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Newbie lighting lumens per square foot question

Discussion in 'Newbie Central' started by bullmoose77, Jan 26, 2007.


    bullmoose77 Active Member

    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.

    babygro Well-Known Member

    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.

    dindy Well-Known Member

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

    yestheoneandonly Active Member

    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

    Nocturn3 Well-Known Member

    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.

    whitenugz Active Member

    + + ++ + + + + vouche

    Quickee Well-Known Member

    haha lol what a bunch of haters

    onenumcat Well-Known Member

    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.
    I call it 'Aerolights'
    I'll never use stationary lights again!

    onenumcat Well-Known Member

    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.

    onenumcat Well-Known Member

    ...but then, you woke up! rofl!

    hobzz420 Active Member

    Best explanation I've seen!!! Thanks Babygro!!!!!

    ropadope Active Member

    I have vented six inch hood with a vortex blowing through it and it allows my hortilux 600 watt bulb eight inches from the canopy. That is as close as you can get with a 600 watt system. here is a light chart that I think has come in very handy for me. Hopefully it will help you. Also I am using a 4x4 tent for the whole process.

    Attached Files:


    atomicronick Active Member

    well.....besides rope and baby, i believe all has been said here that needs to be said. thanks for the charts, and the tips. when my time comes ill keep everyone posted.....got like.....100 seeds just dying to be started.


    samljer Active Member

    Actually his math was a little off as well.
    he didnt take into acount you dont count that first foot of distance, because thats where the original lumen number represents.

    in fact that 430 watt is perfect for that space at 2 feet above the tops.

    so im going to "tsk tsk" too

    riddleme Well-Known Member

    I'd say tsk tsk as well to the 10,000 lumens needed, as the sun gives off 10,000 lumens at the equator at noon this does not happen anywhere else

    ideal is 3000 to 6000 lumens for stress free growth and best advice is get a light meter to dial it in properly, a cheap analog light meter can be had for $29 reads to 10,000

    fonzirelli Well-Known Member

    Dude, just go with 5000 lumens per sq ft. keep the light further away for the babies, closer for when they are getting more mature. check on them twice a day. your plants will let you know if they need more light or not or whether to put the light closer or further away. this would be perfect for you since its your first grow (im guessing its your first cause you say youre a newbie).

    somepotname Active Member

    On a clear day the sun gives 100,000 lumens per square meter on average at sea level.

    Kervork Well-Known Member

    Ok, so if I'm sitting here with a light meter, how do I calculate the optimal distance for my plants over X square feet? Too close the light burns stuff, too far away you grow twigs. Is there a process or equation for this? Some sort of scientific method here? Maybe measure lux across 16 separate points and compare at different heights to see what produces the largest overall average or sum? Set the hottest area to 7K lumens and sacrifice intensity for coverage of outlying areas? I sense this requires things I forgot from calculus.

    Any of you out there have a meter and if so, how do you use it to set your light height?

    ThunkLogic Member

    This explanation shows optimal conditions for any room. Too often people just build a room that feels bright enough and 'slap' their lights as close as possible without burning the plants, when in reality, any good engineer knows that you work backwards through a problem. I tsk tsk tsk your efforts to slander this AMAZING explanation and encourage anyone to ignore anything this slanderous, alleged 'grower' says about his obviously NOT optimal grow room.

    ThunkLogic Member

    Thats probably because 1x1=1 and 56,000x1=56,000, so that first foot is already assumed as the measuring distance in the equation. Algebra I handing out pearls all up in your grille.

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