The rated life of light sources is a measurement of the average hours of use for a lamp. The failure of a lamp occurs when it will no longer start or keep burning. Over any specific group of lamps, some will fail sooner than others, while some may keep burning and burning. The rated life of lamps is defined in the IESNA Lighting Handbook, 9th Ed. as "that time when 50% of a large group of lamps are still in operation." For HID lamps the procedure for determining rated life uses a "burn cycle" of 11 hours of burning and one hour off. While other "burn cycles" will produce different results, this 11on/1off pattern closely resembles the pattern for outdoor lighting. Typically, outdoor lighting is switched on at dusk and off at dawn, so the average burn period is around 12 hours each day, corresponding to 4,380 hours over each calendar year (actual hours are slightly less, around 4,200).
Metal Halide (MH) lamps are sensitive to position, and the lumen output, efficacy and rated life are different for vertical or horizontal lamp positions. In the (leading MH lamp manufacturer) Venture 2000 catalogue (p 21), the 100W MH lamps have 15,000 hours life in the vertical position and 11,250 hours in the horizontal position. This means the horizontal rated life is only 75% of the vertical lamp rated life for these 100W MH lamps.
High Pressure Sodium (HPS) lamps are not sensitive to burning position. From the General Electric 1999 Lamp catalogue (p 3-15), the rated life is 24,000+ hours, which means that at 24,000 hours the testing ended with two-thirds of the lamps still burning.
Using 4,200 hours as a typical annual burn total, the corresponding maintenance period for lamps can be estimated. For these 100W MH lamps, the values are 3.6 for vertical position and 2.7 for horizontal position, and for HPS 5.7 years. 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.
Table 1: Rated Life of HID Sources Source/Description Rated Life (hrs) Maintenance Period (4,200 hrs/yr) MH 100W vertical 15,000 3.6 years 100W horizontal 11,250 2.7 years HPS 100W (any position) 24,000+ 5.7 years (or more)
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