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Confused!! Lumens per sqft /inverse square law / watt per sqft

Discussion in 'Grow Room Design/Setup' started by Cyrate, Jan 31, 2010.

  1. #1 Cyrate, Jan 31, 2010
    Last edited by a moderator: Jan 31, 2010
    Greetings GC!! ... long time lurker first time poster. First let me say, thank you, for all your hard work both in the growroom and on the forums. You guys are truly my mentors and inspiration.


    I had intended my first post to be my Grow Journal, but as i am still in the research phase (2mo and going!!) and found some confusion in my calculations, it is better to ask now, eh?


    My intended grow space:

    Legal Grow under California State Law
    size: 2dx3wx4.5h
    light: 400w remote digital ballast
    reflector: CoolTube 6"
    Vent: 400cfm 6" in-line
    Medium: soil for first couple of runs then moving to DWC.
    Method: 4-way LST and ScrOG.

    TL;DR :)

    FINALLY THE QUESTION!!

    Depending on who one speaks to determining how much light i can squeeze into my 6ft sq area varies greatly.
    some say the rule is:
    1. Lumens per sqft = Lumen output divided by square foot of grow space.
    2. Lumens per sqft = Lumen output divided by the distance of the grow light from the canopy, squared. (inverse square law)
    3. Lumens is an expression of wattage and you want 100 watts per sqft. (sometimes shortened to, 100watt for first plant 50watts for every other rule.)

    so that means my 6 sq ft area 12" under a 400watt HPS (55,000lumens)
    i either have:
    1. 9167lumens per sqft (sun at the equator during equinox ftw)
    2. 55,000lumens per sqft (plant bleaching death even at 2ft 13,750)
    or
    3.67wats per sqft

    Do you see my quandry? if i tailor my lights to one of the rules it negates the other 2 'rules'.....or wait. is this really telling me that i ABSOLUTLY HAVE to have my light 2.5ft above my canopy?
    55,000/(2.5*2.5) = 8800lumens per sq (inverse square law)

    EDIT: i know many people have their lights much closer than that.

    i know it rambles a bit but hopefully one of you math type people can make sense of what i am asking.

    Thank You for your time and patience.

    Cyrate
     
  2. #2 Wasabe, Jan 31, 2010
    Going with experience (my own and others') and foregoing the math, 400w in a 2x3 space is just fine. And, you can put the lights as close to the tops of the plants as you want, provided the temp at the canopy is the correct number.
     
  3. #3 Cyrate, Jan 31, 2010
    yeah i have seen many journals with more lumens in less space. so i know i can get away with it, but to quote -Mu in The best stealth lighting system? ... PL FTW! thread .


    ""Cannabis needs lots of light, around 2,000-7,000 lumen per square foot. Below 1,000 lumen/ft², plants cannot function correctly, cannabis less so; above 7,500 lumen/ft², plants will make extraordinary, usually unmanageable, demands on other environmental factors and generally bleach out and suffer. 3,000-6,000 is optimum.""

    ..this is the heart of my question. isnt my space to small for that many lumens if the above holds true?

    not that i dont believe you or the countless ppl with 400watts in 6sqft. not that at all. im just just confused.

    Cyrate
     
  4. #4 LBH, Jan 31, 2010
    The light rule is easy

    400w=4x4 area

    600w= 6x6

    1000w=10x10


    A space that small though, I would probably run a 250 or a 150.
     
  5. #5 Cyrate, Feb 2, 2010
    Last edited by a moderator: Feb 2, 2010
    Ty LBH, i have been considering the drop to 250w for the last few hours, and will probably continue to think about it until i order my ballast. i saw the guideline you posted somewhere throughout my research. Unfortunatly it doesnt really help me understand the solution. if you look at the lumens per square foot in that chart:

    (i added the 250w to my space for reference)

    250w: 2x3=6 /28,500 =4750 Lumens sqft

    400w: 4x4=16/50,000 =3125

    600w: 6x6=36/92,000 =2555.5

    1000w: 10x10=100/140,000= 1400


    that is the difference between a good spring day and completely overcast.
    without factoring in Falloff or Reflector Efficiency.

    i forgot about Reflector Efficiency!! damn ok,
    does anyone know the efficiency of a CoolTube? lets assume 90% for sake.
    2*3=6/50000/90%= 7500. (full sunlight in summer is 10,000)

    hmm thats Alot more tolerable. And i really dont want my lighting to be my weak link.
    Cyrate.


    i am still considering the step down to 250w. you are most likely correct (swhy i asked your opinion) but if trichs are formed to protect from UV/light i sure dont want to baby my girls in that department.
    thx again.
     
