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Airfield Lighting of in-ground luminaires as a function of inconsequential to many end users, airfield How can we move toward a definition of useful life? luminaires are meant to provide meaningful Understanding the long-term performance of included three red/white directional runway visual guidance to pilots under different LED-based airfield luminaires under realistic centerline luminaires (type L-850A) and three conditions, but particularly in low visibility. conditions is the first step to establishing a white touchdown zone luminaires (type Thus, airfield luminaires can only be functional definition of useful life. With this L-850B). In total, the luminaires were tested considered useful if they meet all their objective in mind, the Lighting Research Center for more than 10,000 hours between October photometric requirements at all times. began a long-term study in the autumn of 2011 and January 2013. These requirements include not only how 2011 with funding from the Federal Aviation much light is produced but also its spatial Administration (FAA) Visual Guidance relative change in photometric characteristics distribution and its colour. In combination, Program to gather initial data on light output as a function of LED operating temperature. these criteria could be used as the basis for a depreciation, colour shift, and intensity During the test, each luminaire was installed functional definition of airfield luminaire life. distribution changes from a limited number in a customised test chamber designed to a 30 per cent drop in light output may be temperature of operation. The samples tested The study was designed to observe the operate the luminaire continuously with a LEDs and human factors Dr John D. Bullough, Senior Scientist at the Lighting Research Centre, Rensselaer Polytechnic Institute nominal 6.6A current and at a predeter- mined temperature. The selected test LED board temperatures were nominally 55°C, 80°C, and 100°C and were kept constant for the duration of the test. Each chamber included the required heating elements, controls, and sensing probes to monitor the performance of the luminaire. Before the beginning of the In addition to the durability, long operating as the flashing light. Effective intensity life and energy efficiency of LEDs, they also predicts the relative visibility of single-pulse have several human factors advantages, flashing signals well 2 , but the FAA’s current which the Lighting Research Centre has been definition of effective intensity for investigating in cooperation with the FAA: multiple-pulse flashes overestimates their ■ Wide range of correlated colour effectiveness when the light pulses are temperature (CCT) for white light, and longer than those produced by xenon more saturated colour than strobes 3 . A definition used by ICAO better incandescent signal lights predicts the visibility of multiple-pulse Instantaneous rise and decay times. flashes, placing LEDs and other sources on ■ the same technology-neutral playing field. The LRC’s research on aviation With regard to the colour characteristics of LEDs, the ability to create white light lighting and LEDs can be found online at: with higher CCT results in more accurate identification of white signal lights. illumination that is regularly confused with colour-deficient observers 1 . consisting of several short pulses often appearing as a single flash, similar to signals using xenon strobes. FAA specifies the performance of flashing lights in terms of ‘effective intensity’, defined as the intensity of a steady light with the same visibility hours using a photometrically calibrated photosensor. To monitor the changes in the colour of the light, the spectral power distribution was measured at least once every 1,000 hours. Figure 1 (page 39) shows a picture of the overall set-up including the six testing chambers and the data logging equipment. What were the results of the long-term test? driving circuit. These failures happened at 2. Bullough JD, Skinner NP. 2013. Conspicuity of flashes of light: Interactions between intensity and duration. Journal of Modern Optics 60: 1193-1199. 3. Bullough JD, Skinner NP, Taranta RT. 2013. Characterising the effective intensity of multiple- pulse flashing signal lights. Lighting Research and Technology 45: 377-390. 560 hours of operation (100°C condition) and at 3,360 hours of operation (80°C condition) for two of the L-850B samples, and at 7,630 hours of operation (100°C condition) for one L-850A sample. These results help make the point that luminaire reliability involves much more than just LED light output depreciation. identifier lights (REILs) and obstruction lights, allows the use of multiple-pulse light flashes of each luminaire was monitored every four operate as a consequence of failures in the LED Bullough JD, Skinner NP, Bierman A, Milburn NJ, Taranta RT, Narendran N, Gallagher DW. 2012. Nonincandescent Source Aviation Signal Light Colors, DOT/FAA/TC-TN12/61. Washington: FAA. For flashing signals like runway end the ability to switch LEDs on and off rapidly the end of the study. The relative light output 1. of LED colour is beneficial for correct identification, for both colour-normal and and kept as a reference to check against at References yellow light. For coloured signal lights, LRC research has also shown that the saturation was captured using a bar goniophotometer Of the six samples under test, three ceased to Incandescent signals, especially when dimmed to lower intensity steps, produce test, the intensity distribution of each luminaire John D. Bullough is a Senior Research Scientist and Adjunct Assistant Professor at the Lighting Research Center, Rensselaer Polytechnic Insti- tute, where he coordinates the Center’s Transportation Lighting and Safety Program. He is a Fellow of the Illuminating Engineering Society and is the author of more than 300 publications on lighting and vision. Figure 2 (page 39) shows the relative light output of the rest of the samples that were operational for the totality of the test. For one type of luminaire, L-850A, even at the lowest operating temperature (59°C) the estimated time to reach an L70 value was between 9,000 hours (for the white side) and 14,000 hours (for the red side). On the other hand, the L-850B sample tested at the lowest temperature only 40 Volume 18 · Issue 2 · 2014