How to define 'light loss factor'?

it's factor that describe the loss of light and how much the lamps is not beneficial by it's total lumen . and it depend on many reasons . like the the dirt of the environment that may exist on the lamp surface that may affect on the total lux of the room , anther point is (the depreciation of the lamp )that describe the decreasing of the light by the time because the lamp's lifetime ,the lumen decreasing by the time . so we must take in our consideration this point .. the third point is the (lamp burnout factor ) that describe that if any lamp from our lamps is burned . so we need sometimes to substitute it . so it can take about1 hour . so it will affect on the total lumen in the room .. so we must take in our consideration this point also .and another factors we must take it in out consideration when we make our calculation of the lux . so we multiply the total lux with the light loss factor 

LLF (Light Loss Factor) or MF(Maintenance Factor) is the depreciation of light output over the time and it can be calculated via the following equation:

MF or LLF= Lamp Lumen Depreciation (LLD) x Luminaire Dirt Depreciation (LDD)

LLF is affected by many factors like; Temperature Dirt&Dust Over time

I believe that LLF must be standardized by Authorities because it can be manipulative criteria if left to manufacturers; as each manufacturer has his own LLF for the products he produces, thus might lead to misleading in communication regarding lighting products