Study on Optical Changes and Dirt Depreciation in LED Street and Area Luminaires

You are here

Pacific Northwest National Laboratory (PNNL) is preparing to commence a study to examine dirt depreciation and optical changes occurring in LED street or area lights over time. For the study, PNNL seeks used luminaires that ideally are nearing the end of their anticipated life.

Stable long-term performance is a selling point for LED luminaires, and for streetlights in particular. However, documented field experiences over a full life cycle are scant, because even the first generation of LED street and area lights installed are just now approaching their initially projected end of life. At 4,100 hours of annual operation, a 50,000-hour expected lifetime extends just over 12 years. 

BACKGROUND

Such lengthy timeframes introduce new challenges for outdoor products. For example, lenses and other luminaire components are exposed to ultraviolet radiation, precipitation, airborne particulates, humidity, and temperature fluctuations for longer periods of time than indoor luminaires experience, and may have longer maintenance intervals, or even no maintenance at all. LED performance over time has been well studied, and dirt depreciation of luminaires has received some attention. In contrast, relatively little is known about the changes in performance that exposure to environmental conditions causes in optical materials. The new study aims to utilize the expanding base of exterior LED installations, some of which have been in place for at least 10 years, to explore and document these changes within the broader context of the cumulative impacts to output from all factors combined.

Long-term changes in performance are important to understand, because they are a significant component of the lighting design and specification process. When designing a lighting system, lumen depreciation and dirt depreciation are used to anticipate performance degradation over time and thereby directly drive the level of luminaire output selected. These de-rating factors can lead to substantial increases in energy use, depending on their values. In contrast, successfully improving performance stability will reduce the need to account for such losses and can result in significant energy and cost savings.

This study aims to help identify different contributors to lumen depreciation and chromaticity shift, while also adding to the evidence of dirt depreciation over time for a wide variety of materials and luminaire designs. The ultimate goal is to provide public guidance on material selection and luminaire design, to better inform the next generation of LED luminaires.

GETTING INVOLVED

PNNL seeks LED street or area lights that ideally are nearing the end of their anticipated life. There is no restriction on the type of luminaire. A key consideration is the ability to establish a performance baseline to which the performance of the used luminaire can be compared. This baseline can come from pre-installation photometric testing data, or from the submission of an equivalent unused luminaire (“shelf stock”) for comparison purposes. The following must be known: total hours of use (within ±500 hours), installation location, and number of cleanings (if any) the luminaire has had. It is preferred, although not required, that the used luminaire still be functional. Only one used and one unused luminaire are needed, although multiple used and unused luminaires may be submitted.

Because it may be difficult to find comparable unused units that are of the same vintage as those nearing the end of their anticipated life, the study will also accept donations of much younger units, if they’ve been in operation for at least 10,000 hours. With such younger units, it should be easier to locate comparable unused units of the same vintage, which may even still be available from the manufacturer.

All submitted luminaires (used and unused) will be sent to PNNL, where they will undergo extensive testing focused on dirt accumulation and changes to luminaire components (optics, LEDs, driver) that affect output over time. The testing process may be destructive, so submitted luminaires will not be returned. Compensation, via replacement luminaires, may be available.

Results for specific products will be shared with the submitter, and aggregate results made public.

For more information, contact Michael Royer (Michael.Royer@pnnl.gov) or Bruce Kinzey (Bruce.Kinzey@pnnl.gov).