LED lighting has achieved widespread adoption in the avionics world, installed virtually everywhere inside and outside all new aircraft designs. Increasingly, it is being retrofitted in existing aircraft. One area that has been slower to incorporate LEDs is the cockpit, but that is now changing as LED backlighting of instrumentation displays is becoming more common. This article will outline why LED adoption on the flight deck was slow, and how to overcome technical challenges such as consistent dimming and color stability.
Figure 1: Advanced avionics systems, such as the Honeywell Primus Epic about to be installed on the next-generation Embraer E-Jet, are using larger LCDs with advanced graphics that require high-quality LED backlighting.
Typical examples of LEDs used for lighting-instrument clusters will be examined, including the Lumex QuasarBrite™ devices. LED backlight drivers are key to backlight quality. Examples appropriate for the rigorous demands of the avionics industry include drivers from Atmel and Semtech. Evaluation kits from STMicroelectronics and Texas Instruments are worthy of inclusion here, as well.
Ticking all the boxes
The aircraft industry has not been slow to recognize and exploit the compelling aesthetic and performance features of LED lighting technology. Their critical demands of low power, small size, high reliability, and mechanical stability are more than met by the characteristics of LEDs, adding long life, flexible form-factor, and improved light quality to the mix. LEDs tick all the boxes. New designs of light, leisure aircraft, business jets, commercial airliners and many military aircraft are now ‘de facto’ incorporating LED lighting both internally and externally, from cabin signage to landing lights.
Not surprisingly, the avionics industry, well used to long product lifetimes, is particularly adept in understanding the benefits of reduced lifecycle costs. Thus the traditional barrier of higher unit costs, and cost of entry that has hindered LED adoption in many other lighting markets, has not truly been a factor here.
Where existing aircraft designs are being re-vamped rather than redesigned, and large elements are re-used to save costs and time for new certifications, there has been some natural resistance to changing to LED lighting technology. However, in many areas, such as cabin lighting, LEDs are being retrofitted. Some military projects have retained existing lighting technology, often because the platforms are not expected to be in service long enough, or the number of flight hours per year is insufficient to recover the upfront costs of the devices and the certification.
Cockpit concerns
However, one area of the aircraft that has resisted the allure of LEDs for longer than most is the flight deck. In part, this is due to the re-use of tried, tested, and certified instrumentation units and layouts. In another part, it is because of the technical challenges associated with the technology change to LEDs. Dimming is critical as bright sunlight turns to night, and consistent dimming across the wide range of screens in the cockpit doubly so. The effort and investment required to integrate a consistent dimming scheme across the cockpit is not a trivial matter.
Color stability is another factor that requires attention. The colors shown by instrumentation in the cockpit have changed considerably over the years. Decades ago, red cockpit lighting was favored as it helped pilots retain their night vision. Today, red lights can only be used to indicate danger. Even yellow/orange lighting is restricted to alert status indication. Most cockpit and legend illumination today is white (cool or blue-white at 6500 to 8000 K) for information panels and displays, and green for normal status indication.
White light LEDs used for backlighting are most commonly created by applying a combination of color phosphors, typically red and yellow, onto a blue LED die. The combination of the die wavelength and the color phosphor recipe generates different white tones, ranging from 1800 K to 5500 K and beyond.
Figure 2: The cockpit of an American Airlines Boeing 777-300. Large-format multicolor displays for complex graphics and video data are becoming common, requiring a careful approach to backlighting and dimming.
However, larger-format LCDs are typically multi-color, and multi-color backlighting is appropriate. RGB backlighting obviates the need for a color mixing area as would be required by single-color LEDs, although slightly more complex drive circuitry is generally required. By mixing, typically, 64% green, 8% blue and 28% red, a white light can be created.
