Alliance for Lighting Information

End-of-Life Lamp Performance Data

by David M. Keith

The cycle of maintenance for lighting systems corresponds to a reduction in light levels due to lamp lumen depreciation and dirt accumulation, until maintenance restores those losses. At the moment before the maintenance occurs, the lighting system is at its most-degraded state, so the lighting levels are the lowest they will be during the maintenance cycle.

Maintenance may occur at any time during the lamp life, but typically the lamps are replaced through either group or single relamping. Group relamping typically occurs at around 70% of rated lamp life, which is significantly after the time defined for "mean" lumens of 40% or 50% depending on the source considered.

Even though the lamps may not burn to their rated life, designers need to have information about lamp lumen depreciation at end-of-life in order to predict lamp performance beyond the time associated with "mean". For single (spot) relamping, the lamp is "in use" until it fails, so the performance at end-of-life is what is the worst-case scenario for lighting designers to consider.

This is especially significant when lighting outdoors, because in most outdoor lighting systems, only one or two luminaires make significant contributions to the light levels at the locations of lowest illuminance - like between roadway poles or in the corner of a parking lot. In such cases it is completely inappropriate to use "mean" lumens for the overall Light Loss Factor unless there is certainty that group relamping will occur at 40% of rated life. Instead the Lamp Lumen Depreciation (LLD) factor should reflect the anticipated maintenance - using the value for group or single relamping as appropriate. For correctly estimating either of these values, designers need to know the lumen output of lamps at end-of-life.

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