Application of Backlit AMLCD

Application of Back-lit AMLCDs to Avionics

Abstract

The ultimate glass cockpit technology has arrived—AMLCDs with improved backlights. Initial expectations for weight, power and volume savings are being realized, as are major advantages of cost, reliability and visual performance that exceed that of CRTs. The unique ability to reflect as well as transmit a colored image makes AMLCDs superior to all other technologies. New fluorescent backlight designs with high luminance and dimmability are the key to adapting AMLCDs to the aviation environment. The major impact of AMLCDs is on system architecture. Panel sources and standardization is still a problem compounded by low-volume consumption by the aviation industry.

Flat-Panel Promise

Ever since the 1950s, engineers have been inventing new flat-panel displays (FPDs) for aircraft. Moving-map and multimode displays were the first objectives which were achieved by scrolling paper, film projectors, electroluminescence and plasma panels, and CRTs in varying degrees of flatness. During this quest, CRTs have served well—even in color, if necessary.

The ultimate promise was a flat panel with the following overall performance:

Flat—Low reflectivity flat image with savings in weight, power and volume
Sunlight Readable—Bright and immune to sunlight from both contrast degradation and color washout
Dimmable—Low light level for nighttime and ANVIS compatibility
Quality Image—Over entire image plane without flicker, “jaggies,” or resolution degradation anywhere in the image plane
Rugged—Immune to shake, rattle and roll without degradation, image breakup or hazard to pilot and crew
Available—Multiple sources and sizes, with 20-year product life at reasonable price
Wide Viewing Angle—When needed for cross-cockpit checking
Transparency—Image independent of display technology, data source or airplane type

With the introduction of AMLCDs for avionics in the mid-1980s, such a technology seemed feasible. In 1986, Boeing publicly announced that they were going to use AMLCDs in their all-new-technology airplane, the 7J7, predecessor to the 777 cockpit [1].

In 1992, DARPA accelerated display device R&D at an unprecedented level. DoD issued Buy-America directives for military displays. The government needed two types of flat-panels, one for primary flight instruments and the other for behind-the-cockpit displays for sensors and air-traffic control needs.

History/Evolution of AMLCDs for Avionics

Development, fabrication and qualification of AMLCDs for avionics started seriously with GE Research’s exhibit and prototype demonstration of a 7J7-type display at the 1986 Paris Air Show. The historical evolution of avionic AMLCDs is a story of the combinations and permutations of many players [2].

The congressional mandate for TCASII caused over a thousand 3-ATI-size (3-1/8″ x 3-1/8″) custom back-lit AMLCDs to be retrofitted in airlines. Flatness and image capability of the AMLCD allowed for the necessary supporting electronics to be incorporated in the available panel space and volume. At that time, the aviation market was still viewed as important and Toshiba, Seiko Epson, Hosiden, et al, agreed to manufacture the 3-ATI custom size displays. AMLCD manufacturers no longer consider the aviation market to be important and, with the exception of Hosiden, Sharp and OIS, will not manufacture custom avionic displays. Toshiba, Seiko Epson, Image Quest and Litton Canada have formally stopped making custom aviation AMLCDs.

Limited availability of custom AMLCDs and U.S. Government procurement policy for consumer off-the-shelf (COTS) displays has driven the avionics community to consider using ruggedized COTS displays. The standard 10-inch diagonal AMLCD, when rotated to become a portrait display, works very well as a D-size ARINC standard display. For a wide viewing angle, the rub-angle must be changed in order to make a portrait instead of landscape display, but this is done for commercial applications in bank terminals, for example. Sharp and Samsung have been making portrait 10-inch AMLCDs.

The ruggedized COTS or custom AMLCD needs a customized backlight for the high brightness and dimming requirements for avionics. Development of a high-performance backlight has its own historical development cycle. The most suitable backlight technology is the fluorescent lamp using a conventional triband phosphor. The three leading construction configurations of the lamp are 1) bent tube, 2) multiple commercially-available straight tubes, and 3) monolithic tube panel.

