Brightness/Luminous Intensity Values for Blue, White, and Green Aviation Signal Lights Using Light-Emitting Diodes

Signal lights at airports and along runways serve a variety of functions, including helping pilots locate the airport, assisting pilots in orienting the aircraft to the appropriate landing runways, and aiding pilots with taxi maneuvers on the airfield. Anecdotal pilot reports suggest that light-emitting diodes (LED) used as signal lights in and around airports are perceived as brighter than comparable signal lights using incandescent sources at the same measured intensity. These subjective reports are consistent with previously published psychophysical data. In general, saturated colors, similar to those produced by LEDs, appear brighter than less saturated lights, similar to those produced by incandescent sources. This technical note describes a series of experimental investigations to quantify the brightness of blue, white, and green LED signal lights relative to incandescent signal lights within the same Federal Aviation Administration (FAA) color boundaries. Both types of signal lights were compared against dark and dimly lighted backgrounds and through simulated fog. The results confirm that LED signal lights are perceived as brighter than incandescent signal lights at matched luminous intensities. Brightness relationships were unaffected by signal light intensity, background light level, or the number of signal lights viewed. The simulated fog reduced the relative brightness difference between the incandescent and the LED signal lights due to light scattering in the fog that desaturated the signal light colors. The results of the present study were not accurately predicted by two previously published models of brightness appearance, but this is most likely due to differences in experimental conditions. A new model was developed for this study that could be used to predict signal light brightness perception for FAA blue, white, and green signal light colors. Except for very short-wavelength blue signal lights, the model was able to accurately predict brightness perception data obtained in the present study as well as those presented in an independent study published 30 years ago. This independent validation lends confidence to the generality of the model for predicting blue, white, and green signal light brightness.

DOT/FAA/TC-TN15/32
Authors: J.D. Bullough, Z. Yuan, and M.S. Rea, and Donald W Gallagher