Unlike the change in average rated life of metal halide based on different burn positions, High Pressure Sodium, another High Intensity Discharge lamp, has the same rated life regardless of burn position.
Typically, High Pressure Sodium lamps, regardless of wattage, are rated at 24,000 hours or more. The HPS lamps that deviate from this are specialty HPS lamps, such as color improved HPS lamps, non-cycling and horticulture lamps.
According to Philips 2011 Lighting Catalog, Improved color rendering HPS lamps have an average rated life of 15,000 hours. Their non-cycling HPS lamps are rated at 30,000 hours and the Horticulture lamps at 15-16,000 average rated hours.
The general lighting HPS lamps, typically with a 24,000 average rated life, gain this mortality rating from a large representation of lamps in laboratory tests under controlled conditions at 10 or more operating hours per start. It is based on a survival of 67% of the lamps, and allows for indivdual lamps or groups of lamps to vary considerably from the average.
In comparison, Metal Halide lamps are given their average rated life under the same test conditions, but with a survival rate of 50% of the lamps tested.
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