I have been thinking more about the question, does the light intensity drop off differently for long linear bulbs than for short "blob" bulbs. I downloaded some log log "paper" from the internet and replotted some of the data I took - the center line of the tank from the substrate level up to near the plane of the bulbs. Here is what it looks like:



I think it is clear that there is a different drop off rate for the two fixtures I tested. And, it appears that the almost linear AH Supply PC bulbs appear to lose intensity inversely approximately proportional to the distance from the bulb. But, the CFL screw-in bulbs seem to lose intensity approximately proportional to the inverse square of the distance. As you get close to the plane of the bulbs, the relationship seems to break down, and that is explainable because the "linear source" is less and less a linear source as you get close to it. The same is true of the "blob" source. I'm sure these relationships break down at the ends of the tanks too for obvious reasons.

This isn't proof of anything, but it does suggest to me that one reason for the effectiveness of T5 bulbs is that they are close to true linear light sources. And, to me it suggests an explanation for why people can successfully use what appear to be way over powered MH bulbs - they are suspended further above the tank, and their very high light intensity drops to a reasonable level as you do deeper into the tank. It makes me also think that the ability of MH fixtures to "penetrate" deep tanks is largely because they are so bright to start with.

I think maybe I have now squeezed out all that I can from the data I took.