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Myth busters its the light

Discussion in 'Indoor Growing' started by desertrat, Jul 16, 2010.


    desertrat Well-Known Member

    What do lst, topping, fimming, top and prune, scrog, sog, stadium, vertical, and all similar indoor training techniques have in common?

    Why do some growers seem to do great with these techniques and others struggle to make them work?

    Are the lighting recommendations of 7,000 to 10,000 lumens per square foot as a target, and 3,000 lumens per square foot as a minimum, good guidelines?

    What growing tool do you not have that is critical to the success of your indoor grow?

    First Myth – we can replicate the sun’s intensity with high intensity lighting. No, we can’t. light decreases rapidly (by the inverse square of distance) as it moves away from the source. the sun is approximately 93 million miles away from the plant it’s shining on and it’s intensity is the same whether at the top of the plant or the very bottom because the percentage change in distance from the sun is so small. Indoors, a totally different story where my 600 watt hps light provides 10,000 lumens per square foot at 21 inches from the light but only 2,000 lumens per square foot at 29 inches.

    Let’s see how this plays out in practice:

    and this is how your plant experiences the light:


    as you can see, only a very small portion of your indoor plant is getting sufficient light for growth.

    Compare this to what your plant sees outdoors:


    the numbers seem similar but the difference in amount of light received is great. To highlight the difference I calculated a new number, lumens per cubic inch, to represent the three dimensional effect of light – the plants are three dimensional so using a three dimensional measure only makes sense.

    I’ll save the detailed calculations for a later post, but the end result is that the sun in my desert climate provides 50 lumens per cubic inch whereas my 600 watt light provides only 12.9 lumens per cubic inch on average over the whole plant. the sun provides almost four times the usable light compared to a 600 watt hps light and I would argue that along with light frequencies probably represents the entire difference between indoor and outdoor grow results.

    Second Myth – you should use an air-cooled light to get as close to the plants as you can without burning the leaves. There still is such a thing as too much of a good thing, and you can have so much light that it reduces the ability of the plant to absorb light. Generally speaking, you should be able to keep your light at the proper distance without air-cooling your light. It can still make sense to air-cool your light as an efficient way to help cool the whole room. Edit - if, as it appears likely, the optimal amount of light is greater than 10,000 lpsf then an air-cooled light is likely to be needed.

    Third Myth – the popular plant training techniques have some major benefit other than shaping a plant to most efficiently use indoor lighting. Maybe they do, but it’s hard to argue that each and every method ends up with more growth tips in a narrow band of vertical space. Maybe the key difference between techniques is the least damage done to the plant to create the desired shape??

    Let’s look at each method:

    1. topping – the benefit of topping is that you get more than one main cola. But that’s just another way of saying you have increased the number of growth tips at the top of the plant and all at roughly the same height. The roughly is important, because the horizontal difference in distance can mean that one cola directly under the bulb is at 70 lumens per cubic inch while a cola at the same height at the other end of the hood is at 21 lumens per cubic inch. I would expect topping to increase yields even further if you managed the top colas to be the same distance from the bulb instead of the same height from the ground.

    2. Scrog and sog – these techniques are inarguably designed to keep the tops of the plants at the same height. Again, I would expect yields to increase even further if you managed the plants to be the same distance from the bulb instead of the same distance from the ground.

    3. Stadium and Vertical – once again these techniques explicitly manage the plant canopy to keep as close to the same distance from the light as possible. In these cases you’re actually measuring distance from the bulb so they should be easier to make more efficient use of light than the other methods, all other things equal.

    Fourth Myth – you can get good results from these training techniques without careful attention to the distance from the light. No, you can’t. you have wasted all of your efforts to train the plants if you fail to keep the growth tips in the narrow window of optimal light (in the case of the 600 watt light, that’s 21 to 24 inches from the bulb).

    Fifth Myth – 7,000 to 10,000 lumens per square foot (50 to 70 lumens per cubic inch) is the optimal amount of light. Maybe it is, and maybe it’s not. I’ve seen no empirical data to support that claim. Let’s take my calibrated light meter on a search for the optimal light. I’m going to be running a series of experiments to test the light sensitivity of plants and will post the results here. Stay tuned.

    And if you haven’t guessed it already, a light meter with a reading up to 20,000 foot-candles (lumens per square foot) is an invaluable tool for good indoor growing, although I suspect experienced growers have learned to judge their light without the aid of a meter. Please be aware if purchasing a meter there is a wide range of quality and to choose carefully. I am not willing to endorse anything short of a nist certified meter.

    riddleme Well-Known Member

    Awesome post my friend, been yelling for a long time kudos and scribed, +rep if it lets me
    desertrat likes this.

    Murfy Well-Known Member

    i need a light meter-

    income comin soon!

    rzza Well-Known Member

    i was just playing with my light meter today actually. outside i was shocked at how low it read compared to inside my tent.

    under the 400hps the needle was maxed out until dropped to about 14 inches, under the 1000ps it didnt drop from the max till like 20 or more inches.
    outside without the sun put it like 3/4of the way an wth the sn maxed it out aywhere i stood

    desertrat Well-Known Member

    Sounds like a 10,000 foot-candle meter and I'm going to jump out on a limb here and bet that our plants can use more light than that, how much more is an open question.

    rzza Well-Known Member

    some cheap thing ...

    Attached Files:

    • LM.jpg
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    riddleme Well-Known Member

    I had one of those, think it only goes to like 3000 lumens, maybe 5000

    desertrat Well-Known Member

    thanks, if you nail the temperature/light frequency questions in your balls to the wall thread then with this I think we've got most of the difference between indoor and outdoor grows covered.
    grasshoper031 likes this.

    desertrat Well-Known Member

    Not much good at all for growing needs - just a teaser
    Jer La Mota

    Jer La Mota Well-Known Member

    Very informative !

    xivex Active Member


    desertrat Well-Known Member

    useless - it only goes up to 4,000 fc (foot-candles=lumens per square foot)

    xivex Active Member

    Damn! Wow, thanks for saving me from buying that! :) Have any other suggestion for a good meter?

    EDIT -- Found this: http://www.tequipment.net/ExtechEA30.asp?Source=Google
    Up to 40,000 FC! Sounds great, but damn its expensive. $250 :(

    desertrat Well-Known Member

    Maybe I should buy some stock in a light meter company although somebody could go through the trouble of measuring the key distances for each kind/wattage of light and post a table.

    riddleme Well-Known Member

    My little meter was only $29 and goes up to 10,000 lumens (foot candles) works for what I do, though admittedly Deserts is a beter meter


    desertrat Well-Known Member

    We can calibrate your meter against mine with identical lights and a couple of rulers. We'd at least know if it was better than +\- 25%, if so then that's a great price.

    riddleme Well-Known Member

    It claims a 2% margin of error, I was gonna do some pics of different CFL's with it this weekend so that might work to check it against yours

    desertrat Well-Known Member

    that'll work

    sixstring2112 Well-Known Member

    Nice thread, i would like to know what the meter says at 12" for a 600 eye super hps .i try to keep my canopy from 12" to 30" from my lights. most everything below 30" gets cut off, too fluffy for me.also would your numbers be different if you put the meter under 4 600 hps right in the middle?I would think your lumens per sq ft would be way different.another good test would be to take the meter outside and cover it with one big fan leave for a reading. then do it under a hps.i gotta get me a light meter, you just cost me money;) I'm baked.

    riddleme Well-Known Member

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