Although LED technology is improving, it has been noted that different color devices age at different rates, which can make color stability over time difficult to maintain. Some LEDs exhibit slight color changes at higher temperatures as the efficiency of the phosphor can drop. The human eye does not always perceive this, although the luminous flux of red LEDs has been noted to decrease noticeably and may need to be compensated. In addition, to complicate matters further, although the change in color temperature when LEDs are dimmed can be small, this may not always be desirable. Pilots may be used to the effect produced by the dimming of traditional electroluminescent or fluorescent (CCFL) backlighting technologies.
An early concern among instrument panel designers when moving to LEDs is the conical shape of LED emitters, compared to spherical for the incandescent lamps they typically replace. This can mean a smaller area of the panel is lit, unless the LEDs are retracted behind the front panel. Some early devices also projected a black dot at the center of the lighted area.
Obsolescence is a further major drawback, although this affects many technological aspects of aircraft design. On the one hand, aircraft instrument designers are keen to see the fast-paced developments that make LEDs eminently suitable for their applications. On the other, they recognize that, as with many immature technologies, device choice and availability changes rapidly. However, military and aerospace customers require continuous supply and support for 10 to 20 years.
Overcoming the hurdles
Consequently, it has taken time for manufacturers to demonstrate that LEDs are suitable replacements for the range of technologies used in cockpit applications. One of the more obvious solutions is binning, which now means that devices can be tested for color stability at higher temperatures. Another solution, developed by several manufacturers, is the ability to dim LEDs in a way that mimics incandescent or halogen luminaires. Latterly thinner, wide-angle LEDs have emerged, as backlighting applications require the widest possible angle, at least 120°.
LED drivers are proving to be a critical component in backlighting instrument panels and displays. A PWM (pulse width modulation)-based driver is typically used to provide color balance over time and varying current conditions. An Application Note¹ from Osram Opto Semiconductor explains precisely how this works and what devices are needed (microcontroller, PWM driver, phototransistor/RGB sensor, and more).
However, the rationale for LED lighting becomes more forceful as the technology improves. LED backlighting for large-format displays, control panels, instrument clusters, and even for individual illuminated switches, is increasingly common. The LEDs typically now last longer than the instruments themselves, which was not always the case. Less heat and glare has made a noticeable contribution to a more comfortable and less tiring environment. Smaller size, particularly in depth, can lead to thinner displays, giving cockpit designers greater flexibility in terms of layout.
The development of heads-up display guidance systems for commercial aircraft, such as has been installed by Rockwell Collins in Boeing’s 757/767 airliners, has only been possible with ultra-thin LED-backlit LCDs. (See Figure 3 below and Figure 1 above.)
Figure 3: This Boeing 767 cockpit shows a mix of green screens for information, white backlit switches, as well as multicolor displays. Consistent dimming as nighttime approaches is critical.
Display size is a complicated business. On the one hand, new large-format displays may require more panel space than the original equipment they replace (except perhaps when they replace CRT-based displays). Yet these new displays are more flexible and can provide considerably more information than their predecessors, and one unit may replace several older displays.
Despite the slow start to LEDs in the cockpit, the retrofit market for LED backlit instruments on flight decks is now a growing business. The desire to take advantage of new instrument functionality, and the vastly improved display of flight information, is encouraging operators to upgrade their aircraft cockpits and extend their service life with a comparatively modest investment. Some smaller aircraft may be limited on how many large display units can be installed, but this only gives rise to demand for smaller versions of mainstream avionics instrument displays.
Meanwhile, new instruments are emerging. The so-called ‘NextGen’ avionics applications include synthetic vision systems (SVS), automatic dependent surveillance broadcast (ADS-B) traffic systems, high-resolution terrain-rich maps, airport surface and taxi maps, approach charts, and uplink graphical weather depictions. While offering additional functionality, they demand larger, high-resolution screens to display complex information clearly and logically. Displayed and backlit effectively, this information can lead to greater situational awareness, and therefore safety.
Direct approach
Effective backlighting is the key. It is well known that the quality of the cockpit display unit is highly determined by the quality of the backlight unit. The choice of LEDs, white and/or RGB, and the choice of a direct or indirect backlighting scheme can make a significant difference to the quality of the backlight.
