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Rear-Projection Helps New Avionics Displays Fit into Old Cockpits

Photonics Spectra
Dec 2007
Hank Hogan

A plane’s cockpit is a tight space, full of displays that are rectangular, square, circular, trapezoidal or something else, all of which convey vital information to the pilot. Because most displays in use today are based on older technology, there is a push to replace them with something newer so that the relatively small avionics market can take advantage of the latest technology and benefit from its cost structure. But the cost of tooling up to make such specialized shapes would be high and the market small, since there are not that many airplane cockpits.

LED_SID_fig1.jpg

This schematic depicts a novel avionics display that uses rear-projection technology. Three LEDs provide light that reflects off a liquid crystal on silicon microdisplay and then is projected onto a viewing screen. Images reprinted with permission from the Journal of the Society for Information Display.


There is a problem, however. Most displays are rectangular, whereas not all avionics displays are. Bridging that gap must be done economically.

Now a team from the Italian research institution CESI Ricerca and from the avionics company Logic SpA, both in Milan, has demonstrated an inexpensive solution based on rear-projection technology. The investigators’ display is scalable and more easily can match the shape of an instrument panel. “These advantages are related to the projection display technology, regardless of the display device technology,” said Riccardo Grassetti of Logic.

LED_SID_Fig2.jpg
The rear-display prototype fits within an allotted space in a cockpit and weighs 15 percent less than a traditional display.


As a result, he added, the technique could be used by active matrix LCDs, liquid crystal on silicon chips, digital mirror devices and organic LEDs. The choice of which display technology to use can be based on the particular application.

The researchers selected a liquid crystal on silicon microdisplay for their demonstration because the technology has multiple suppliers — not true of digital mirror devices — and is considered more reliable than organic LEDs, which is a key avionics requirement. They illuminated the device with red, green and blue diodes, using a polarizing beamsplitter, an aluminum-coated prism, dichroic mirrors and lenses to project the device output onto a viewing screen. They chose the multiple diodes instead of multiple display chips to reduce space requirements and energy consumption. The resulting display was 15 percent lighter than a traditional avionics device of the same size.

They tested the display for luminance, as well as for its ability to withstand temperature swings and vibration. The latter was challenging for the prototype, as it had to withstand root-mean-square acceleration of 5.4 g. That the system passed was not surprising, Grassetti said. “The prototype was designed paying great attention to these requirements.”

LED_SID_fig3.jpg
Shown here are the trichromatic coordinates of the experimental rear-projection avionics display (solid line) and the required specifications (dashed line). Note that the display met requirements for this specification.


With regard to luminance, however, the results were more disappointing. The maximum luminance target was 340 cd/m2, but the prototype produced only 301 cd/m2. The researchers attributed this deficit to the light sources, which they said were not bright enough. Plans call for testing another technology. Grassetti noted that one possibility might be RGB solid-state lasers, which already exist at a prototype level and will be available in the market in the near future.

Although the demonstration is promising, he noted that a solution to the luminance issue must be found before the technology can move out of the prototype stage. “Before considering commercial prospects, we need to improve the projection display luminance.”

Journal of the Society for Information Display, October 2007, pp. 799-803.


GLOSSARY
liquid crystal
A type of material that possesses less geometrical regularity or order than normal solid crystals, and whose order varies in response to alterations in temperature and other quantities. Liquid crystals are characterized by phase varieties, including cholesteric, nematic and smectic. The optical properties of liquid crystals are familiar from their use in displays, known as LCDs.
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