instantly. This not only allows the picture to appear immediately with full color and brightness, but (when properly synced with the DLP chip) can also help improve contrast and picture quality. And the simple optical design of PhlatLight LEDs, which use a single emitting area per color instead of an array of small LEDs, allows for shallower cabinet depths.

But maybe the biggest advantages of LEDs are in color reproduction and brightness. Because individual LEDs are used to create the primary colors, they can produce a wider, richer range of colors than sets using a white-light bulb and filters or color wheels to split the light into separate colors. For example, the PhlatLight PT120 chip set used in the Samsung RPTVs can produce about 40% more color than traditional lamp-based sets, according to Luminus. They can also help reduce or eliminate the visual artifact known as "the rainbow effect."

In a conventionally lit single-chip DLP microdisplay TV, light from a lamp passes through a spinning color wheel before it's beamed onto the surface of the DLP chip's digital micromirror device (DMD), which is essentially a microchip covered with millions of tiny mirrors. The wheel - made up of red, green, and blue color segments - is what produces color. Because the wheel is accurately synchronized with the DMD chip, colors are displayed sequentially on the chip, but at a speed fast enough so that the eye perceives them as a composite, full-color image. However, color wheels can produce the aforementioned rainbow effect, a color-sequencing artifact that appears as a quick flash of color, typically in scenes where bright objects are juxtaposed against a dark background.

The PhlatLight technology, meanwhile, uses individual red, green, and blue LED light panels instead of a color wheel. In a single-chip DLP RPTV with LED panels, color is also displayed sequentially: Each of the colored panels is synced to the DMD and sequentially switched to produce the full-color image. But the colors are cycled 16 times faster than they are with a standard color wheel. This produces smoother motion and essentially eliminates any rainbow effect. Since the LEDs individually create the pure, primary colors, TV sets using LED backlights can produce deeper, more saturated colors than conventional lamp-based TVs.

One reason LEDs haven't been more widely used is that they have a hard time creating enough brightness to illuminate a large screen. But Luminus's development of the photonic lattice structure for the PhlatLight chip set's emitting surfaces has been key in allowing them to produce the brightness needed for RPTVs.

Photonic lattices are complex microstructures embedded in the LEDs' dielectric material that affect the propagation of light. In conventional LEDs, light is dispersed laterally as well as vertically, causing some light to escape from the panel's edges. As a result, that light needs to be redirected via reflectors. But lattice-based emission surfaces are self-collimating. In other words, the shape and dimensions of the material corral any light that might have been lost via lateral dispersion at the panel's edges, instead collecting it and focusing it directly on the DMD. Also, since no additional collection lenses are needed, self-collimation simplifies the design and increases the overall efficiency of the light engine.

The PhlatLight PT120 chip set used in Samsung's RPTVs was developed and optimized for Texas Instruments' 1080p DLP chips. Each chip set includes three LED panels - one each for red, green, and blue - that have an emitting surface with a 16:9 aspect ratio that matches the TV display, eliminating the need for tapered light tunnels to reshape the projected image to the correct aspect ratio. The PT120's design allows the chip set's LEDs to produce enough lumens to illuminate a 61-inch rear-pro HDTV. The next generation of PhlatLight chip sets will likely be bright enough to light even larger screens. And the PhlatLight's low-profile design, which allows the primary optical elements to be placed very close to the device's emitting area, helps manufacturers shave inches off the depth of their rear-pro cabinets. One of Samsung's current Slim LED DLP sets, for example, is just 13 inches deep.

Laser, which promises even greater clarity and color, will likely one day be used in many high-def TVs. But it's still largely unproven commercially and very expensive. LED is already here, and it's getting better and cheaper. The PhlatLight LED adds only about $300 to the price of Samsung's models - equal to the cost of replacing one UHP lamp, which you have to do many times during the life of a set.

(A year ago, the same LEDs carried a $1,200 premium.) And the price difference will likely narrow - which is necessary if rear-pro TVs are going to compete in an increasingly flat-panel world.

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