According to a useful Application Note² from Osram Opto Semiconductor, direct backlighting is preferred for modules with high brightness, medium size, and limited space characteristics. It gives the example of a 10.4 inch TFT display fitted with 336 white Power TOPLED® devices with a brightness of 600 to 1000 cd/m² that is not sensitive to vibration and has a lifetime better than 10,000 hours. The height of the entire module, comprising reflector and PCB is just 20 mm.
Lumex is taking a different approach to LED backlighting, with its QuantumBrite range of flexible LED panels. They are supplied in a low-profile, ultra-slim format, measuring just 0.125 mm thick compared to 5 mm for conventional COB (chip-on-board) packaged devices, and 2 to 3 mm for edge-lit technologies. Claimed to be durable and shadow resistant, performance in terms of brightness is comparable to edge-lit devices, although not always up to the levels of COB backlighting.
The company maintains that brightness levels are sufficient for visually-challenging applications, including display backlighting, switch and dashboard illumination, and in aircraft cabins. Its primary advantage however, is its ability to be fitted to curved surfaces, and to include cut-outs for switches or other components. It can be integrated with other LEDs, LCDs, light pipes, and other panelware components.
While the QuantumBrite technology is available in customer-specified solutions, Lumex also separately offers the QuasarBrite range of RGB LEDs used in the QuantumBrite modules. Claimed to be the smallest available, the devices are contained in the compact 0404 package, measuring just 1 x 1 x 0.25 mm. Designed for use in restricted spaces, the QuasarBrite 0404 RGB LEDs are destined for backlighting LCDs and switches, among other applications.
The standard device provides 30 mcd red, 40 mcd green, and 20 mcd blue, with forward voltages of 1.95 V, 2.85 V and 2.73 V, respectively. With an operating temperature range of -40 to 85°C and a 120° viewing angle, the LEDs are rugged and have already been used in applications including dashboard lighting in armored military transportation.
Driving forward
There is a wide choice of LED driver devices and modules available for backlighting applications, and selection can depend on a number of factors, including the number of LEDs connected in series, the number of strings, board space, and thermal management issues. Some drivers are more adept at dealing with challenges such as color stability and consistent dimming, taking into account temperature changes and aging.
Atmel offers a broad portfolio of multistring LED drivers for backlighting applications. Features such as ‘efficiency optimizer’ technology, flexible dimming options, and scalability across a wide range of power levels ensure that these devices are suitable for both direct and edge-lit backlighting schemes for industrial, military, and avionics displays.
Of potential interest to cockpit designers, the devices provide solutions for local and global dimming, power management, signal conditioning, and smart interfacing. The efficiency optimizer enables multiple drivers to communicate to control overall operating voltage, to keep the LEDs in current regulation. This, combined with on-chip programmability, allows designers to compensate for LED aging and provide thermal control.
A digitally-controlled feedback loop manages the LED supply and regulates LED current up to 30 mA per LED string, for up to twelve LEDs per string. LED dimming is achieved through an external PWM signal or an internal 8-bit PWM engine. Analog dimming of LED string current is available for use with an ambient light sensor (ALS) and/or temperature management with a thermistor sensor, and does not require external compensation.
A typical Atmel LED driver for LCD panels is the MSL1064, which incorporates a current-mode PWM boost regulator with 50 V internal switch and 4.75 to 36 V input voltage range. It is capable of driving six LED strings at 30 mA, up to 48 V, allowing up to seventy-two LEDs per driver.
Figure 4: Atmel’s MSL106X LED driver with digital PWM for brightness control.
Semtech takes a similar approach. The SC4541 is one example in an extended range of drivers designed for a variety of backlighting applications. It is a fully integrated, high voltage boost (step up) and buck (step down) LED driver. The input voltage range is 2.9 to 22 V, with output voltage of up to 25 V. The device is capable of driving up to seven white LEDs connected in series. Additional features include current-mode control, direct PWM dimming, and integrated Schottky rectifier.
