Article Published on LED Lumen Maintenance and Light Loss Factors
January 20, 2014
An article has been published in LEUKOS: The Journal of the Illuminating Engineering Society of North America (IES) that may be of interest to the solid-state lighting community. Entitled "Lumen Maintenance and Light Loss Factors: Consequences of Current Design Practices for LEDs," the article was written by Michael Royer of Pacific Northwest National Laboratory and discusses complications related to the lamp lumen depreciation (LLD) light loss factor and LEDs.
Light loss factors are used to help lighting systems meet quantitative design criteria throughout the life of the installation, but they also have other consequences, such as influencing first cost and energy use. Because of the unique operating characteristics of LEDs and lack of a comprehensive lifetime rating—as well as the problematic relationship between SSL lifetime and lumen maintenance—determining an appropriate LLD factor for LED products is difficult.
The IES recommends using an LLD of no greater than 0.70 when the quantity of light is an important design consideration. This approach deviates from the practice of using the ratio of mean to initial lumen output—which is typically used with conventional sources—and may misrepresent actual performance, increase energy use, and inhibit comparisons between products.
The article discusses the issue in detail, compares the performance of some conventional and LED products, and examines alternatives to the currently recommended approach for determining LLD factors for LED products.