Below is a plot of my data yesterday for my 45 gallon tank fixture with AHS light kits, measured along the length of the fixture at the depth of the top of the substrate. I added the data for the 10 gallon fixture with the screw-in CFL bulbs for comparison.


I think the drop off in intensity results because as you move towards the ends of the fixture the light contribution from the opposite end drops off rapidly, and there is less contribution from the shorter end in the opposite direction.

It should be clear that there is no one number that characterizes the light from a light fixture, no matter what bulbs or reflectors are used. So, we might as well just use wattage and bulb/reflector type as our descriptive "number" for such fixtures. i.e. 110 watts of AHS Bright Light kits with GE 9325K bulbs.