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Introduction

Texas Instruments (TI) launched a study in May of 2002 to investigate Digital Micromirror Device (DMD) field reliability and learn how Digital Light Processing™ technology compared to competing technologies. The goal was to obtain objective data on both DMD and other data projector light modulators in a simulated end use environment as a gauge for comparisons in future marketing and reliability activities. A sample of projectors using DMD and LCD light modulators were placed in a “head to head” life test. The purpose of the test was to gain insight on optical performance over time. Parametric data measurements and image quality evaluations were performed at periodic intervals to gauge useable lifetime, or time to unacceptable performance (based on picture quality), for each projector tested.

Munsell Color Science Laboratory (MCSL) at Rochester Institute of Technology carried out this study under the direction of Texas Instruments. MCSL was responsible for data collection as well as projector use and maintenance. TI was responsible for evaluation and interpretation of the data.

Our end goal was not only to validate the reliable performance of the DMD when compared to LCD, but to learn how DLP™ technology performs over time when assembled within its final projector configuration. Based on internal lab tests, we knew the DMD performance did not degrade for many thousands of hours. We lacked the end-use data that confirmed what we saw in the lab. We also suspected that LCD could not make that claim. Therefore, we rolled this technology into this study to validate DLP™ technology has a competitive advantage in picture reliability. The QRA group within TI volunteered to perform a high level study, to see how data projectors perform over time, and to determine how competing technologies compared.

Though intended as a preliminary investigation, the following assumptions were targeted in the study:

• That optical performance and image quality of LCD technology will degrade over time.
• That optical performance and image quality failure modes of LCD technology are unrecoverable.
• That there is an optical degradation in LCD technology that translates into an unacceptable image quality defect.
• That both optical performance and image quality of DLP™ technology is only affected by lamp performance, and is thus recovered as lamps are replaced.

Methodology

TI selected a sample of the latest data projectors incorporating the following light modulators: five LCD and two DMD. All units were placed in a darkened lab at MCSL and run in a continuous operation mode.

Using the standard, ANSI/NAMP IT7.228-1997 (Audiovisual Systems – Electronic Projection - Fixed Resolution Projectors), MCSL collected the following parametric data at periodic intervals:

• Luminance
• Full On/Full Off (FOFO) Contrast
• ANSI Contrast
• Uniformity
• FOFO and ANSI Contrast for red, green, and blue colors
• Color Chromaticity for white, red, green, and blue

All units received their input signals from the same source material. Lamp replacement was determined by either an on-board system, or by 1/2 starting lumens (ANSI standard) where no on-board system existed.

Results/Observations

After approximately 4700 hours of operation, the following observations and trends were noted in the data:

Full On/Full Off (FOFO) and ANSI Contrast degraded over time for LCD, but remained steady for DMD, as expected (see Figure 2). The optical degradation seen in LCD washed the screen image out and raised the dark levels. Trends in color contrast supported conclusions drawn in FOFO and ANSI for white screen. Graphing just the Dark Levels for all projectors also supported the conclusions drawn from the contrast data.

Significant changes in Color Chromaticity values supported image quality degradation seen on the screen for LCD (see figures 3, 4, 5, and 6). Color Chromaticity remained stable for DMD.

The term Picture Reliability was selected to refer to picture quality over time. Within this study we defined Picture Reliability as "The time to unacceptable image quality attributed to the light modulator subassembly". A graphical representation of Picture Reliability was included in this study (see figure 1). As you can see, the first blemish attributed to optical degradation, occurred at 1368 hours for LCD (see figures 7 for sample baseline, and 8 for sample image defect at 4700 hours). Most LCD projectors exhibited a subtle yellowing of the screen image, then later developed a blue blemish (see figure 9 for sample of “yellowing”). Picture quality remained unchanged for DMD (see figures 10 for sample baseline, and 11 for sample image at approximately 4700 hours of use).

Conclusions

Data shows definite signs of supporting our base assumptions. There is a marked difference in optical performance and picture reliability over time between LCD and DMD projectors.

Image quality defects, caused by the optical degradation were as spectacular as anticipated and occurred quite early in operation, but slower than we hypothesized. Assuming as much as 100 hours of operation per month, we would expect to see the earliest occurrence of an unacceptable picture for LCD at just over one year. The average time to an unacceptable picture for LCD would be at about two years.

Continued downward trends in optical performance for LCD are showing in all parametric data.

Changes in visual "on screen" performance for LCD was not gradual. Its occurrence was sudden and unacceptable under normal viewing conditions. Even the subtle yellowing of the image was judged unacceptable at the point it became apparent to the naked eye.

Data collected for both on-screen lumens and uniformity did not yield any solid conclusions, as far as the two light modulators tested were concerned. These two parameters seem driven more by the lamp than the modulator.

In Closing

The results of this study proved an eye opener, not only where our competition is concerned, but in overall projector performance as well. It pointed out both that our competitors have areas of improvement to explore, and that DMD picture reliability remains steady beyond the expected serviceable life of a data projector. This one fact alone is important for any end user requiring a data projector to last for more than a couple of years. LCD light modulators appear to have a similar life expectancy as the lamps they employ, therefore, preventing user serviceability of projectors based on this technology. One must return the projector to the manufacturer to restore it to its original performance levels. DLP™ technology allows user serviceable lamp replacement as the only life limited item in the projector.