From the data accumulated at http://www.apsa.co.za/board/index.php?topic=4454.0
(in 2010) it is calculated that, on average, you get about 1 PAR/W from tube/fluorescent lamps (T8 & T5). (T5s are only marginally better for output and this is due, mostly, to geometry.) So a 20 W CFL will put out as much PAR as a 20 W T8 tube (because the physics of light emission process is the same). Of course the CFL is all twisted and a lot of light is lost among the twists and turns of the tube (especially as about 30-40% of the light emitting surface is facing another light emitting surface and gets lost by "squishing").
The major factors (that will vary with each aquarium setup) are the turbidity of the water (the more light absorbing dissolved solids the more light is absorbed with depth), the amount of light lost to reflection due to surface disturbance or angle of lamp to water (90% of the light hitting the water at 60o is reflected and it gets worse the shallower the angle). You need the lamps as close to the water as possible to keep the angle of incidence steep. The quality of your reflector is also critical. With a bad reflector you are probably getting less than 50% of your light into the tank. With no reflector the value is closer to 30%. A model to determine reflector efficiency is available at http://www.apsa.co.za/board/index.ph...41393#msg41393
Essentially, if you can engineer a reflector with high efficiency and place it so that as much light as possible hits the water's surface of the tank, then you can, with some certainty, use W/m2 to estimate PPF at specific water depths.
So, to summate by example: if you have two 30 W T8 tubes they are emitting approximately 60 PAR. If you have a good reflector that can focus all the light into the tank (which is impossible but lets just pretend) which is 36 x 12 inches in surface area you will get 222 PPF (PPF = PAR/(0.9 x 0.3 m)). Assuming 70% of the light actually enters the water, this will mean you will have about 99 PPF at 15 inches of water
depth (not tank
depth). More than enough for Glosso and HM and unless you have ample CO2 and nutrients you will get an algae soup. Without reflecors the value would be closer to 30-40 PPF which attests to the old adage that 2 full length tubes are enough to grow just about any plant.
I'm having a hard time understanding this and it's relation to the chart. I have a 5.5 G tank with a 13w 6500k CFL about an inch from the surface in hood with no reflector. So I'm reading this noticing I'm losing a ton of light due to it being parallel to the surface and without a reflector but unless I'm reading the chart wrong since my bulb is probably 8 inches from my tanks lowest point my par is extremely high. I'm just trying to figure out what my par is and what category it falls in. Judging by the coloration of my plants I always thought it was low to medium-low at best. Am I opp operating at 1 p/w? So I'd need a 30w cfl for my 5.5g tank. My betta would probably think he's in the interrogation room