Light Loss Factor

A factor used to account for the degradation in luminous output over time.

Light loss factors (LLF) are the factors that need to be considered when calculating the Lumen Method. The most important factors to be considered are:

  1. Lamp Lumen Depreciation (LLD): Lamp lumen depreciation values reflect the overall performance of a lamp over its life. LLD = (mean lumens/rated lumens) Those values can be found from the lamp manufacturer data
  2. Ballast Factor (BF): Compares the ratio of the light output of a lamp working by a specific ballast to the light output of the same lamp working by a standard reference ballast. The BF is given in the ballast manufacturer data.
  3. Luminaire Dirt Depreciation (LDD): It is the light loss prior to cleaning dust. LDD is estimated from tables in the IESNA Lighting handbook.
  4. Room Surface Dirt Depreciation (RSDD): This value accounts for dirt or dust that accumulates on all of the room surfaces — especially on the upper walls and ceiling. RSDD is also estimated from handbook tables.

The product of all these factors is the light loss factor (LLF) which then is used in the Average Illuminance equation.

Key Terms

light loss factor
light output

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Sources & Credits

Last modified on December 8 2019
Content adapted from Wikipedia
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