  6. #6 LBH, Feb 2, 2010
    The lumens here are irrelevant. Those will come into play when you decide which size u want, then u go find the one that fits your needs and has the most lumens. ( a shopping criteria)

    With lights, (in a grow that is small enough to only need 1) its more about heat. By all means, use the 400 if you can move the heat, even the 600 if you can dissipate the heat.

    Think of it this way,....you can't have too bright of a light, these plants really want the natural sun which is faaaaaaar brighter than any bulb so get the biggest light that you can while still keeping temps in the 65-75 optimum range

    As long as you get an hps light, it won't be ur weak link ;)
     
  7. #7 Norcali, Feb 2, 2010
    Id just like to add a little tid bit from my personal experience thus far in here, im in a 2x4x6 grow space with a 250w HID, and a pretty god damn strong blower, no cool tube though, and heat is still around 80-82*F. For that small of a space, I would go with the 250w or 400w if the cooltube really has a huge impact on heat (I have no expereince with one, but LBH or someone can comment more. LBH ur the shit btw.)

    Hope that helps give you a realistic idea of what you will be dealing with! Good luck to yah :D and keep us updated.
     
  8. #8 Cyrate, Feb 2, 2010
    Thx guys, deffinatly some more food for thought. the room the cab is in stays pretty chilly 70f on most days. and if this thermodynamic equation thing is right, i SHOULD be able to keep it 78f in there. tho i know... math lies. :) or just misunderstood. either way thank you :)


    Norcal, it is hard to see from that picture, but it looks like your carbon scrubber is venting into your grow space. and how much CFM do you have in ventilation? input/output? length of duct etc?
     
  9. #9 kerenski, Feb 2, 2010
    Last edited by a moderator: Feb 2, 2010
    a cooltube would definitely be recommendable in a situation like this IMO...
    i run a 600w hps in a tube and i can hang it about 1.3ft above the tops (i'd say 2.5ft min without one), but the tube only takes care of the heat the bulb creates, the heat from the rays is another matter...
    good luck
    great input as always LBH... :hello:
    just feel the need to senselessly nitpick at one point:p
    it is theoretically possible to give even mj too much light... that would happen at intensities above 90,000 lumens (noon on a hot, clear day close to equator), which can cause "solarisation", whereby chlorophyll is destroyed and growth is severely stunted...
    however it's pretty much impossible to even get close to those intensities without seriously overdoing things... :D

    so as long as you can handle the temps, more watts=more bud :smoke:
     
  10. #10 Norcali, Feb 3, 2010
    Last edited by a moderator: Feb 3, 2010
    That pic of the room was when it was incomplete, at least the ventilation part. Its an 190cfm blower (so much more than I need) exhausted to under the door, the scrub isnt attached yet as it really isnt needed and with the strains or this early, probably wont need it until well into flowering. The "intake" is just the negative air pressure created by the heavy exhaust. Works well for my grow space, but if you were going to go bigger Id get a legit intake. The ducting I used was a mere 10ft. Also not shown in there is a 12'' fan blowing on the girls, you will want one of those too.
     
  11. #11 Cyrate, Feb 3, 2010
    Last edited by a moderator: Feb 3, 2010
    nice. ill have 400cfm thru a cool tube. 2ft of duct at the very most. 1 slow 90 bend. (maybe) i may just straight duct it from end to end. but that would take reworking the passive intake box.
    you give me hope :). Check out Mechs DIY cooltube. :) probably shave 5deg easily off of your space.


    Krenski!!!

    thx man, thats what my original concern was about. to many lumens per square foot. I know this now to be a misnomer. its actual 'foot-candle power', or intensity that had me confused. not the bulbs total luminosity.

    aint MJ great. i have learned so much in chase of a dream.

    Cyrate.
     
  12. #12 mochadog, Feb 12, 2010
    1000 watt for 10x 10??? I see most people doing 4 1000k on that same space. More light the better denser nugs you get. It's better to have more lights then less, and don't buy cheap crap bulbs either. Buck up and send the $100 for a good bulb
     

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