EL displays show sunlight readability advantage

June 1, 1998
MISSISSAUGA, Ontario - A new flat-panel display design approach that enhances sunlight readability and reduces glare received a healthy vote of confidence with two recent contract wins.

By Chris Chinnock

MISSISSAUGA, Ontario - A new flat-panel display design approach that enhances sunlight readability and reduces glare received a healthy vote of confidence with two recent contract wins.

Officials of Luxell Technologies Inc. of Mississauga, Ontario, who created the display technology they call a sunshine legible black layer, received new contracts from GEC Marconi Avionics Ltd. of Rochester, England, and Oceaneering Space Systems of Houston. The superior contrast of the displays in high ambient lighting conditions was a principle reason for the contract awards, Luxell officials say.

Luxell is a relative newcomer in thin film electroluminescent (TFEL) display manufacturing. Fred Prins, the company`s director of international sales, says the Luxell patented display technology can achieve a 2-to-1-contrast ratio in direct sunlight. "I defy anyone to beat that," Prins challenges.

Apparently, officials of Planar Advance in Beaverton, Ore., have heard the call. They have teamed with experts at the Georgia Tech Research Institute in Atlanta, Ga., to develop displays with even higher contrast in high ambient conditions.

Tom Curran, the Planar manager for advanced programs, says his engineers have tested two high-brightness display configurations in sunlight. In one display, they used a standard aluminum rear electrode with a night-vision filter and achieved a greater than 3-to-1 contrast.

The second configuration included a patented technology they call Integral Contrast Enhancement (ICE), but without a night-vision filter. This produced a 4.5-to-1-contrast ratio. Planar experts are building several thousand displays per month using their ICE process.

A band pass optical night vision filter suppresses light beyond about 600 nano-meters. The filter goes over cockpit instruments to enable pilots to use night vision goggles.

Without the filter, the infrared energy from these instruments would swamp sensors in the night vision goggles. A night vision filter also reduces the internally emitted light of an electroluminescent display.

Engineers from Luxell and Planar have developed what experts generally call "black layers" to improve the performance of the monochrome TFEL displays in bright sunlight.

The back electrode in a TFEL stack normally comes with a highly reflective aluminum metal. For indoor applications this helps improve brightness by providing a surface to reflect internally generated light from the phosphor layer. Outdoors however, the black electrode of this display also reflects sunlight and can overpower the display phosphor, degrading contrast. Light also can leak into adjacent pixels causing blooming and additional contrast degradation.

While officials from both companies are guarded about the composition of these black layers, Luxell experts revealed that their black layer has vacuum-deposited absorbing and dielectric materials.

This layer goes between the back electrode and the phosphor material in the EL stack. Although the absorbing material reduces the intensity of incoming light, designers adjust the thickness of the black layer so that it is about a one-fourth wavelength thick. In this way, light reflected from the back electrode will be 180 degrees out of phase with light reflected from the top of the black layer, thus canceling each other out. The thickness is optimized to reduce reflections at the emission wavelength of the phosphor, 585 nanometers. Prins notes that this technique has produced displays with 14 percent total reflectance.

Planar`s ICE technology uses a graded index material between the phosphor and back electrode. This absorptive material provides a smooth index gradient, matching the indices of the end materials, thus eliminating reflections.

Both techniques produce a display that appears black instead of gray when the display is off. The trade-off is less emitted light because of the loss of the back reflector. Consequently, the displays are often driven at a higher refresh rate. With TFEL displays, the higher the refresh rate, the brighter the display - but at the expense of more power consumption.

In April, Luxell officials won a contract from Oceaneering Space Systems, a division of Oceaneering International Inc. The display will be for the Trace Gas Analyzer System on order from NASA. The instrument is a mass spectrometer for determining the partial pressures of ammonia, hydrazines, nitrogen, and oxygen. Astronauts will use it aboard the future space station.

Last February, Marconi officials awarded Luxell a $1 million contract to deliver displays for their Sky Guardian 2000 Radar Warning Receiver. The display features a 150-by-155-pixel arrangement packaged in a 3ATI form factor. Marconi engineers are upgrading their current radar warning receiver, which is CRT-based, and are marketing the new receiver for use in new and retrofit fixed-wing aircraft and helicopters. Marconi officials expect to show the new receiver in England at the upcoming Farnborough Air Show in September.

AMLCDs could not meet the 10 watt total power requirement because of the need to incorporate resistive heaters to operate the display down to -54 C, according to results presented at last April`s Aerospace Sensing Conference in Orlando, Fla. FEDs were judged promising, but currently immature. Only TFEL was mature enough.

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Electroluminescent displays from Planar Advance and Luxell Technologies are using a special black layer to improve sunlight readability and reduce glare.

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