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Tomorrow's TV

A dark-horse technologythe Grating Light Valvemay join the competition to dethrone the CRT

7 min read

Erico Guizzo is IEEE Spectrum's Digital Innovation Director.

The cathode-ray-tube TV set has ruled the consumer electronics world for decades: since its introduction, more than a billion of them have been sold. In some countries, households are much more likely to have a TV than a refrigerator.

From a baroque box with a bulbous little black-and-white screen, TVs became big, bright, and ubiquitous. Today they not only dominate entertainment rooms, they also perch on dressers, hang under kitchen cabinets, fit into pockets and purses, and pop down from the ceilings of automobiles. They got their start snatching signals out of the air, but TVs are now apt to be fed from cables, satellite dishes, VCRs, and DVDs, and, increasingly, computers.

But what kind of TV will be our portal into the digital, thousand-channel, high-definition world we've all been waiting for? Nobody can say at the moment. But a dazzling dark-horse candidate, the Grating Light Valve display, exploits several extraordinarily promising recent advancesmicroelectromechanical systems and advanced semiconductor lasersto offer some of the most brilliant, sharpest pictures ever to grace a glowing screen.

Ironically, amid this blossoming of the TV market, one thing has become clear: the CRT is destined for a slow but sure decline. The CRT is losing market share because for TVs, more and more, size does matter. For their entertainment rooms, consumers want big screens, and CRTs can't satisfy: the bigger a CRT screen is, the deeper the glass tube must be. The set becomes impossibly heavy and unwieldy when the diagonal measurement of the screen goes beyond about 36 inches. That problem, and a few others, have opened the way for a host of contenders to replace the CRT as the centerpiece in the home of the future and to reap untold billions in sales.

Like candidates in a presidential primary, a field of these possible successors appeared in January at the 2004 International Consumer Electronics Show, in Las Vegas, Nev., creating a pulsating cacophony of big, bigger, and gargantuan images. Liquid-crystal displays, plasma displays, non-CRT projection displays using digital-light-processing chips or liquid-crystal-on-silicon technologyeach had its moment in the spotlight. All featured impressive technical developments and big pictures. And all are vying for a place in your living room.

But the best may very well be yet to come. Developed by Silicon Light Machines, Sunnyvale, Calif., a subsidiary of Cypress Semiconductor Corp., San Jose, Calif., the Grating Light Valve (or GLV) display [see photo] relies on microelectromechanical systems technology to control and direct light from semiconductor lasers to form a TV picture on a screen or even a wall. It is licensed for display systems by Sony Corp., Tokyo. Silicon Light, which now manufactures and sells about a thousand GLV modules a year to the printing industry, doesn't know when GLV displays will reach the market. And Sony isn't saying.

Sony did, however, offer a tantalizing demo of a GLV-based projection television at a recent Combined Exhibition of Advanced Technologies (CEATEC Japan), a trade show held outside of Tokyo.

The colors were awesomely bright and vivid. During a viewing of the trailer for the movie Spider-Man , the red of the hero's suit fairly throbbed; the blues of the sky and water scenes, requisite for any high-definition TV demo, were captivating.

I had to know more about it. "A prototype," said the Sony spokesman manning the company's show booth. Those words pretty much exhausted his English, and I don't speak Japanese.

I moved on to look at blue-laser disc recorders. Turns out that wasn't a completely wrong turn.

Blue lasers will be essential to a commercial GLV TV, along with green and red lasers. (TV 101: a television picture is made up of red, green, and blue dots.) But it wasn't until I got home to Silicon Valley that I found out what a GLV actually does. Each GLV television contains three of these special semiconductor devices. The light valve was originally developed at Stanford University, in California, by electrical engineering professor David M. Bloom, along with Raj Apte, Francisco Sandejas, and Olav Solgaard. In 1994, Bloom and several others founded Silicon Light Machines to develop and commercialize the technology. The company, now wholly owned by Cypress Semiconductor, licensed Stanford's patents (and went on to receive 39 related patents, with some 100 more pending). So that's where I went to check it out, with Silicon Light's product marketing manager, Robert Monteverde.

The GLV is an optical microelectromechanical systems device made using a conventional CMOS fabrication process in 0.6-micrometer technology, without any exotic steps or materials. Each sticklike silicon chip has 1080 "elements." An element consists of six flexible silicon nitride ribbons arranged in parallel like a tiny Venetian blind [see figure, " Color Controller"]. The ribbonseach a couple of hundred nanometers thick, 3.65 micrometers wide, 220 µm long, with a 0.6-µm gap between themare coated in their center regions with a reflective top layer of aluminum. They are attached to the top and bottom of a silicon substrate, then pulled taut and suspended over it, like guitar strings. A television system uses three of the silicon "sticks," one each for the red, green, and blue lasers [see " Laser TV"].

