Newbie lighting lumens per square foot question

Originally Posted by bullmoose77

New to indoor growing and am trying to determine which light would be sufficient for a 6X3 foot grow room. I am interested in the Son Agro 430W HPS bulb which has an added blue spectrum to the bulb. It has 53000 lumens, which at the recommended 2500 lumens per square foot ratio, would be enough light for 21.2 feet squared. However I keep seeing on here that a 45000 lumen light is only enough for a 3X3 foot area, that is actually 5000 lumens per square foot, so which is it. Also if it helps, I plan on doing a scrog grow, and using a 4 or 6 ft light mover. I know a light mover is a good tool for reducing shaded as well as hot spots, but how much additional light coverage do you actually think this yields. Thanks in advance for help, and I know I put a little bit of info into one posts, so if any clarification needs to be done, I'm ready for it.

Okay. So you have an 18 square foot space you want to illuminate, first you need to understand the light requirements of plants and then the inverse square law.

Plants in seedling and clone stage require about 400-500 lumens per square foot. In vegetative growth they require about 2,500 lumens per square foot and in flowering, they require about 10,000 lumens per square foot.

The most important thing about light, is to understand that it diminishes rapidly the further away it is from the target. Now, As I'm sure you know, HID systems output a fair amount of heat and that means they cannot be used particularly closely to the plant tops for fear of burning them, so for a 400w (or 430w) system it would need to be about 2 feet from the plant tops. So if a 400w system outputs 53,000 lumens at source, it's represented as 53,000 lumens at a distance of 1 foot from the target. The Inverse square law is light intensity (output in lumens) divided by distance (distance the target is from the source) squared. So for a 400w system outputting 53,000 lumens we now know this will be outputting 53,000 lumens divided by 2 foot squared (53,000/2x2) which is 13,250 lumens at the plant tops.

So if we wanted to give the plants 2,500 lumens per sq foot for vegetative growth, we need to work out how far the lamp could be away from the source and that would provide us with the sq area. 53,000 lumens at 3 foot is (53,000/3x3) 5,888 lumens, at 4 foot is (53,000/4x4) 3,312 and 5 foot is (53,000/5x5) 2120, so we know that to deliver 2,500 lumens per square foot allows the bulb to be about 4.75 feet from the source which would illuminate a square area of 4.75 x 4.75 or 22.5 sq feet.

Light movers increase coverage by about 25%, so it looks to me like if you wern't using a light mover you would probably need 2 x 400 systems for flowering and vegging and perhaps 1 x 600 for both using the light mover. Now you have the math, you can work it out.

dude you are so fucking wrong...i keep my 400w about 6 inches above without burn....tsk tsk tsk...

Originally Posted by dindy

dude you are so fucking wrong...i keep my 400w about 6 inches above without burn....tsk tsk tsk...

dude, you dont know gold when you see it. The best and most simplified explanation i have seen. and you "tsk tsk" him about one comment he had about burning plants. You might have a vented 400w and then next guy to come around might not and he WILL burn his plants.

Babygro is handing out pearls

Personally, i'd say 12 inches from the top of the plants is fine for a 400, provided you have a fan blowing across the canopy. Maybe less, but not much. I wouldn't say anyone is particularly "fucking wrong" though.

Originally Posted by yestheoneandonly

dude, you dont know gold when you see it. The best and most simplified explanation i have seen. and you "tsk tsk" him about one comment he had about burning plants. You might have a vented 400w and then next guy to come around might not and he WILL burn his plants.

Babygro is handing out pearls

+ + ++ + + + + vouche

haha lol what a bunch of haters

bullmoose, you'll also be able to set your light closer than most if you use the light mover.
stationary light project intense heat just below them, a mover will diffuse that heat.
I use one, my own design, and it works great...see how low I put two 400w HID lights.
http://www.youtube.com/user/onenumcat
I call it 'Aerolights'
I'll never use stationary lights again!

Originally Posted by babygro

...so we know that to deliver 2,500 lumens per square foot allows the bulb to be about 4.75 feet from the source which would illuminate a square area of 4.75 x 4.75 or 22.5 sq feet.

Light movers increase coverage by about 25%, so it looks to me like if you wern't using a light mover you would probably need 2 x 400 systems for flowering and vegging and perhaps 1 x 600 for both using the light mover. Now you have the math, you can work it out.

that was just a sweet explanation. I already know about that stuff, but felt like I got something from it anyhow, because it was so clear.
thanks.