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3000 lums ffor veg is the minimum, but I have found ffor me personally that I get very good response from my vegging plants when I get about 4500 lumens. Anymore than that doesn't seem to increase the benefit so I just stay at that level for veg and I try to get 6500 lums for flower. Anymore that that on either of them, I believe is a waste of watts.
Keeping the lights close together is good as it allows you to concentrate the available light energy ffrom those bulbs into a smaller grow area. That is good because the bigger the grow area for a given set of lights, the lower the amount of lumens per sqft. This is the concentration of energy or "saturation". Some would say this amounts to the intensity as well. But if you are only needing to achieve 4500 lum per sqft and your lighting for that space is giving you 10,000 then you can either lesson the light to the 4500 or you can open up the space until the rate of dispersion lowers the Lum per sqft to the 4500. Opening the space up some will allow more penetration on the plants while allowing better air circulation for the plants, and it will lower the heat that is concentrated in the grow space.
Light from a single source versus light that is spread out, is a moot comparison. because the key for this is lumens per sqft and the amount of penetration that the lights produce. You can have 1 600w light that gives the right amount of lumens per square foot for a given space like a 4'x4' tent, and you will have no problems. Or you can set up a scrog and have 2 400w lights setting side by side but being a little closer to the plants so that they get the maximum output of the lower wattage lights, and the scrog keeps the canopy at one level so penetration is no an issue. With the 2 400w lights, you will achieve the same results or maybe a little better depending on your grow methods as you would do with the single 600w light.
Now penetration is the distance the photons of light will travel without loosing more energy than the plant needs for photosynthesis. As light travels, it loses some of its initial energy given to the photons at the light source. This is because the photons meet air molecules and anything suspended on them as they fly out from the light source, and that creates resistance which drags away some of the energy. Also the photons disperse into an open space and the more space they have, the more they spread out, which gives them more time to lose energy.
Plants need a certain amount of energy from each photon that hits their leaves or they cant do the chemical processes that they need to do. Iff those protons travel too ffar and lose enough energy to drop below the threshold of usable energy by the leaf, then that energy is wasted as heat on the surface of the plant. The distance that photon can travel before its energy level drops below the usable threshold is its level of penetration.
When ever light has to travel to a wall and then bounce back to the plant, it is losing that much more of its energy before making contact with the leaves.
Now with floros, you don't have as much penetrating energy as with the HID lights, but you have a lot of it being evenly dispersed. That makes the floro ffixtures good for small vegging plants because you have many plants that you want to keep close enough to the light source to get good even lighting energy to all parts of the plants. The T5s are cool enough and even dispersing enough that you can place them closer to the plants without burning the plants with intensity or heat. So you can spread out your lights a little and open up the space to allow for better air flow which is important, but still get all of the lumens that the smaller plants need.
When vegging or flowering bigger plants, I prefer 2 light sources to one as that will give you deeper penetration off the HID light but also give you light hitting the plants from 2 angles which reduces shadowing on lower growth. I hope all of my "lighting 101" ramblings makes sense to you rather than confuse you more.