I was fortunate to be able to borrow a Par meter this week. My tank is somewhat tall, with lights well off the surface, and I always wondered what the actual Par values are. Well now I know so I thought I would share my results.
Frist here are the pertinent details. The tank is a 120 gallon, at 60 x 18 26. There are two fixtures, with total light being 4 X 54W, with 48 bulbs and good quality polished reflectors. The back and front bulbs are Zoo Med Ocean Sun 10,000K, center bulbs are Truelumen Fresh Flora red. All are about a month old.
So with a 60 tank, and 48 bulbs, I have 6 of tank on both sides with no light directly above. You would not really notice it all just looking at the tank, but I expected the Par would drop off quickly, which is why I focus on Crypts there.
Light fixtures are mounted up inside a canopy, so lights are about 10 from the water surface, and 28 from the top of substrate.
I put together charts to show the values at 10, 19, and 28 from the light.
A few things I found interesting. I expected a drop off in intensity as you move away from the center of the tank, which was true. What I wasnt expecting was how much the drop off was at the ends of the tank. I expected it at the very end, as there no light directly above. But there is also a significant drop off in the last 1 foot or so of the bulb. Might have to consider going to 5' bulbs? At least it's food for thought.
I also found it interesting how quickly the light intensity fades from surface to substrate. I imagine if I re positioned my lights closer to the top of the water, it would increase the readings. May consider that as well.
So here are the readings. They were all taken at least three times. So am I high light, medium light, low light? I guess that depends on where in the tank you are measuring.