06-28-2018, 07:19 PM
Join Date: Apr 2005
Location: Brentwood, CA, USA
iTrader Positive Rating: 100%
| | Re: LED Lighting for 20G long
Originally Posted by dwalstad
I got a PAR reading of about 400 at the water surface just 2.5" under this fixture, which has ~142 diodes. Some of the other 30" Beamswork have only 42 diodes; for them I measured only 200 at the same position. I used a high quality PAR meter (Apogee) that was lent to me by the local aquarium society.
Most plants start their life at the substrate, and it is during that early growth period that plants either grow well or don't grow well. Based on that it is much better to measure light intensity at the substrate, not at the water-air interface. If you have a tall tank, like a 20 high tank, a high PAR reading at the top of the water column, with the light sitting on top of the tank, doesn't mean you have adequate light at the substrate. And, a high PAR reading at the top of the water column can mean much too much light at the substrate, if you have a low tank, like a 20 long tank. I suspect that some day someone will come up with a better way to use a PAR meter, when choosing a light fixture, but for now I strongly suggest always going by the PAR reading at the substrate.