LED displays turn to light pipes to increase luminance

At this year's Society for Information Display symposium, Global Lighting Technologies introduced a 15-in. backlight that uses the company's MicroLens lightguide technology.

Aug 1st, 2002
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At this year's Society for Information Display (SID) symposium, Global Lighting Technologies (GLT; Brecksville, OH; www.glthome.com) introduced a 15-in. backlight that uses the company's MicroLens lightguide technology. Such a backlighted display provides uniform 2700-cd/m2 output (see Vision Systems Design, July 2002, p. 6). According to David DeAgazio, director of worldwide sales at GLT, such panels could be used to effectively replace flat area-array light-emitting-diode (LED) based illuminators in certain machine-vision applications.

In the design of automated smart-card inspection system, Gemplus abandoned standard fiberoptic backlights in favor of edgelit backlights from Phlox. According to the company, this has eliminated maintenance costs and increased machine efficiency.
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Another backlight supplier, Phlox Corp. (Provence, France; www.phlox-gc.com), has finalized several patents based on its edgelit-backlight light-pipe technology initially developed for liquid-crystal-display backlighting applications. Manufactured of polymethyl methacrylate (PMMA) by Rohm and Haas (Philadelphia, PA; www.rohmhaas.com), the PMMA surface of the backlight is machined using a CO2 laser, according to Phlox's proprietary scattering and refraction model of the material. Now, the company has diversified the use of the technology with 10 standard products developed for machine-vision applications.

"When light is injected on a side of a Phlox light pipe," says Stephane Francois, executive vice president of Leutrek Vision (Burlington, MA; www.leutrek.com), Phlox Corp's distributor in the United States, "more than 90% of the light is re-emitted uniformly on the surface, making it an efficient way to emit light uniformly on a large surface." Of the 10 standard products, Phlox's 6080890 is the largest display, measuring 8 X 8 in. with a luminance of 4500 cd/m2.

Instead of PMMA, Advanced Illumination (AI; Rochester, VT) uses a film-based refracting material from a leading manufacturer of solid thin films. According to Bill Thrailkill, AI's founder and chief executive officer, power-balanced arrays of LEDs must be precisely aligned and targeted around the refracting material to provide an even illumination. Using this technology results in devices with comparable illumination of those manufactured by Phlox.

Already, Phlox's displays are displacing the standard fiberoptic backlights powered by halogen lamps currently used in many machine-vision applications. Gemplus Gemenos (Gémenos, France; www.gemplus.com), for example, makes magnetic-stripe cards, microprocessor-based smart cards, software-development tools, and readers. To inspect the smart card electrical contacts that are precut from conductive metallic sheet, the company used a vision system that has a fiberoptic backlight powered by a 150-W halogen lamp.

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According to Chris Blanc, chief executive officer of Phlox, this lighting system was performance-limited. "First, poor lighting uniformity required lighting compensation that increased the cost of the machine," he says. "This, coupled with poor light intensity and short lamp lifetime required extended image integration time and regular maintenance, all of which increased the operation cost."

To overcome this challenge, the company was faced with two alternatives. While metal-halide or xenon lamps could have been used, they would add to the cost and maintenance of the machine without resolving the uniformity issue. LED matrix technology had also presented a possibility but its heat generation made integration difficult and costly. As an alternative, the company chose the 2i-BL 4 X 4-in. 6040400 display from Phlox. With more than 80% light uniformity and long lifetime, this choice eliminated maintenance costs and increased machine efficiency.

"Because of the initial success of the illumination source," says Blanc, "Gemplus has upgraded all 50 of its inspection systems with the Phlox backlight." And, to increase the throughput of the test systems further, Phlox has just introduced a 4i-BL series of the 4 X 4-in. backlight that will provide twice the illumination power of the 2i-BL model.

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