The U.S. Department of Energy's GATEWAY program has released the third report in a series documenting the performance of LED luminaires in the Yuma (Arizona) Sector Border Patrol Area, on the U.S.-Mexico border, which is a high-temperature, high-solar-radiation environment. The first report compared six LED luminaires installed in February 2014 to the incumbent high-intensity discharge (HID) lighting system, and the second discussed the performance of the LED system after 2,500 and 5,000 hours of operation, including unanticipated illuminance changes.
 
The new report summarizes the results of further investigations to explain those changes. The distribution of light produced by the LED luminaires changed considerably in the first 2,500 hours of operation, and the trend continued after 5,000 and 7,000 hours of operation. Comparing the 7,000-hour measurements to the initial measurements reveals several important changes in the illuminances delivered by the lighting system, including significant decreases in the average horizontal and vertical illuminance.
 
The suspected cause was dirt depreciation, so two luminaires were removed and tested in a photometric laboratory, first dirty and then after cleaning. The laboratory measurements confirmed the effect of dirt not only on lumen output, but also on the distribution of light exiting the luminaire. The change in distribution raises an important question for manufacturers, specifiers, and researchers: How should lighting system designs account for distribution changes due to dirt accumulation?
 
For a closer look at the findings, download the full report.