Re: Light levels

Aquatic plants saturate their photosynthetic rates at between 300 and 
1000 umol per m2 per second of Photosynthetically Active Radiation (PAR). 
Anything below 100 umol/m2/s PAR should be considered low light as 
aquatic plant light compensation points are in the range of 15 to 85 
umol/m2/s PAR. Full sunlight on a cloudless, clear day at high noon in 
the midwestern US is about 2000 umol/m2/s PAR. Thats about as precise as 
you can get ... unfortunately, few people have acces to a quantum light 
meter with underwater sensor. Next best is Lux which is lumens per m2. If 
you know your lumen output and the surface area of your tank you can 
estimate lux. Unfortunately, the amount of PAR delivered per lux changes 
for different bulbs which produces a lack of precision. Finally, that 
leaves us with Watts ... not very precise or informative, but certainly 
adequate for most of us since most beginners realize from the 2 to 4 W 
rule that they are woefully underlighted.
If you pack as much lighting as possible under your hood you will 
approach the Ps saturation intensity (using fluorescent). Metal Halide 
lights will of course provide even more.

Dr. dave.