A 400w hps grows nice buds at the edges of my 3ft x 3ft tent and their par is not good, especially in the corners.
People been growing for years with "low PAR" and it grew damn fine award winning bud... with 500-700 ppfd you will rock it and get a totally satisfying result
In Europe 600W SE HPS is more standard in those sizes, american tradition is 1000W DE hung higher. (edit: well, not in a 3x3, slightly bigger)
I mean, it was a time people wondered why MH doesn't hit HPS yields watt for watt
Duh, MH puts just out
way less light than HPS
1000 PPFD without CO2 is a fad. It's nice to have the option but I have seen plenty of plants that don't like it so much, or it changes the morphology in a undesirable way
I like what a guy called LostLeaf says: if you're hitting your plants with 1000 PPFD most of them are done photosynthesizing after like 9 or 10 hours.
And to add something to the "Selfishness". If you take a plant and prune some of the big fan leaves, you'll notice that the shoots which are supported by those leaves you removed will grow very poorly compared to the ones where you didn't remove the leaf. Even if they are right next to it. I see every shoot as a new iteration of the same plant, a recursive thing. It just grows itself in itself in itself, as much as it can. All the shoots of the plant appear to compete against each other, looking for light. Not by choice, it's just what happens. If the left shoot gets 50µmol/s more than the right shoot, the left shoot always wins over time.
So yes light spread and light diffusion are king. It's better to have 600µmol/s everywhere than 1000 in the middle and 300 at the edges. This is what you can achieve to great success with DIY methods. But to be honest, the light spread of most commercial modern LEDs should be an improvement over single point sources of light such as HPS.
When I used single point light sources, I would always see the leaves at the edge pointing inwards toward the light. With diffuse light sources the leaves are always pointing up. I see this as a sign that every part of the canopy gets adequate lighting. The quality of the tops is high, and larf is reduced. (More light can reach more spots because of the diffusion)
