Alliance for Lighting Information

Light Loss Factors: Initial vs Mean vs Maintained

by David M. Keith

Light Loss Factors (LLF) are multiplier values used to estimate the overall performance at different times during the life of the lighting system. LLF values reflect the performance of the lamp and luminaire as well as the maintenance level of a lighting system. The prediction of these different components can be relatively accurate or difficult. The use of LLF values does require making assumptions about the performance of the system, particularly maintenance.

LLF is calculated as the product of a series of factors, each typically between 1.0 and 0, although in some exceptional cases a factor may be greater than 1. Since the individual values are all multiplied together, each has a proportional effect on the system as a whole. Therefore, if one factor in the LLF is decreased by 10% of its value (e.g. from 0.90 to 0.81) the overall LLF will also decrease by 10% of its own value (e.g. from 0.60 to 0.54).

LLF is typically considered to have two groups of factors: recoverable and non-recoverable. Recoverable factors are those that can be recovered by proper maintenance, such as Lamp Lumen Depreciation (LLD). Non-recoverable factors are those that either occur whenever the system is on - such as ballast-lamp losses - or produce progressive degradation that is not improved by maintenance - such as Luminaire Surface Depreciation (LSD).

Furthermore the factors may be applied in distinct combinations, to reflect the initial, mean or maintained condition of the system. Initial factors are those that occur from the moment the system is first lighted, and typically do not change value over time. The difference between the actual ballast and the ballast used for rating the lamp corresponds to the Ballast-Lamp factor, typically less than 1.0 but sometimes over, and always applicable when the system is on. By comparison, LLD is a maintained factor. Mean LLF values can be developed by combining the factors for initial condition with "mean" versions of the maintained factors.

In general non-recoverable factors appply to the initial condition, but may not (e.g. LSD). In comparison, all recoverable factors are always excluded from the initial condition but included for the maintained condition.

LLF is applied to the rated lamp lumens, so it directly and proportionally effects the system's points' magnitudes - maximum, average and minimum values. However, since LLF is a multiplier to the rated lumens, it has the same proportional effect on every point, so ratios of values stay exactly the same. One way to consider this is that the lamp is being dimmed. This is certainly the appropriate way to account for LLD, but may not be the appropriate way to account for Luminaire Dirt Depreciation (LDD), which may not have the same effect on all the intensity values of the luminaire distribution.

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