Today someone at APC posted a link to Reef Tank, which shows PAR measurements using T5 bulbs. These measurements clearly do not show an inverse square drop in intensity. So, another APC poster did the math and figured out that my mistake was treating a tubular light as a spherical bulb instead of as a linear tube. When you treat it as what it really is, the drop in intensity is inversely proportional to the distance (not squared) from the bulb. A pair of pendant MH lights mounted side by side would also not have an inverse square drop in the area between the two bulbs, and a group of MH pendants mounted so the are all touching in a line, would give the same result for most of the length of the group. As you get near the end of the tubular light or the string of MH lights, the drop approaches an inverse square relationship again.

So, that is good news! Good, because it better explains how we manage to ever get enough light at the substrate for deep tanks.

Another couple of thoughts: First, we have long known that a deep substrate gives better growth than a thin substrate. Much of that could be because the deep substrate moves the plants closer to the light. And, when we first plant our tanks, the plants are short, no where near the top of the water. So, their growth rate has to be adversely affected by the lower light that results. That explains what I always see - relatively slow growth for a couple of weeks, followed by furious growth to the water surface. And, it suggests that our EI dosing could be reduced until the plant growth starts to accelerate.

Finally, all of us who couldn't afford or didn't want to mess with a MH pendant light can finally say we knew what we were doing - T5's have big advantages over them.