Final answer:
Mean lumens to rated lumens in evaluating existing fluorescent lighting systems refers to lamp lumen depreciation, which is the reduction of light output over time as the lamp ages.
Step-by-step explanation:
When evaluating an existing fluorescent lighting system for a potential LED upgrade, the term mean lumens to rated lumens refers to lamp lumen depreciation (LLD), which is a measure of the loss of light output of the lamp as it ages. As lamps age, they produce fewer lumens than when they are new; this loss of luminous flux over time can significantly affect the lighting level of a space. This is a particularly important consideration when transitioning to LEDs because LEDs maintain a more consistent lumen output over their lifespan compared to fluorescent lights. The other options given—ballast factor, voltage drop, and luminaire dirt depreciation—are factors that can affect the lighting system's performance, but they are not directly related to the reduction in lumens a lamp emits over time.