Factors Affecting Efficiency and Output of LEDs Used in Display-Backlighting Applications for Aircraft Cockpits
LEDs bring a new dimension to LCD screens in cockpit applications.
by Francis Nguyen
ILLUMINATING information displays in aircraft cockpits sounds like a straightforward task. But having the right illumination levels, colors, and display quality means better safety and security when flying 300,000 pounds of heavier-than-air aircraft filled with several hundred passengers. The ability to access information quickly and reliably is critical. Avionics instrumentation has kept pace with the increasing demands for access to technical and operational information in flight, and now LEDs bring a new dimension of light to LCD backlights and indicators for aviation cockpits.
One element of the design of commercial aircraft displays that is sometimes overlooked is the correct level of light. For the human visual system to be able to read displayed information efficiently, it must have the proper balance of light. For best results, this light must meet several criteria, including the appropriate luminance and color spectrum that enhance legibility without causing undue eye fatigue.
Many aircraft with "glass cockpit" displays – those that use electronic instruments rather than mechanical gauges – still rely on fluorescent lighting for the LCD-panel backlights as well as other light sources, including incandescent lamps. The trend in cockpit design, however, is to move away from fluorescent lighting for LCD-panel backlights and toward the use of LEDs (Fig. 1).
Fortunately, the industry is rapidly integrating solid-state lighting into airliner cockpits and display backlight units. Advances in LED technology give solid-state lighting distinct advantages for aviation applications, such as increased reliability, reduced maintenance, and improved visibility.
Advantages of LEDs
The avionics industry is well aware of the limitations of fluorescent and incandescent lighting in cockpit applications and is embracing LEDs as the backlighting technology of preference. LEDs address many of the technical challenges of aircraft displays and offer a significant number of advantages compared to incandescent and fluorescent lamps:
• Solid-state robustness compared to incandescent and fluorescent lamps, which have glass envelopes prone to breakage.
• Wide operating temperature range of – 40° to +100°C. Fluorescent lamps, including cold-cathode fluorescent lamps (CCFLs), typically have a low temperature limit of – 20°C, which requires a warm-up time before providing any useful light if the aircraft is parked in sub-zero temperatures. This becomes critical for tactical and military aircraft, which require quick take-off.
• Lifetimes of at least 50,000 hours. This compares to a few thousand hours for incandescent lamps often used in backlit switches and indicators. FAA regulations require parts to be changed out at rated life, even before actual failure, resulting in substantial maintenance costs and aircraft downtime.
• LEDs are more efficient than incandescent lamps for applications requiring colored light, such as red and yellow indicators. The colored light is achieved by putting a color filter in front of the miniature incandescent lamps with a resultant efficiency of 1–6 lm/W. On the other hand, indicator-sized LEDs have efficiencies upward of 50 lm/W and they work with and without color filters. LEDs not only reduce power consumption in the cockpit, they also reduce the heat generated by lamps.
• LEDs require a low-voltage DC drive (2.0–3.8 V) compared to the higher voltage (typically >20 V) for AC drives for CCFLs. Careful shielding is needed to separate sensitive navigation equipment from any EMI generated by the AC drive.
• LEDs are RoHS (reduction of hazardous substances) compliant and rarely require replacement, thus reducing their impact on the environment.
• LEDs provide excellent color saturation and a wide color gamut of over 125% NTSC in LCD backlighting when used in red-green-blue (RGB) combinations compared to the typical 72% capability of CCFLs. The wider color gamut and higher brightness achievable with RGB LED backlights enables the development of multi-function displays (MFDs) that provide brilliant colors so that images can be viewed in sharper detail, even in direct sunlight. This brings new levels of situational awareness, simplicity, and safety to the cockpit with integrated navigation, weather, terrain, traffic, and engine data.
Wide Range of Lighting Conditions
The biggest challenge for cockpit displays, however, is the wide range of ambient lighting conditions within the cockpit. Cockpit information-display systems have to deliver content at the appropriate light level. For the LCD panels that make up the majority of modern glass cockpit instrumentation, this poses a distinct challenge. Where a typical desktop-computer monitor might have a luminance of 200 cd/m2, a cockpit LCD panel typically needs to produce up to 1000 cd/m2 in order to be readable under high-ambient-light conditions. Because an average LCD panel absorbs about 95% of the backlight output even when showing a full-white screen, this requires an extremely bright backlight.
