Light loss factor: IES versus manufacturer recommendations

Engineers should understand appropriate light loss factors, as recommended by the Illuminating Engineering Society.


The lighting industry has agreed that the minimum life of an LED luminaire shall be at least 70% lumen maintenance (IE: L70) at 50,000 hours of operation. Based on these standards, the Illuminating Engineering Society (IES) recommends using a light loss factor of not more than 0.7. However, some manufacturers identify products as having hours of operation longer than 50,000 hours. Others indicate that their luminaire will have 90% lumen maintenance (L90) for more than 50,000 hours. A few manufacturers recommend a light loss factor to use in calculations for their product.

In these cases, what is considered an appropriate light loss factor? It boils down to individual judgment based on the application of the LED luminaires, the lighting requirements of the space, and the level of confidence in the manufacturer’s data. As it is unlikely that most LED products on the market have been fully tested to their rated life to prove their output, it seems reasonable to err on the side of caution. The ultimate light loss factor selected for a given LED luminaire will have a direct impact on its evaluation when compared to other sources.

Brian Fiander is an electrical engineer at Harley Ellis Devereaux specializing in lighting design and specification. He serves as one of the firm’s sustainable design champions, acting as a resource for other staff members regarding green design, LEED certification, lighting, and electrical systems. He has participated as a panel speaker for the IES Detroit chapter on LED installation and application due to his understanding and specification of LED luminaires.

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