Alliance for Lighting Information

Lamp Lumen Depreciation

by David M. Keith

Lamp lumen depreciation values reflect the overall performance of a lamp over its life. As lamps are burned, their lumen output decreases. How far and how fast are sometimes unclear.

Lamp manufacturers publish values for rated lumens, "mean" lumens and rated life, and the following chart shows that data for the 100W lamps presented in those discussions. The solid lines represent the published data. The dashed lines represent the linear extension of that data out to the reported rated life.

The linear extensions are "questionable", but without more published information it is hard to know what else to use. Manufacturers report that the curves are typically shaped more like the second figure (for a 400W MH vertical lamp), but the actual values of course would differ. In the second figure, the lumens at 40% of rated life - "mean" lumens for MH lamps - is about 22% below initial, while for the 100W lamp the drop to "mean" is about 33%. By extension, using the "400W shape" for the 100W lamps would lead to a lumen value at end-of-life that is "half-again" of the 400W value of 40% loss. This would lead to a estimate of 60% loss for the 100W lamp at end-of-life, or a LLD value of 0.40. This corresponds to 3600 lumens for the vertical and 3240 lumens for the horizontal at "end-of-life" (15,000 hours and 11,250 hours respectively.)

The HPS lamp has 8550 lumens at 12,000 hours and around 7600 lumens at 24,000 hours.

Some "generic" source comparisons are useful. The following diagram shows information on LLD for different sources, with no specific lamps referenced.

A report for the Arizona Department of Transportation (AZ 522) investigated the lifetime costs for lighting roadways with different sources and the following is figure B-2, which is described as showing the LLD for end-of-life conditions for 400W lamps. The HPS lamp data is for "standard" equipment while the MH pulse start is "latest" technology with electronic ballast.

The AZ 522 values are reportedly for horizontal position pulse start metal halide lamps. For comparison, a different manufacturer's data for vertical position MH pulse start 250W lamps is shown below, although only for 8,000 hours, despite the note that rated life is 15,000 hours. This is about the same value of LLD at 8,000 as the "generic" diagram above shows for 20,000 hours. Compared with the data from AZ522, the difference in LLD between around 0.55 for the vertical position and around 0.73 for the horizontal is unexpected but not entirely surprising, since horizontal lamps typically have shorter life and lower rated lumens.

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