Abstract
Light loss factors are used to help lighting systems meet quantitative design criteria throughout the life of the installation, but they also carry ancillary consequences, such as influencing first cost and energy use. As the type of light sources being specified continues to evolve, it is an appropriate time to evaluate the methods used in calculating light loss factors and understand the broad effects of performance attributes like lumen maintenance. Because of the unique operating characteristics of light emitting diodes (LEDs) and lack of a comprehensive lifetime rating—as well as the problematic relationship between lifetime and lumen maintenance—determining an appropriate lamp lumen depreciation (LLD) factor for LED products is difficult. The IES recommends using an LLD of not greater than 0.70 when quantity of light is an important design consideration. This approach deviates from the typical practice for conventional sources of using the ratio of mean to initial lumen output and may misrepresent actual performance, increase energy use, and inhibit comparisons between products. This article discusses the complications related to LLD and LEDs, compares the performance of some conventional and LED products, and examines alternatives to the current recommended approach for determining LLDs for LED products.
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