The cycle of maintenance for lighting systems corresponds to the reduction of the light levels, until maintenance restores the losses due to lamp lumen depreciation and dirt accumulation. 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. For HPS lamps, the "mean" is defined for 50% of rated life or 12,000 hours, while 70% of rated life corresponds to 16,800 hours. This difference is over a year of dusk-to-dawn operation, during which time the lamp lumen output continues to depreciate below the "mean" lumens published by manufacturers. Even though the lamps do 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 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. 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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last changed on 4 Apr 03 by