Comparative Evaluation of Internally and Externally Lighted Taxiway Guidance Signs

Airport installations of both internally and externally lighted taxiway guidance signs are presently eligible for Federal Airport Improvement Program (AIP) funding. However, there is some basis for believing that the internal form of sign illumination is superior because of the more uniform light distribution attainable and the reduced maintenance required. In view of this, the Federal Aviation Administration (FAA) is considering a change to advisory circulars making the internally lighted sign the sole standard for AIP funded installations.
A comparative evaluation of the two types of illuminated airport signs was conducted at The Federal Aviation Administration (FAA) Technical Center. The purpose of the evaluation was to determine if one sign illumination technique exhibited significantly better performance than the other technique. To obtain performance data applicable to signs presently in service, samples of both internally and externally illuminated signs were obtained and evaluated. Results of the evaluation indicate that the performance of the internally illuminated taxiway guidance signs can be expected to be significantly better than that of externally lighted signs, especially at greater viewing distances and under conditions of reduced visibility.