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Endurance test on photometric performance for first generation LED luminaires.

Manufacturers of LED luminaires promise very long lifetimes for their products. This is a main advantage compared to traditional light sources. Long operational lifetimes are particularly interesting for reducing maintenance efforts and to ensure the ecological and economical relevance of this technology. However, Light Emitting Diodes typically show a gradual depreciation of their characteristic photometric performances. The key point is thus to make sure that the luminaires still meet the expected performances after many hours of operation. The main objective of this study is to identify the relative light output of luminaires over long running times in realistic conditions and to identify the course of the depreciation. Secondly it is intended to extend the lifetime evaluation to other indicators. The current standardized lifetime definition takes only the variation of light flux into account as a parameter. Nevertheless, complementary performance issues such as colour shift and the changes of the input power should also be considered. Just as the lumen depreciation these characteristics can also render a product gradually obsolete. This paper addresses the results of an experimental endurance test on photometric performances and electrical parameters for a selection of luminaires of various types and price categories. It was found that the lumen depreciation shows a relative consistent path over time, while colour shift tends to show more irregular patterns and thus it is more difficult to predict. Colour shift is also much more perceptible than lumen depreciation in any typical application case so more attention should be paid to this problem. Meanwhile it was observed that the energy efficiency of the luminaires depreciates in a similar way to the loss of light output. As different causes for depreciation coexist, lifetime claims should always consider the whole luminaire system in order to determine the operational service time.