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.



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 TI optical semiconductor chip that makes DLP possible.
The TI optical semiconductor chip that makes DLP possible.
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 yellow pixel, that mirror will be switched off while the blue filter is in position, and the red and green flashes will alternate so rapidly, our brains will blend them together and we'll see yellow. 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.

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