Step 3: How DLP TV Technology Works

The Basics: DLP TVs are rear-projection units, meaning they create pictures by manipulating light, which originates from a centralized source, as it's projected onto a screen. If you've read our article on LCD displays, you already know that they work by blocking light. DLP TV monitors, by contrast, work by reflecting light: That is, they utilize a complex system of mirrors to reflect or deflect red, green, and/or blue light through a optical projection lens and onto the screen in front of you. These mirrors can switch between ON (where light is fully reflected) and OFF (where light is fully deflected) states. And, by switching between light and dark states at great speeds, these mirrors are able to reproduce the gray scale with staggering accuracy.

The Digital Micromirror Device (DMD): The functional core of DLP technology is the DMD semiconductor, which TI defines as a binary spatial light modulator. That's tech speak for an "on-" and "off-" (binary) moving (spatial) light regulator (light modulator)--a light switch, if you will.

Let's have a look at one of these semiconductors that Texas Instruments claims has "changed everything."

The principle behind these semiconductor chips is fairly straightforward. DMDs contain millions of microscopic mirrors--one for each pixel in their display--that can be directed to tilt 10° back and forth on their axes. These micromirrors are made of aluminum for maximum reflectivity and durability. They measure just 16 µm by 16µm (1 µm = 1 millionth of a meter).

Let's just say a strand of your hair is about five times the width of one of these mirrors.

These micromirrors are arranged into rectangular or square grids (depending on the DMD's aspect ratio), with each mirror separated by 1 mm. To the unaided eye, though, this 1 mm of space is invisible. So, what looks like one, solid thumbnail-sized mirror is really millions of micromirrors arranged in a grid pattern, like so:

DMD™ with Mirror Removal

The top left view shows nine mirrors. The top right view shows the central mirror removed to expose the underlying, hidden-hinge structure. The bottom right shows a close-up view of the mirror substructure. The mirror post, which connects to the mirror, sits directly on the center of this underlying surface. Lastly the bottom left view shows several pixels with the mirror removed.

DMD™ and Straight Pin

Micrographic photo of the tip of a pin on the surface of a DMD. Each mirror is 16µm per side, with a 1µm separation between mirrors.

The actual number of micromirrors present on a given DMD is equal to the number of pixels on your TV's screen (a k a its resolution). A display with a resolution of 1280 x 1024 contains a DMD consisting of 1,310,720 micromirrors. This "one pixel, one mirror" setup lends itself to exceptionally precise digital imaging.

The Mirror As Switch: Since each micromirror on a DMD chip operates independent of its neighbors on the DMD grid, each aluminum mirror can reflect light in one of two directions--either 10° toward or 10° away from a lens. The light the gets reflected is directed towards an optical lens, which projects that light onto a screen in the form of a lit pixel. A light absorber absorbs the light that is deflected away from this lens, so no light reaches the screen at that particular pixel, producing a dark, square pixel image. In short, each micromirror either illuminates a pixel (i.e., switches it ON by moving +10°) or darkens a pixel (i.e., switches it OFF by moving -10°). When these mirrors aren't in use, they remain "parked" at 0° on their axes.

Addressing the Switch: A DMD operates according to the data input into the static random access memory (SRAM) cells located beneath each micromirror. The data directing each mirror's tilt angle comes in the form of binary bit planes (i.e., ones and zeroes), where 1 = +10° tilt = ON and 0 = -10° tilt = OFF. The incoming video or graphics signal is decoded; the address electrodes are activated; and they produce the electrostatic torque necessary to rotate the mirror. If the mirror reflects light, that light is directed toward a projection lens, amplified, and then cast onto the back of a screen. If the mirror deflects light, it gets absorbed, which leaves a pixel-sized portion of the screen dark.

Note: With DLP technology, each pixel of information gets mapped directly to its corresponding micromirror. If the signal has a 640 x 480 sampling structure, the central 640 x 480 mirrors on the DMD will be active in broadcasting the picture. All the extraneous mirrors outside the 640 x 480 area will be tilted -10°, or "off," which preserves the integrity of the source image as it's processed and displayed.

Recreating the Gray Scale: With DLPs, grays are produced through the rapid oscillation of micromirrors switching between light and dark states. Grays are encoded in bit planes that represent the ratio of light states to dark ones, "on's" to "off's", ones to zeroes. A pixel's relative brightness is written in 8- or 10-bit segments, with each bit representing successively longer on/off durations. In an 8-bit word, for example, the time durations have relative values of 2⁰, 2¹, 2², 2³, …, 2⁷, which allows for 256 unique combinations of ones and zeroes or lights and darks. Each pixel is thus capable of displaying 256 or 28 equally spaced shades of gray or relative brightnesses. 10-bit image processing boosts conventional gray scale gradations by a factor of four to 1024 or 210. Lighter shades of gray consist of more "on's" than "off's," and darker grays consist of more "off's" than "on's." The gray scale you see is effectively the analog version of a purely digital light signal. This technique for producing the sensation of the gray scale before a viewer's eyes is called binary pulsewidth modulation.

Coloring the Gray Scale: DLPs utilize a color wheel composed of red, green, and blue filters, which direct individual pulses of colored light toward the DMD for gray-scale processing. This means that, at any given instant, only one of the primary light colors is hitting the DMD, but when the filter system spins fast enough (~120 time per second), the colors appear blended into a full-color digital image. DLPs display color images as rapid sequences of red, green, and blue light signals, which the brain integrates or reads this sequential color data as an analog or "whole picture" signal. The result is an image teeming with colors. [After all, 256 shades of red x 256 shades of green x 256 shades of blue " 16.8 million different colors!]

All this color blending and/or image formation, known as "averaging," is performed on the viewer's end, so DLP TV technology respects the digital integrity of video/graphic signals from start to finish. This ensures that the image you see on your TV screen is an exacting rendition of the source material, as it were, bit for bit.

Note: Because DLPs "stack" red, green, and blue components of light over a period of time--albeit an exceedingly brief period of time (milliseconds)--it is theoretically possible to see a "rainbow effect" on your TV screen. This is an instance where the colors apparently fail to layer properly, so that viewers perceive discrete transitions from red to green to blue on the screen (hence the "rainbow" euphemism). Few people are genuinely bothered by this phenomenon, and it seems mainly to occur on the earliest DLP models. Second- and third-generation DLPs have been reengineered to mitigate such color separation.

Still, the only way to figure out if you're sensitive to the speed of the color wheel and can see artifacts generated as it spins is to do some viewing tests of your own. You should try out a variety of video material because this effect tends to be content-specific, i.e., more prevalent in the dark areas around a moving bright spot.

8 Steps to Buy a DLP TV

• STEP 1: DLP Television Today
• STEP 2: DLP: Rear Projection Television for a new generation
• STEP 3: How does DLP technology work?
• STEP 4: What are the advantages of DLP displays?
• STEP 5: DLP TV purchasing considerations
• STEP 6: DLP Television placement options
• STEP 7: How and where to buy a DLP TV
• STEP 8: Find a reputable online DLP TV dealers/vendors