Another issue that the measurements on the 10 gallon Perfecto modified hood could resolve to some extent is whether or not the light varies per the inverse square rule as you move further from the bulbs. To try to answer that I plotted the PAR values measured under the center of the fixture, and tried to fit an inverse square equation to the data. Of course it isn't possible to fit such an equation as you get too close to the bulb, because eventually the light source is just too far from an ideal source. At a quarter inch from the center of the outer surface of one bulb I measured 1000 on the meter (vs. 1700 outdoors in sunlight at 9:00 a.m.) I settled on fitting an inverse square equation to just the points near the bulb, but not that near. This is what I found:



I have to conclude that for this fixture you can use an inverse square relationship to approximate the difference in light intensity between two places at different distances from the bulb, but you can't use it to judge the loss of intensity starting from right at the bulb itself. So, it is accurate to say that a tank that is twice as deep as another tank will have about one quarter the light intensity at the middle of the tank, for the same fixture on both tanks. And, much more interesting to me is that as a plant grows in this tank, the amount of light it receives grows in inverse square with its nearness to the bulb.

If I generalize this, it explains why my plants, in any tank, grow slowly for quite awhile, then accelerate in growth as they get taller.