(Image: DOE) As noted by Jim Brodrick, manager of the U.S. Department of Energy's (DOE's) solid-state lighting program, the DOE'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 GATEWAY tests are evaluations of LED lighting in real-life conditions outside the lab (see http://energy.gov/eere/ssl/gateway-demonstrations). The first GATEWAY report in the series of tests at Yuma compared six LED luminaires installed in February 2014 to the already existing high-intensity discharge (HID) lighting system, while the second discussed the performance of the LED system after 2500 and 5000 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 2500 hours of operation, and the trend continued after 5000 and 7000 hours of operation. Comparing the 7000-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.Dirt! 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? As noted in the report, "Careful optical design may lead to superior illuminance uniformity during pre-installation photometric analyses; however, the results from this investigation demonstrate that the uniformity will change as the luminaires accumulate dirt. In practice, this finding indicates that the effects of an optimized optical design may be negated by dirt accumulation, and sustained performance may require regular maintenance."Tolerance to dirt? While this conclusion may sound obvious, the light-extraction optics of lighting LEDs are much more complex than any needed for other types of lighting such as HID, sodium, or mercury arc, and their presence encourages some innovative optical designs that provide very directional and yet uniform LED lighting. However, it appears that some LED optical designers need to keep in mind that real-life conditions may at least partially undo the effects of a beautiful design; maybe a slightly less elegant but more smudge-tolerant design will be the result. For a closer look at the findings, download the full report at http://energy.gov/eere/ssl/downloads/trial-demonstration-area-lighting-retrofit.