look at it this way, each fixture is made up of lots of small lights, and as you said they don't have narrow focus optics - more like a flood light than a spotlight.
If you put one old fashioned incandescent floodlight, say 25 watts, in a dark room and point it at the ceiling it will light up the room dimly. Add a second and the room will be brighter - even if it is pointed at a different part of the ceiling (because they are wide angle floods). Add 30 of these lights and you now have overlapping coverage all over the ceiling - and 800 watts total.
Say the room is very long and you started at the right end. Even as you add more light to the left side of the line of lights the overspill is going to increase the light hitting the right side too, and (jumping back to an aquarium) give more PAR to the right end as well as the left.
In lights that have narrow optics this effect would be much less (more like a line of narrow focused spotlights)
So going by your math, the PAR of all fixtures would be greater at the middle than on the ends? Sounds reasonable. If that's the case, where are Finnex's numbers being measured, on the ends or in the middle? It also means that in order for the LED's on one end of a 4ft fixture to affect the PAR on the complete opposite side, one single LED has to have a spread of 4ft (at 18" from measuring point). Is that true? That seems like a huge waste on any fixture that is smaller than 4ft, which is every single one of their fixtures, minus the 4ft one.
Edit - I realize nothing you said claims that the LED's on the far right of a 4ft fixture are affecting the PAR on the far left side of the fixture, just that the spread of the LED's is greater than 2ft, thus creating more par on a 4ft fixture than the 2ft fixture.
I guess what i'm really looking to find is the angle of spread on each LED. Aren't most LED's 120°?
Last edited by Shark_; 03-26-2014 at 06:05 PM.