Typical specifications of an MFD in a modern aircraft require a minimum dimming ratio of 1000:1 (ratio of the brightest setting to the dimmest setting). A CCFL system with careful design in combining an analog control (current only) with pulse-width modulation (PWM) can only achieve a dimming ratio of 300:1 with some risk of causing interference to the display sync signals and audible noise from the power-supply transformer. On the other hand, LEDs with PWM dimming control can easily achieve the necessary dimming range.
Fig. 1: Shown is an aircraft cockpit with digital displays.
Advances in LED Technology
Advances in LED fabrication technology have led to high-brightness LEDs that provide a wider color gamut and higher efficiency while offering an optimized display brightness-to-thickness ratio. New Thinfilm LED fabrication technology developed at OSRAM is one example of industry advances being made in the area of more-efficient LED light sources. Conventional LEDs have substrates usually composed of silicon carbide, sapphire, or other material. These substrates absorb some of the photons generated by the LED, reducing efficiencies. Thinfilm LED technology emits over 97% of the light from the top surface. Conventional LEDs emit from the four sides besides the top surface, whereas Thinfilm chips are similar to conventional AlInGaP and InGaN wafers, except that they add a sacrificial layer under the epi-layer. The wafer is then inverted and bonded to a germanium substrate with a highly reflective metal mirror surface. Next, the original substrate is lifted off using a laser or chemical etching. The resultant wafer has a Thinfilm active layer that is less than 10 μm thick. Standard processing is then employed to fabricate individual chips with contacts, after which the wafer is singulated into individual LED chips and packaged (Fig. 2).
One example of a high-brightness LED utilizing Thinfilm technology is the new Advanced Power TOPLED Plus (Fig. 3), which also utilizes a new lens specially designed for high efficiency in backlighting applications. At an operating current of 100 mA, these LEDs achieve 14 lm (red), 24 lm (true green), and 28 mW/sr (deep blue). The wider viewing angle of the new lens ensures absolutely uniform backlighting. The chips used are fabricated with ThinGaN and Thinfilm technologies for high efficiency: 65 lm/W (red), 70 lm/W (true green), and 36% overall efficiency (deep blue). CCFLs can achieve similar efficiencies but cannot match LEDs in providing high-brightness (1000 nits) capability.
More than Flat Panels
There is more to information display in an airliner cockpit than LCD panels. The cockpits are wonders of technological versatility. The same display can be used to show information on different systems, from navigation to engine management, and from radio communications to weather radar. Different types of information are available to the pilots on demand, and when an urgent situation requires it, the aircraft's information system can deliver content to the display for the pilots' attention.
A typical cockpit also has a myriad of switches and indicators that are used to control and monitor the status of many different systems. Information is communicated through the switches and indicators as well. At the most fundamental level, the legends on the switches must be legible in light levels from direct sunlight to nighttime. More than that, the legends and other indicator lights must be visible when they change color, as they do when indicating system status or the position of the control. Urgent conditions are often signaled by a switch legend or a monitor light that changes color. A good solution for illuminating switches and small indicators in applications where multiple changing colors are required is an LED with a small package size, directed light output, integrated reflector, and a homogenous and wide viewing angle.
LEDs are advantageous in switch and indicator applications as well. The low power draw, wide range of dimmable light levels, long life, small form factor, and rugged design all are valuable attributes. In addition, color control is also important, as the hue of the light can be altered for varying conditions. This feature can be especially helpful in choosing a color for cockpit illumination that is least likely to interfere with night vision. Color stability and uniformity emitted from various switches at different brightness settings is important in visual ergonomics, e.g., when a red flag pops up among a sea of yellow indicators and backlit switches.
One other area where LEDs excel over fluorescent and incandescent lamps for cockpit and display illumination is in nighttime surveillance applications (Fig. 4). LED colors can be chosen such that they do not interfere with night-vision imaging systems, whereas fluorescent and incandescent lighting would not work.
