Re: Light levels

 | From: Elizabeth Worobel <eworobe at cc_UManitoba.CA>
 | Date: Tue, 2 Apr 1996 08:07:50 -0600 (CST)
 | Subject: 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.

I used in the beginning 2X20W PPUT on a 29 gallon tank. That makes for
2760 lumens/.2323 m2=11900 lux. Amano, referring to CO2, states that
one good indication if the plants are photosynthesizing is to observe
them 1-2 hrs after CO2 addition. Interpolating this to "lights on"
you would think that you are saturating the photosynthetic rates if
the plants are bubbling 1-2 hrs after turning the lights on. Well,
I've achieved this with the above mentioned 2X20W PPUT, which
coincidentally would be in total agreement with the Dennerle
recommendations. It's still hard to believe that 2 bulbs or 1.37W/G 
is enough. 

 Didi Soichin
 didi at wwnet_com
 Westland, MI, USA