At the Fourth International Conference on Solid-State Lighting, organized by SPIE, Honeywell Lighting and Electronics (Urbana, Ohio) presented a paper describing the development of its Astreon position lights targeting the business jet retrofit market.
Light sources for position lights
Aircraft position lights consist of forward-facing red and green lights on the left and right wing tips, respectively, and white aft-facing light on the trailing edges of the wings and/or the tail cone. In the US, Code of Federal Regulation (CFR) documents closely define the color of position lights, as well as angular distribution and intensity requirements of the light sources.
Usually, the required colors are achieved by filtering white light from a halogen or incandescent lamp through colored glass. This is an inefficient approach with relatively high power consumption. Other disadvantages include:
* Limited operational lifetimes
* Thermal shock of the glass lens
* Failure of the lamp in high vibration environments
Clearly, the narrow emission and solid-state nature of LEDs should circumvent these deficiencies, reducing the time and cost associated with unplanned maintenance.
Honeywell chose to focus on a design for forward position lights (red and green) that could be used as a retrofit on several types of business jet from different manufacturers. Of course, this meant that the overall package size, mounting configurations and input voltage were all defined.
Optical design
The design team determined that it could achieve the desired optical performance using for each light three 1-W Luxeon LEDs from Lumileds. A single lambertian emitter was sufficient to meet the requirements of the region between 20 and 110 deg from the forward direction. The much higher intensity requirements in the 0-20 deg region were realized with two side-emitting LEDs and a specially designed reflector.
The different distribution patterns of the two types of LEDs meant that the reflector shape and mounting angle was a compromise that would work for both red and green lights. Cut-off shields were added to prevent the output beams from violating intensity restrictions in regions of overlap of the emissions from the different position lights.
Other design aspects
Based on an operational life of 3000 hours minimum and a nominal 28-V DC input voltage, a low cost resistive circuit was chosen, despite this solution's inefficiency (which impacted thermal management) and the need to design different circuits for the red and green lights. The mechanical design of the lamp is such that the electrical module can be replaced if the light needs to be used in a high-temperature application where an inefficient electrical system (with a power drop of around 6 W to drive 3 LEDs with a total power dissipation of 3 W) cannot be tolerated. However, in general the product is likely to experience only minimal periods of time at high temperatures and is also connected to a metal structure, aiding heat transfer. As described in the paper, Honeywell was able to demonstrate a high level of correlation between predicted and measured performance for both optical and thermal characteristics.
Challenges for LEDs in aviation market
Several issues were identified concerning the use of LEDs in this application:
1. Difference between InGaN (green) and AlGaInP (red) LEDs
Ideally, the different-colored lamps would allow identical packaging solutions to be employed. However, the difference in forward voltage required variation in drive circuitry. Also, where chip shaping is employed to increase light extraction, the red and green packages can exhibit different radiation patterns. Moreover, the Luxeon packages had different values of luminous flux; 44 lm for red and 30 for green. Also, the AlGaInP devices showed more rapid degradation of lumen output with elevated temperature or with time.
2. Variations within LED batches
For either type of LED, variations are noticed in lumen output, forward voltage, color and spatial distribution. Although binning allows the management of these variations to some extent, there are often situations where only a portion of a bin will meet the associated design specification (as defined, for example, by the CFRs).
3. Environmental testing
The aviation market calls for very stringent performance, and compliance is closely monitored. Product sheets for LEDs usually indicate testing to -40 deg. C while aviation applications require tests to -55 deg. C and sometimes below. Significant additional in-house testing is therefore often required.
4. End-of-life
Unlike traditional filament-based lamps, for which failure is easily detectable, the effective end-of-life of an LED is when its performance falls below the minimum specifications in force. There are several different ways to identify end-of-life: manufacturer-defined replacement intervals, a strict time limit based on hours of operation, active feedback of LED characteristics, regular operator measurement, or algorithm circuitry based on time and temperature of use.
Conclusion
Honeywell has successfully implemented a design for LED-based aircraft position lights for the business jet market. Astreon lights are now in flight on aircraft such as the Citation X and the Gulfstream G550, and were recently selected for use on the new Citation Sovereign.
It is estimated that the entire aircraft lighting market for 2004 was $800 million, including new products, spares and repairs. Although this market could be seen as an "early adopter" of LED lighting technology, the volume of lamps used is, and will remain, very small compared with other LED market segments.
