I first saw a DLP (Digital Light Processor)* demonstration in 1989 at a Society for Information Display (SID) conference in San Jose. The domo was given by Texas Instruments, Inc., the inventor of the DLP. Demonstrations at SID conferences represent the very cutting edges of new display technologies. Therefore, the demos are comparatively crude exhibitions of concept prototypes. However, in this instance the DLP demo was particularly crude. It exhibited a comparatively dim, low-resolution projected monochrome image with several black areas caused by "stuck" pixels. Of all the prototypes shown, DLP seemed to have the least promise. A few years later, at the urging of Dr. Jim Carnes, then Director of the David Sarnoff Research Center, I traveled to Washington, DC, to see another DLP demo at a Defense Advanced Research Projects Agency (DARPA) conference. What I saw was now greatly improved. The image was larger, brighter and in color. Although there were still a number of "stuck" pixels and a slightly misconverged image, the three chip system now showed great promise as a possible consumer electronics product, albeit very expensive. Subsequent visits to Texas Instruments in Dallas indicated an ever-increasing level of performance.

Today, DLP is one of the prime projection technologies used for HDTV. Many major manufacturers have adopted DLP for most or all of their HDTV projection models. And no wander, DLP delivers a very cost effective, bright, high-quality HDTV image using an elegant, simple projection system design. Here is a brief review of how it works:

Figure 1 is a photograph of a mounted HDTV DLP chip. This chip consists of over two million individual pixels plus driving electronics. A photomicrograph (Figure 1A) of the chip surface shows the actual pixel array.

Each pixel consists of a tiny movable mirror, which flips back and forth upon application of a very small voltage provided by the driving electronics.

Figure 2 illustrated the anatomy of a DLP pixel. The moving mirror is mounted on a hinge that allows it to pivot approximately ±10° upon application of the "address" voltage. This voltage is applied between the mirror surface and one of two electrodes located on either side of the pixel base. When the address voltage is applied to either one of the base electrodes, the static charge generated causes the mirror to pivot toward the respective address electrode. Figure 2 illustrates the mirror pivoted in a position that reflects light away from an associated lens system that projects the light to the screen. Thus, this pixel's reflected light does not appear on the screen, generating a projected "black" pixel.

When the pixel is flipped to the opposite position (Figure 3), light is directed into the projection lens system, generation a "white" pixel. This bi-stable light switching action allows the image to be generated digitally. By varying the amount of time each pixel reflects light into the lens system (the "on" time), the brightness of the pixel's reflected light can be varied, thus creating a varying gray scale, i.e. the less time the pixel is "on," the darker the reflected pixel, and the more time it is "on," the brighter the pixel. This digital addressing scheme is very similar to the driving process used with plasma technology. (See my previous article, "Plasma.")

There are three methods today's DLP systems can generate a color image. One method is to use three separate chips, one for each primary color. The incoming light is split into the primary colors (Red, Green and Blue), modulated by the respective chip, and converged in the projection optics. Since the light reflecting efficiency of the DLP mirrors is very high, the resulting projected image from a three-chip system can be very bright. This solution, however, is relatively expensive, but is successfully employed in commercial front projection applications.

The second method is to employ a rotation color filter (wheel) with a single DLP chip. The filter wheel itself (Figure 4), consists of a disk segmented into the three primary colors, plus some additional color "mixes" to increase the projected color gamut. The primary color components of each video frame sequentially modulate the DLP chip in synchronization with the respective light color passed by the rotating color wheel. The result is a full color image projected onto the screen (Figure 5).
[Historic note: This technique is actually a modern adaptation of the CBS color wheel scheme first used in early 1950's but later replaced by the all electronic RCA tri-color system.]

The color wheel method is the one used today in consumer electronics applications of the DLP technology, as well as in some less expensive commercial front projection systems. Using one chip with a color wheel allows a very cost effective, low weight projection "light engine," with the added advantage of eliminating the complexity and cost of optical color convergence.

The third method is new, having been first publicly demonstrated at the 2006 CES. This method eliminates the mechanical color wheel by utilizing switched Red, Green, and Blue LEDs (Light Emitting Diodes) to generate a sequential primary color light source. This system allows an all-electronic solution with all the advantages of the color wheel method. It can be anticipated that soon all DLP systems with employ LED's.

The primary DLP competitors for consumer electronics applications are LCD and LCoS technologies, both of which will improve performance by the use of LED's. From a performance standpoint there is parity among these technologies. At the end of the day, the winner will be decided by systems cost. And that's a good thing.

Ed

*Also known as DMD - Digital Micromirror Device

(Illustrations courtesy of Texas Instruments Inc.)

Posted by Ed Milbourn, February 6, 2006 10:39 AM