While I do not pretend to even halfway understand lighting. I do understand a few things, even though I cant explain it in my own words.

But....

Plants DO NOT USE lumens. We use lumens. Plants use something called Plant Growth Lumen(PGL) which is the plants version of lumens and what THEY see. As far as lumens goes its the light(spectrum) that humans can see. The lumens that plants see are pretty much in spectrums that we cannot. So, just because it says 1650 lumens on your bulb does not mean its the right thing for a plant. You could buy a bulb that says 1650 lumens, that is what you will see when you turn it on, but the plant may only see 200 lumens.

I am going to run over to our Agriculture department tomorrow and see if I can find something that totally spells it all out in terms us normal people can understand. In the meantime here are some links.
Towards a Plant Growth Lumen
Lumens… Please. « Calling all photons…
foot-candles, lux, lumens, sunlight, PAR

Hmmm! This is really interesting. It makes sense though, I mean compare the lumens of a 26W CFL to a 42W. The difference is small, yet the 42W is remarkably more effective.
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