The ribbons control how much red, green, and blue light reaches the viewing screen and thus the actual color of a pixel there, the same way the three colors of phosphors function in a CRT color set. The intensity of each of the three colors that strike the screen depends on the position of the ribbons when the laser light hits them.

This signal, in turn, depends on the TV picture to be reproduced. What's particularly unusual about the approach is that the GLV does not scan a single line at a time horizontally across the screen, as in conventional TV. Rather, the GLV projects a vertical line of 1080 pixels all at once and sweeps the line across the screen to produce the TV frame. This is done 60 times a second.

When the power to a six-ribbon valve is off, each ribbon is stretched flat and rests in the same plane. They form a mirror that reflects light straight back to its source; no color gets to the screen. A voltage applied between the ribbon and the substrate creates an electrostatic attraction that pulls the ribbon toward the substrate.

When alternate ribbons are pulled down, a portion of the light aimed at the ribbons diffractscreating a wave front of greater or lesser intensity depending on the positions of the ribbons; other light is reflected. Increasing the voltage pulls the ribbons further down, causing differing amounts of light to diffract and reflectthink of cranking open that Venetian blind. It's the amount of diffracted red, blue, and green light that determines the color of a pixel on the screen. Each GLV element is switching at about 115 kilohertz a second for each of the 1080 pixels in the vertical stick.

The ribbons move a quarter wavelength at most (which, for green light, for example, is about 130 nm) and never make contact with the substrate. That, Monteverde says, makes the system quite durable and very fast.

Next, an optical projection system containing a Fourier filter collects the diffracted light and rejects the reflected light. The collected light is sent on to a projection lens and then to a scanning mirrora flat oscillating mirror that directs the 1080 vertical elements across the display a column at a time to produce a two-megapixel image. (For simplicity, the figure shows only one pixel, but all 1080 pixels pass through the optics at the same time.)

Besides the GLV chip itself, a GLV projection TV contains a number of other components, including a processor that adjusts the video data that determines the voltages applied to the chip so the colors can be faithfully produced. In addition, because laser light tends to speckle, the light is also processed through a despeckling filter so it appears consistent in brightness.

The technology most similar to GLV is digital light processing, currently making a big splash on the television market. It also uses a microelectromechanical systems device, in this case a chip with hinged mirrors in a two-dimensional array that tilt toward and away from a light sourceon and offcreating light or dark pixels. The system uses a single strong white lamp, but the light is put through a color wheel to filter it into red, green, and blue. It's cheap but relatively inefficient, and it can't produce images as sharp as those of a GLV-projection television.

The GLV-projection display is sharp, but it costs a lot at the moment. The GLV chip itself is not the reason for the high cost. Rather, the issue is the laser diodes, in particular the blue and green. In fact, the GLV was conceived before a blue laser diode was available at all, in anticipation of the technology. In the original prototype, made about five years ago as a proof of concept, the green laser is an intercavity-doubled neodymium-doped yttrium vanadate (Nd:YVO 4 ) laser. It is a 1064-nm laser, which is then frequency doubled to produce 532-nm green light. The blue laser is also an intercavity-doubled Nd:YVO 4 laser, but it uses the 915-nm laser line, which is frequency doubled to produce 457-nm blue light. Commercial products would likely use gallium nitride diodes, which came on the market in the late 1990s for the blue. Prices of those diodes are still high, and the brightness of the laser is not yet what it needs to be for a projection display.

Manufacturers are finding applications outside of projection displays for GLV chips, which will help accelerate economies of scale. A prime opportunity is in commercial printing. GLVs are illuminated with infrared light to paint an image onto a resist-coated aluminum plate of both pictures and text coming directly from a computer publishing system. This plate, attached to a press, is used to do the printing. Agfa-Gevaert NV, of Mortsel, Belgium, and Dainippon Screen Manufacturing Co., of Kyoto, Japan, are currently selling commercial printers that use the technology. It may soon be used in the manufacture of integrated circuits, printed circuit boards, and other electronic components; Silicon Light has demonstrated printing of micrometer-scale images. And a team at the Massachusetts Institute of Technology, in Cambridge, is incorporating the GLV device into its maskless lithography system, which will print into the submicrometer region.

Another application is in telecommunications, where the GLV can act as a dynamic gain equalizer or as a reconfigurable blocking filter. Both are types of tunable spectral filters used in optical communications. One GLV can take 1080 channels of communications and adjust the output of each independently.

Meanwhile blue-laser-diode prices are falling rapidly, so commercial display products may soon arrive, although putting those products on the market is a decision for Sony to make. This is a technology for very big screens, intended for displays at least as large as the wall of an average conference room. It will probably first go into commercial theaters and the business market, for conference rooms and corporate theaters, then trickle down to your home theater just in time for you to replace that big-screen television one more time.

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