A Bright Idea: Automatic Dimming
Light management within the cockpit is critical to maximize the pilots' ability to gather information from both inside and outside the aircraft. By matching the light output of the information displays to the ambient light conditions, the pilots are able to read instruments and displays quickly and reliably. One challenge is to keep light output at optimal levels without adding to the flight crew's visual load.
Fig. 2: The use of Thinfilm LED fabrication technology results in LED chips with smaller form factors and more-efficient light emission than conventional LED devices.
Fig. 3: Advanced Power TOPLED Plus LEDs have a low profile – 2.25 mm high – that enables the design and production of very thin backlights.
An approach to this challenge is to automate the light output, causing displays to be dimmer or brighter based on ambient lighting conditions. This would seem to be a simple task – use a silicon light sensor to detect light levels in the cockpit and adjust the light output of the LCD panels, switches, indicators, and cockpit illumination accordingly. Digital control of brightness settings via PWM can be stored in memory for instant retrieval by the pilot. Automated dimming to maintain optimum ambient light conditions can reduce cockpit-crew fatigue.
However, ambient-light sensing and adjustment are not as simple as they sound. The human eye is sensitive to a relatively narrow band of the light spectrum, generally between 400- and 700-nm wavelengths, which shifts to blue at a lower light level. A typical silicon photosensor is sensitive to a much wider spectrum, starting at about 350 nm and extending well into the near-infrared (IR-A) range to 1100 nm or more. As a result, the silicon device can read light levels that are invisible to the pilots, which could result in the lighting levels being set higher or lower than they should be. A better solution is to use an ambient-light sensor (ALS) that better matches the sensitivity of the human eye (Fig. 5).
Some silicon devices are designed to move the peak sensitivity down to about 550 nm, similar to that of the human vision system. These are far more sensitive to near-IR radiation than human vision, however, and are still subject to significant error.
The optimum solution is a hybrid device that combines an optimized photodiode with an integrated circuit that incorporates signal amplification, a logarithmic converter, and temperature correction. The result is a sensor that most closely matches the performance of the human eye. The curves shown in Fig. 5 compare such a hybrid device (SFH 5711) to an ambient-light sensor, a standard silicon photosensor, and the human eye. The logarithmic amplifier IC enables a large brightness range to be detected with great accuracy without the need for various series resistors. The net result is a solid-state integrated lighting system that adjusts to match the light level needs of the flight crew.
Fig. 4: In darkness, the levels of instrument, control, and interior lighting must be balanced to maintain the pilots' night vision, yet still ensure that all information sources are readily visible.
Fig. 5: Ambient-light sensors (ALSs) most closely match the light sensitivity of the human eye, compared to standard silicon detectors at the same brightness.
The Future Is Up in the Air
Industry experts agree that LED lighting is becoming the standard for cockpit-display light sources and will be so for the foreseeable future. LED technology will continue to improve and to migrate into other applications in the cockpit and throughout the aircraft.
Technology advances will continue to increase the LEDs' efficiency, decrease their size, and lower their production cost. This in turn will help make it possible for aircraft-cockpit designers to make improvements to avionics display and cockpit design. For example, by expanding on the multi-functional abilities of LCD panels, expect more touch-screen technology to be implemented into aircraft cockpits. The versatility of LCD screens will also make it possible to present touch-screen controls for many systems. This will help reduce clutter in the cockpit design by eliminating switches and controls, improving reliability, and encouraging quicker pilot decision-making and response time.
LEDs also can be used in backlights for conformable displays; it may be that curved panels will prove to be more ergonomic and effective at displaying multiple sets of information to the pilots. By making better use of the flight crew's peripheral vision, it may be possible to reduce response times in certain urgent conditions. High-brightness LEDs can also be used effectively for head-up displays (HUDs).
As a result, the role of LEDs in cockpit displays and lighting will continue to expand. Reliable, compact, and efficient, LEDs can help the flight crew stay on top of the many systems involved in getting a flight safely from here to there. •