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Comparing Rear-Projection Display Technologies -- Part 2

AP/LCD, polysilicon LCD and DLP have important differences By Lisa Duckett
Editor's note: Did you miss part 1 of this feature? Click here to read it.

Digital Light Processing (DLP) was developed by Texas Instruments, Inc., which is the sole supplier of this display technology. The core of the DLP engine is the Digital Micromirror Device (DMD), an array of more than 500,000 hinged, microscopic mirrors, with each mirror corresponding to a single pixel in the display. Using a light source and a color wheel, colored light is reflected off the mirrors; the relative amount of time each mirror is in the "on" or "off" position determines the hue and shade of the pixel it generates.

DLP is growing in its acceptance for rear projection due to its widespread use in portable presentation projectors. But it too has several shortcomings. First, the available resolutions are limited to SXGA and lower. Second, the native contrast can be much lower than that of AP/LCD. Third, the color sequential architecture uses a mechanical rotating color wheel that is susceptible to failure, and can exhibit the image artifact characteristic known as "color breakup". Finally, the video performance of DLP is limited in the low level dynamic range due to the pulse width modulation scheme used to achieve gray shades.

These limitations are being addressed as DLP is increasingly being marketed to the consumer television market. New techniques for reducing the image artifacts and improving the contrast are on the horizon, and are anticipated to be available in future generations of projectors.

Comparing technologies
The tables included on the last page of this article present the comparative attributes of the three technologies. Users should weigh these attributes in relation to their application needs. Equally important, they should consider each technology with respect to its cost of ownership over time. For example, in the case of AP/LCD versus polysilicon LCD, the typical brightness of polysilicon LCD might be specified as higher than that of AP/LCD; however, users should consider what is required to maintain that brightness over time. Also, when comparing the failure rates of polysilicon LCD devices (at 16,000 hours maximum) to those of AM/LCD at 80,000 hours maximum, the main differences between the LCD technologies become all the more apparent.

This article has discussed the three leading digital technologies for rear projection displays: AP/LCD, polysilicon LCD, and DLP. Those examining DLP in relation to polysilicon LCD have pointed out the shortcomings of polysilicon LCD for applications requiring high reliability, and the author agrees with these concerns. As DLP continues to mature, its limitations of SXGA resolution, color breakup, video performance and contrast are also expected to be overcome.

AP/LCD, on the other hand, does not currently have these same limitations and also represents a most reliable approach in rear-projection systems as a result of its high image quality and resolution, contrast, and brightness uniformity.

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Related Keywords:part 2, two-part feature, Clarity Visual Systems, Lisa Duckett, Digital Light Processing, DLP, Digital Micromirror Device, DMD, rear projection displays, AP/LCD, polysilicon LCD, DLP

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