A large infrastructure of U. S. ruggedized high-performance backlight manufacturers has evolved. The same design issue exists for backlights whether they are used for COTS or custom AMLCDs.

AMLCD storage, startup and operating temperatures were problems. The LC materials supercool and remain operable, although slower, down to -40°C and lower. Speed of operation can be increased with heating via a transparent Indium-tin-oxide (ITO) thin-film heater applied to the back of the display. Also, the backlight acts as a heater because of its power dissipation.

The upper temperature is limited by the clearing point of the LC material. Above the clearing point temperature, an immediate but reversible loss of image occurs. Modern LC materials can be fabricated with a clearing point temperature of over 90°C and without sacrificing other properties.

Historically, the viewing angle of an AMLCD has been limited and has impacted cross-cockpit reading. Cathode-ray tubes are lambertian emitters and, as a consequence, their viewing angle was never an issue. This is an issue with AMLCDs which must be specified in the requirements document and tested for during qualification. However, the present generation of AMLCDs can achieve an adequate viewing angle out to ±60° horizontally. This is now done with a combination of optical compensating films and multigap cell thickness using the normally white mode. The Boeing 777 AMLCD uses the normally black mode with half-tone pixels made by Hosiden/Honeywell [3].

The new in-plane twist AMLCDs have an excellent wide viewing angle comparable to that of a CRT. The introduction of in-plane twist AMLCDs for avionics may be delayed because of availability. Also, NEC’s in-plane twist displays on the market have reduced optical transmissivity. Presently, for avionics, the transmissivity is more important than further improvement in viewing angle. As will be explained later, avionic AMLCD transmittance to maintain immunity to ambient illumination is of primary importance.

Aviation Grade Primary Flight Instruments

The flat-panel “glass” display for primary flight information has the highest performance requirements of any display application. Tempest displays may have a higher restriction on electromagnetic emission and submarine displays may have a higher requirement for shock. There may be other environmental requirements in special applications, but, clearly, the cockpit has the most hostile readability issues. These readability issues can be outlined as follows:

  1. Readability in ambient lighting of from a low of zero to a high of 6,000 to 20,000 fL of diffuse and/or collimated illumination from one or more directions.The display is mounted to minimize the impact of the ambient illumination. The display is typically mounted under an eyebrow and tilted slightly downward in a cockpit with reduced windows when possible. All reflective surfaces inside the AMLCD front surface are minimized.At the air/display interface on the front surface, there must be antireflective treatment and, to scatter any remaining reflections, there must be a front diffuser treatment.
  2. The pilot’s eye may be highly desensitized because of exposure to the outside bright environment of sunlight, clouds, haze, snow, etc. To meet the outside brightness requirement or to avoid the black hole phenomenon, the display luminance must be increased to as much as is practical. The luminance is limited by the backlight power and manageable heat dissipation.
  3. The pilot’s eye position is limited and the display orientation is tied to the aircraft attitude. The display must always be readable immediately, without a change in the viewing angle. Glint reflections of the sun or specular reflections must be avoided by making the display a lambertian reflector of ambient illumination. Light that could not be transmitted into the display by the antireflective coating and absorbed or recycled must be scattered.

Other requirements of ruggedness, storage, EMI, ANVIS compatibility, etc., are achievable with back-lit AMLCDs. The market for aviation-grade displays is limited to the aviation industry—thus, very small.

Why the Back-lit AMLCD is So Good

The back-lit AMLCD is unique in several ways that prove to be highly desirable attributes for avionics use. The following objectives for primary aviation instruments are satisfied by the AMLCD: 1) Reasonable flat or low volume with low weight and power preferably without forced air cooling; 2) Logarithmic dimmability from luminances of 0.05 fL to as high as 200 fL; 3) Color primaries of saturated red, green and blue with approximately eight shades of gray in each primary; 4) Wide viewing angle of ± 60° in the horizontal direction and +30° to -10° in the vertical direction; 5) Readability in high ambient illumination with contrast ratios of at least ten to one; 6) Uniform resolution through the image plane with at least 160 lines per inch addressability; 7) Video speed for at least 20 new frames per second.

