I'm jumping in here a bit late, but reading the thread and the questions and difficulties that people are having are the product of a serious misunderstanding best encapsulated in the following statement:
PAR isn't something that you put into the tank. It is the intensity of the light at various points in the tank.
PAR is something you can put into your tank. It is measured in microEinsteins which is a measure of the number of photons (these are real things!). What is being measured with PAR-meters, and what Hoppy is talking about is Photosynthetic Photon Flux (PPF), also called Phososynthetic Photon Flux Density (PPFD). This is measured in microEinsteins per square meter per second. So, to summate,
PAR = uE
PPF = uE/m2/s
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"). PAR output of LEDs varies from 1.55 to 1.03 PAR/W (depending on bias current) but the geometry of the lamps greatly increases efficiency as the light in concentrated over a narrow beam focus. We suspect that LEDs are, in the end, 3 times more efficient than tube lamps based on the number of LED watts needed to get plants to pearl compared to T5 tubes.
Using this data, and Tom Barr's PPF data
for actual planted aquariums and scientific journals for attenuation coefficients*
we worked out the following: http://www.apsa.co.za/board/index.ph...9062#msg109062
(and have been fine tuning it since 2010 as we gathered more data). This is our best guess for how many watts you need over a tank to achieve 40 PPF at a certain depth.
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.
The issue here is that light moving through water is subject to the law of attenuation, not the inverse square law, and there is significant internal reflection of the light moving from the water to the glass. Light out of a fishtank follows the inverse square law just fine meaning the higher your lamps above the tank the less your photon density becomes with distance from the source.