The LLF is a depreciation factor based on how the light output of a fixture decreases with time. All photometric calculations for illumination design should use the LLF that corresponds to the luminaire used.

For LED the LLF is based on how well the luminaire dissipates heat. An LED luminaire that dissipates heat well will depreciate slower, and will have a higher LLF than a luminaire that does not dissipate heat well. For TxDOT LEDs the LLF is between 0.70 and 0.90, and is posted with each luminaire on the MPL. The method for determining a LED luminaire's LLF is in DMS-11011.

For HPS, the LLF is based on the depreciation characteristics of a HPS lamp. TxDOT's design LLF for HPS is 0.65.