07-03-2018, 04:16 PM
Join Date: Apr 2006
| | Re: LED Lighting for 20G long
Originally Posted by hoppycalif
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.
Yes, but you should be able to calculate PAR at the substrate based on surface measurements. As light passes through a certain distance of water, it loses its intensity in a straightforward way.
I have established tanks so overhead light is absorbed by floating plants and plants already in the tank. So substrate measurements are misleading, especially since I now have bare glass bottoms with plants in pots.
That said, I measured about 5-20 PAR at the glass bottom of my 20 gal Long (12" high).
Lighting is complicated. We could split hairs all day, but I hope the information that I have provided will be useful.