How can i find the true lumen efficacy of a given fluorescent lamp? Luminous efficacy tables place them on a broad range from 45-75 lm/W.
Many fluorescent lamps state the wattage as 40w yet the lumens in all different quantities. From as low as 2000 to as high as 3800. Is the stated wattage and lumen package derived from IF the bulb were being driven by a ballast with a ballast factor of 1?

Question

How can i find the true lumen efficacy of a given fluorescent lamp? Luminous efficacy tables place them on a broad range from 45-75 lm/W. 
Many fluorescent lamps state the wattage as 40w yet the lumens in all different quantities. From as low as 2000 to as high as 3800. Is the stated wattage and lumen package derived from IF the bulb were being driven by a ballast with a ballast factor of 1?