The rated life of light sources is based on the continuing operation of the lamps. Over the rated life of the lamp, the lumen output drops from the rated value toward the value just before the lamp fails, called end-of-life. Since the rate of this decrease in lumen output can vary between individual lamps and source types, the industry has developed definitions of "mean" lumens. For different High Intensity Discharge (HID) sources, the definitions of "mean" has been different.
For High Pressure Sodium (HPS) sources, "mean" used to be defined as corresponding to 50% of rated lamp life (and for mercury lamps too). Now some lamp catalogue's footnotes define "mean" for HPS as occuring at 40% of rated life, while some still use 50%! Since HPS lamps over 35W consistently have rated life of 24,000+ hours, the mean for HPS would now correspond to 9,600 hours of operation - over two years of dusk-to-dawn operation.
The definition of "mean" for Metal Halide (MH) lamps is at 40% of rated lamp life, but the rated life for MH varies with lamp wattage and position. For the 100W lamps in the example discussion, the rated life is 15,000 hours for vertical burn and 11,250 hours for horizontal, so "mean" corresponds to 6,000 and 4,500 hours respectively.
Using 4,200 hours as a typical annual burn total, the corresponding mean period for lamps can be estimated. For these 100W MH lamps, the values are 1.4 for vertical position and 1.1 for horizontal position, and for HPS 2.9 years. Rated lumens and lamp life for other wattages of MH sources will be different than for these 100W lamps in this example, with smaller wattage lamps having equal or shorter life and higher wattage lamps having longer life, up to 20,000 hrs. HPS lamp life is consistently "24,000+" hours, which typically means that 67% of the lamps are still burning after 24,000 hours.
Table 1: Mean Hours of HID Sources Source/Description Rated Life (hrs) Mean % Mean (Hrs) (Yrs) MH 100W vertical 15,000 40% 6,000 1.4 100W horizontal 11,250 40% 4,500 1.1 HPS 100W (any position) 24,000+ 50% 12,000 2.9
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