Another device in the family is the SC4509, described as a highly integrated, step-up DC/DC converter, targeted at white LED and OLED backlighting of smaller instrument displays. Capable of driving up to five LEDs in series, it has an input voltage range of 2.7 to 10 V, with an output voltage to 20 V. It operates at a constant 1.2 MHz PWM frequency.
Texas Instruments has also adopted the tried and tested approach of using PWM dimming as it allows the light’s color temperature to remain constant irrespective of its intensity. This is achieved by keeping the amplitude of the LED current constant, but periodically turning on and off the LED driver, thus varying the LEDs’ average current and light intensity. TI has developed a reference design that automatically adjusts display brightness for changing ambient light conditions. The design is based on the PowerWise LM3423 LED driver together with five LEDs connected in series, running from an input voltage of 4.5 to 12 V. The LED current is nominally 100 mA and the output voltage is 15 to 20 V.
TI has developed an evaluation board showcasing the LM3423 used with a boost current regulator, to drive up to twelve LEDs at a current of 700 mA from a DC input voltage of 10 to 26 V. The LM3423BS2LYE/NOPB allows the designer to try out PWM dimming, overvoltage protection, and input under-voltage lockout.
STMicroelectronics has the STEVAL-ILL020V1 demonstration board for backlighting LCD panels, based on its LED7706 devices. Implementing a high-efficiency monolithic boost converter and six controlled current generators, the device can manage an output voltage up to 36 V, equivalent to ten white LEDs in series. Usefully, the generators can be externally programmed to sink up to 30 mA, and can be dimmed via a PWM signal. Additional features include internal power MOSFETs, 5 V LDC for device supply up to 36 V, and constant-frequency peak current mode control.
Figure 5: ST’s LED backlighting demonstration board.
Remote phosphor future
Planning ahead, the avionics industry is keeping a close eye on new technologies, such as organic OLEDs and remote phosphor backlights. OLED technology is expected to allow displays to be made thinner. However, observers in the aircraft industry have been reported to be unsure of the aging characteristics of OLEDs. Meanwhile, a number of manufacturers are experimenting with remote phosphor technology, which uses a blue LED, mixing chamber, and phosphor-coated secondary optics, physically separated from the LED source.
Research is underway in Europe³ into remote phosphor displays, which they believe have the potential to deliver substantial benefits on system simplicity, using color-conversion techniques to improve quality, reliability, and power efficiency.
Under the transport theme of the European 7th RTD Framework Program, the concept proposed is an evolution of current LED backlights, based on RGB or white LEDs.
By using a blue pump light source (LED) and an external fluorescent phosphor layer, the researchers expect that the blue light will be converted to a very-stable, customized white light. One of the challenges will be the tuning of the fluorescent phosphors to the color filter of the LCD. The project will monitor the optical behavior of the unit over time and evaluate the suitability of a remote phosphor backlight concept for both direct- and edge-lit applications, specifically in avionic cockpit displays.
Summary
LED and LED driver technology have advanced sufficiently to be widely accepted for backlighting LCDs and instrumentation panels in the aircraft cockpit. Programmability and the use of standard interfaces and dimming techniques will aid cockpit designers to ensure consistent dimming. Binning is becoming more common to ensure devices can be selected that are rugged and especially stable at higher temperatures. Some manufacturers are now committing to ‘long life’ programs to ensure the continued supply of LEDs and device drivers over time.
However, new technologies, such as OLEDs and remote phosphor, may well make inroads in the LCD backlighting market, particularly for high-reliability, high-quality applications.
References:
- Application Note: Color Stablization of RGB LEDs in an LED Backlighting example, Osram Opto Semiconductor
- LEDs, new light sources for display backlighting, Osram Opto Semiconductor
- Remote phosphor for avionic cockpit displays research project
