The U.S. Department of Energy has released a report on the longer-term performance of an LED lighting system that was installed on the I-35W Bridge in Minneapolis in September 2008 and represents one of the country’s oldest continuously operated exterior LED lighting installations. The report is a welcome addition to the literature on LED lighting, which is lacking in the area of longer-term field performance.
 
Prior to installation, two of the LED luminaires were tested, along with a third luminaire that was not installed on the bridge but was tested for 6,000 hours in a laboratory for comparison purposes. Follow-up testing was conducted in April 2013. The two pre-tested luminaires were retrieved from the bridge after roughly 20,300 hours of operation.

Among the findings was an average 4% reduction in input power of uncertain cause. This issue was a partial contributor to an overall 18% reduction in light output (independent of dirt accumulation) after that same interval, but other factors – such as normal LED lumen depreciation and an optical gel bubble issue that the manufacturer subsequently resolved – also contributed. Luminaire efficacy was reduced by a corresponding 15%, and there were some measured changes in color properties as well. Dirt depreciation at 20,300 hours measured an average of 12%, suggesting that periodic cleaning of the luminaires may be important in applications where maintaining road illuminance is critical.
 
Overall, the bridge’s LED lighting system continues to offer effective operation, with the few issues encountered not unexpected given the early stage of SSL development at the time of purchase. The system is still providing much value for the Minnesota Department of Transportation, which reports satisfaction with its performance.
 
The full report and a report brief are available on the GATEWAY Results webpage.