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Illumination & Displays

Low-cost long-life solution to the high-pressure mercury lamp

A new reflector approach, the dual-paraboloid reflector system, makes a 1 to 1 real image of the arc at the output lightpipe.
30 March 2006, SPIE Newsroom. DOI: 10.1117/2.1200602.0142

As the price difference between flat-panel displays (such as liquid-crystal displays—LCDs—and plasma display panels, PDPs) closes with microdisplay rear-projection TVs (MD-RPTVs), the preference in the marketplace is clearly for the former. The challenge for MD-RPTV manufacturers is to maintain the current price difference by reducing the bill of materials (BOM) for their displays. One sure method of doing this is to reduce the size of the microdisplay chips used, as smaller chip sizes require smaller associated optical components, and these are lower in cost. However, when the chip size is reduced, the étendue of the system is also lowered. This makes it harder to reflect enough light from the smaller chips using existing reflector technology: ellipse or parabolic reflectors.

The technical problem arises from the fact that, at the focus of traditional reflectors, the image is fuzzy and the size of the image is larger than the input aperture of the system. In order to create a small image, the lamp manufacturers have to shorten the arc of the lamp. However, when the arc of the lamp is shortened, the lamp life suffers because the electrodes are more susceptible to sputtering. This causes an increase in arc gap and the deposition of electrode material inside the glass bulb. Both of these effects reduce the light output as observed at the screen.

Currently, lamp manufacturers are using lamps with a 1mm arc, and claiming lamp life of 6,000 to 10,000 hours, without consideration of microdisplay imager sizes. In actuality, the life is closer to 2,000 to 3,000 hours when used with small microdisplay imagers: less that one year of normal television usage. With replacement costs close to $300, the actual cost of ownership for MD-RPTV is significantly higher than represented by the retailers.

We have invented a new reflector approach, the dual-paraboloid reflector (DPR) system, which makes a 1 to 1 real image of the arc at the output lightpipe.1Figure 1 shows a schematic diagram of the DPR system, with a tapered light pipe (TLP) at the focus channeling light to illuminate the microdisplay imager. Since the input of the light pipe is larger than the arc for a typical system (see Figure 2), as the arc lengthens due to normal sputtering in the arc lamp, the brightness remains the same until the arc gap outgrows the input face of the lightpipe. In addition, because of the image correction of the DPR reflector, a longer arc and brighter lamp can be used (e.g. an 1.8mm, 200W lamp as opposed to 1.0mm, 120W lamp) while preserving the naturally-longer life of the longer-arc lamp: 10,000 hours.

Figure 1. Schematic diagram of DPR system.

Figure 2. Focused image of the arc produced by the DPR overlaid with original 1.0mm arc in yellow.

In contrast, the elliptical-reflector system is not an imaging device. It focuses the light from the arc lamp from one focus to the other with variable magnification as shown in Figure 3. The magnification depends on the angle of light emitted from the arc. The net result is a fuzzy spot at the focus as shown in Figure 4. As the arc lamp ages, the arc gap grows and the fuzzy image grows even fuzzier, reducing the output as seen on the screen, shortening the arc lamp's effective life.

Figure 3. Schematic diagram of elliptical reflector showing variable magnification.

Figure 4. Fuzzy image of the arc produced by the elliptical reflector overlaid with the original 1.0mm arc in yellow.

The next generation HD5 digital light processing (DLP) imager is 0.443in: smaller than the current HD4 (0.55in) imager. Recent research has determined that traditional reflectors are simply not able to illuminate the smaller chip with reasonable lamp life and sufficient light output. The Wavien DPR 200 lamp puts 6,000 lumens at the output end of the light pipe with an étendue of 8.5 and a 10,000-hour life suitable for driving an HD5 display.

The same holds true for the coming generation of liquid-crystal-on-silicon- (LCOS-) based chips. In addition, LCOS requires polarized light. This makes the problem of reducing the chip size for LCOS manufacturers ever more difficult. The DPR system is immune from this effect and is able to maintain the brightness on the screen despite the increase in the arc length over time. Effectively, the life of the lamp is ‘extended’ in terms of observed brightness at the screen.Figure 5 shows the relationship between lamp lifetime and DLP-imager sizes showing that the lifetime decreases with smaller imagers using elliptical reflectors. For the 0.443in DLP imager, the lamp lasts only 3,000 hours even though the specification is 10,000 hours for larger imagers.

Figure 5. Graph showing the lamp lifetime dependence on DLP imaging panel sizes using elliptical reflector and the DPR.

The DPR system is able to maintain this long life for all imager sizes. Thus, our DPR technology is able to provide the RPTV market with a long-life lamp for low-cost MD-RPTVs. The cost/performance enabled by the DPR will create and surpass the industry-forecasted volume.

Kenneth Li and Seiji Inatsugu
Wavien, Inc
Santa Clarita,California