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Digital light processing

Digital light processing

When one first hears about digital light processing (DLP), it seems almost impossibly complex, even magical -- millions of tiny mirrors on a chip the size of your thumbnail, each of them capable of moving thousands of times per second to create a digital image. In fact, DLP (a trademark of Texas Instruments Inc.) gives new meaning to the phrase 'smoke and mirrors' as it applies to computer-related technology.

When one first hears about digital light processing (DLP), it seems almost impossibly complex, even magical -- millions of tiny mirrors on a chip the size of your thumbnail, each of them capable of moving thousands of times per second to create a digital image. In fact, DLP (a trademark of Texas Instruments Inc.) gives new meaning to the phrase "smoke and mirrors" as it applies to computer-related technology. How DLP works

In essence, DLP is a nanotechnology implementation of the old survival technique of using a mirror to signal for help -- its purpose is to shine a controlled series of light flashes on a target to send a message. The mirror in this case is part of an optical semiconductor called a digital micromirror device, or DMD. The DMD chip contains not one but an array of up to 2.1 million microscopic mirrors, each just 16 micrometers square (less than one-fifth the size of a human hair) and 1 micrometer apart.

The DMD chip is driven by a digital video or graphic signal in which each digital pixel corresponds to a single mirror on the DMD. Add a light source and a projection lens, and the mirrors can reflect a digital image onto a viewing screen or other surface. Each mirror is mounted on tiny hinges, so it can be tilted 12 degrees toward or away from the light source, creating a light or dark pixel on the projection surface.

The control electronics direct each mirror to tilt -- in other words, to switch on and off -- up to 5,000 times per second. When a mirror is switched on more frequently than off, it reflects a light gray pixel; a mirror that's switched off more often reflects a darker-gray pixel. This lets DLP project up to 1,024 shades of gray.

To get color, such as for a TV set, a rotating color wheel (with red, green and blue filters) is put between the white light source and the DMD. The control input delivers separate signals for each of the three colors, and each mirror (i.e., each pixel) is switched on and off as the filter rotates each color between the lamp and DMD.

For example, to project a yellow pixel, a mirror will reflect only red and green light to the projection surface. To project a purple pixel, that mirror will be switched off while the blue filter is in position, and the blue and yellow flashes will alternate so rapidly, our brains will blend them together and we'll see purple. This process allows a DLP system to produce up to 16.1 million colors. Older DLP systems also included a clear segment to bump up overall brightness at the expense of color saturation.

Consumer-grade television monitors use the system described above. For very large projection, such as in movie theaters and auditoriums, a more sophisticated system uses three DMD chips, one for each color, plus an optical prism. The prism splits white light into colors and then recombines the three images before sending them through the projection lens. This system, called DLP Cinema, can produce 35 trillion colors.

In most applications, DLP competes directly with LCD projection. DLP typically offers greater contrast (up to 5,000-to-1 vs. LCD's 800-to-1), with better blacks, while LCD produces greater color saturation. Side by side, an LCD display looks slightly sharper than a DLP in text display applications, but DLP has the edge with moving video, reducing pixelation, or the "screen-door effect."

The brightest projectors still use LCD technology, which is slightly more efficient, but the smallest, lightest projectors use DLP. In 2003, DLP systems accounted for 13 percent of the market for large-screen televisions (over 40 inches), according to The NPD Group Inc., a Port Washington, N.Y.-based consultancy. In the past year, the number of models of DLP TVs has tripled.

DLP's origins

The DMD chip was invented in 1987 by TI scientist Larry Hornbeck, who had been exploring the manipulation of reflected light since 1977. In 1992, TI started a project to explore the DMD's commercial viability. A year later, it named the new technology DLP and formed a separate group (now called the DLP Products division) to develop commercial display applications.

In 1994, TI demonstrated prototype DLP projectors for the first time. The technology's promise was quickly recognized. In 1997, the Academy of Motion Picture Arts and Sciences chose DLP to project film at the Oscars, where the first three-chip DLP technology was demonstrated to the Hollywood community.

In 1999, DLP Cinema was first demonstrated to the public with the release of Star Wars Episode I: The Phantom Menace. By December 2002, TI had shipped 2 million DLP subsystems.

DLP Products has also received two Emmy Awards, for broadcast excellence in 1998 and for technology and engineering in 2003. In 2002, Hornbeck was elected a fellow of the International Society for Optical Engineering and received the David Sarnoff Medal from the Society of Motion Picture and Television Engineers.

Kay is a Computerworld contributing writer. You can reach him at russkay@charter.net. -- Computerworld (US)

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