Quote Originally Posted by LIP
As many as poss
LoL. You just made me have an idea..

"As many as possible. All right!"

I could imagine someone trying to place a kilowatt of HPS on a square foot of space almost right up to the bulb; then try to put a fan between it and the light to solve the 'heat' problem only to come to realize it's actually the intensity of light turning the once green plant into a tanned pile.

As many as possible is right, but there is a limit. Although I doubt this scenario is remotely possible [ no one's that stupid ], it's still in the realm of possibility.

LIP and Swizzy are both right. Simulating the same environment the plant receives outside, light wise, is a good start with any grow.

The Earth-Sun distance tells us, on a cloudless day, that there will be roughly ten thousand lumens per square foot at ground level. Let's try to get as close to that number as possible, but don't exceed it by much -- that is, don't put more than maybe 12k lumens per square foot or there's a chance you're going to bleach the plant, and 12k may still very well be too high.

I've got a 150 watt HPS that can and will bleach parts of the plant that are immediately under the 16,000 lumen bulb [ well, 13,500 mean lumens ]. The damage isn't a result of the heat so much as it's a result of just being, well, bleached.