All flat-panel technologies meet most of the requirements for avionics but most have problems in luminous efficiency and sunlight readability. When the display technology is made bright enough to be sunlight readable, power requirements are prohibitive due to thermal management problems and display self-destruction. Further, they may be made sunlight readable in luminance contrast ratio but the color becomes washed-out.

Because of performance under high ambient illumination, AMLCDs are superior to all other electronic display technologies. Simply stated, “THE LIQUID-CRYSTAL DISPLAY REFLECTS THE IMAGE PORTRAYED.”

This is the single most-important attribute of the AMLCD over all other technologies in the application to avionics. Because of the polarizers, aperture size and color filters, transmittance of AMLCDs is only about 8% and, after reflecting back, only about 0.64% of the ambient light is reflected to the viewer. However, the contrast ratio is high and the colors remain saturated since the normally-broad band ambient light passes through the color filters twice. If it were not for the first surface reflections the contrast ratio would be well over ten to one. After aviation grade first-surface treatment the reflectivity is approximately 0.5%. The resulting contrast ratio is approximately two to one and the colors are still clear and approximately 50% saturated. The image is discernible but not satisfactory because it is too dim. When reading an image with low luminance contrast, the color contrast is very important. Now, add the backlight to the appropriate luminance level and the resulting image is spectacular.

The second advantage that AMLCDs have over all other technologies is the separation of image and luminance. THE AMLCD IMAGE AND BACKLIGHT ARE SEPARATE ENTITIES. With separation of the backlight from the image generation, there are several unique advantages: 1) The best and most-efficient lighting technology can be used. Thus far, this has been found to be fluorescent lamps; 2) Luminance, uniformity, color and dimmability are delectable independent of the image generation; 3) Thermal management of the back-lit AMLCD is isolated to the backlight; 4) The backlight is a low-cost replaceable assembly compared to the cost of the AMLCD image generator.

Problem Areas for Back-lit AMLCDs

All the technical problem areas have been solved. The difficulties that remain are not readily solvable. The biggest, perhaps, is the lack of a significant market demand for the high-performance aviation-grade displays compounded by the fact that there is little or no standardization of size, pixel arrangement or resolution. Fortunately, most all of the high performance issues can be added to commercial displays. Thus, ruggedized COTS is readily feasible for avionics.

There are many design details that will be improved upon as the technology evolves. One is the first surface treatment and minimization of specular reflections. Another is efficiency of the overall optical path of the luminance from the backlight and better control of the viewing angle to prevent wasted and/or stray light.

Conclusion

The back-lit AMLCD is ideal for aviation. There is no display technology that comes close to competing with it. The back-lit AMLCD creates an image that is transparent to the airplane, electronic and navigational sensors and the pilot. The product life is not limited to any particular size or configuration. In the future, advanced versions of back-lit AMLCDs can be substituted at will.

The back-lit AMLCD accommodates the universal display image concept. The image can change via software changes only. The feasibility of the ultimate universal primary flight images may be here sooner than we think [4]. The display is so small that inertial sensors and GPS receivers can be integrated into the same box as a panel-mounted display system. The next step in avionic architecture will be to make display boxes more independent with more self-contained information allowing for better situation awareness, failure analysis and confidence in the displayed image.

References

  1. J. Rupp, “Color Flat-panel Displays in the Commercial Airplane Flight Deck,” Japan Display ’86 IDRC, September 30-October 2, 1986, Tokyo, p. 406.
  2. L. Tannas, Jr., “World Status of Avionic AMLCD Panels,” SPIE Cockpit Displays, Volume 2219, April 7-8, 1994, Orlando, Florida, pp. 202-212.
  3. R. McCartney, E. Haim, & C. Kucera, “Performance Testing of the Primary Flight Instruments for the Boeing 777 Airplane,” SPIE Cockpit Displays III, Volume 2734, April 10-11, 1996, Orlando, Florida, pp. 86-93.
  4. L. Tannas, Jr., “Laws for the Design of the Universal Cockpit Displays” (Paper 3363-501),” SPIE Cockpit Displays V, April 15-17, 1998, Orlando, Florida.

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