What is the mean lumen output of a fluorescent lamp, and how is it different from the initial lumens?

Since fluorescent lamps have such a long life compared to incandescent lamps, eventually the brightness (lumen output) of the lamp will begin to decrease.

Because of this, fluorescent lamps have an initial lumen rating and a mean lumen rating. The mean lumen output is measured at 40% of the lamp life.

NOTE: Decorative or colored bulbs do not have a lumen rating since they are not considered a primary source of lights, only decorative.