It's my turn to ask a question of you, Hosebomber.
I've been considering expanding my LED lighting, and I've been comparing PAR grids and area coverage, etc. I find that my one 540 watt panel doesn't have much of a spread. At a 24 inch height, its off-center PAR drops 40% to 400 umols. So it really only has effective PAR in a 2 foot square/oval. The solution for my 4x4 is to add another 100 watts in each corner. If I'm reading the grids correctly, that should get me 500+ umols across the entire room at 24 inches.
At what intensity do we start overloading the plants? How many umols is too many? I see growers blooming large plants under 1000 umols now.
I've found this common on all the LED's I have tested so far, they have excellent intensity just below the light (within the dimensions of the light) but more than 1' outside the direct overhead lighting, the lighting drops off quite a bit. I have found with LED it seems that more panels with less wattage is better than 1 large panel with lots of wattage. All of my different brands of LED do this, some carry the footprint better than others but they all drop off quite a big outside of a 2'x2' area for the most part. I think you are on the right track with more lower wattage lights to help extend the footprint.
As far as what I have read, cannabis should have at least 500 umol (at bottom of canopy) for best optimal lighting, meaning the tops are receiving much more than this. Hosebomber is right that plants can receive up to 1500 umol or so, before it starts negatively affecting photosynthesis rates (unless temps are ideal and supplemental co2 is added). Generally you should have a good 500+ umol across the entire footprint of your grow area for ideal conditions, as far as my research has showed.
A great way to look at plant lighting (and in my opinion, one of the most accurate) is DLI or Daily Light Integral. Basically this says that cannabis plants need 22-30 moles per day of light. Now the Umol that this translates to is defined by the length of the daylight period, hence why vegging plants need less intense light, and plants under a 12/12 schedule need a much higher umol. Basically it goes like this...
DLI requirement of 22-30+
(255 Umol - 347 Umol) PPFD on 24 hours daylight schedule.
(383 Umol - 371 Umol) PPFD on 18 hours daylight schedule.
(510 Umol - 694 Umol) PPFD on 12 hours daylight schedule.
Now these numbers generally are measured from the bottom of the canopy per my research, as well as how lighting specialists measure for greenhouses and commercial agriculture. Hope this helps ya out!
I would say just make sure to look at the PAR charts/over footprint to determine which lights will give you the footprint you are looking for, above 500 umol at the distance you are looking to hang them
