# 1000w lumens by square foot (inverse law of dispersion)



## Tact (Nov 21, 2009)

I am trying to figure out some math regarding my grow area. The room in question is 6' wide/8' long/6' tall, though the light hanging down would make the height from the bulb to the floor more like 5.5' tall., though this is irrelevant to the space the canopy can occupy in order to achieve the requisite lumens, please keep reading.

I have a 1000w light, the MH bulb gives off 110,000 lumens, the HPS bulb gives off 140,000 lumens. 

*Plants would not be on the entire floor space of this grow area, they would be in an area under the light that is guaranteed to get the required lumens for their respective phases. Therefore what is being calculated is the CANOPY area within the grow room, not the grow room area itself. *

There would be two stages for lumen requirements, vegetative and then flowering. Assuming I need 2,500 lumen per square foot for vegetative and 10,000 lumens per square foot for flowering, is this math or formulaic approach correct?

First the approach for flowering, as this is more demanding then vegetative.

For HPS flowering phase (base requirement of 10,000 lumen p/sqf):

_Putting the light at 3.75' feet from the plants (45" inches above the canopy)
_
*140,000 / 3.75'x3.75' = 996 lumens (per square foot)*

This would be illuminating an area of 14 feet under the light (3.75'x3.75'), in what I assume would be the shape of the hood that is reflecting the light, which is rectangular more then square shaped. So an area 2.8' wide by 5' long (2.8'x5' = 14'), would be where the plants should rest under the hood. This being more efficient than a simple square arrangements of 3.75'^2, which would have parts of the plants going out of this 'sun spot' that the lamp is simulating.

For MH vegetative phase (base requirement of 2,500 lumen p/sqf):

_Putting the light at 5' feet from the plants (60" inches above the canopy)_
*
110,000 / 5'x5' = 4,400 lumens (per square foot)*

Even getting the light as high as it can go practically, will result in almost double the required amount of lumens for the vegetative phase. Literally I can't get the light to go any higher then this due to the lower basement ceiling height. This would cover a much greater area of 25 feet, or (4' long by 6' wide) for the more efficient rectangular shape.  

Was this approach correct mathematically or am I missing something? *

Having setup my approach, these are my questions:

If this math is correct, then during the flowering phase should an interior partition of Panda/reflective material be put up on the perimeter of the maximum 'sun spot' effected area of 2.8' feet wide by 5' feet long, which would serve to keep all the light within this area bouncing around? 

Or should I simply take note that this is the area that light will get reach the most effectively, and there is no need for an internal partition around this perimeter, only to arrange the pots accordingly within this area? 

Around the 6' x 8' grow room itself Panda reflective material is up already.

And finally, how many plants, and in what pot sizes could I fit in the 14 foot illuminated area during the flowering phase by just TOPPING the plants? 

I understand SOG/SCROG/Lolipopping would increase the usable area dramatically, but just with the topping approach how many plants do you think I could fit in this area, if indeed this math is correct?

Thanks for any answers 
*


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## ozzydiodude (Nov 21, 2009)

Hey Tact You are figuring your square footage wrong A room 6' x 8' has 48 square feet(ft2) 2- 1000watt lights with 110,000 and 140,000 lumens would give you 2292 lumens/ft2 for the 110,000 lumens light and 2916 lumens/ft2 for the 140000. In a area thix big you would need to run both lights for enough lumens/ft2.

Most of us try to use 3000 lumen/ft2 for veg and 5000lumens/ft2 for flowering


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## nouvellechef (Nov 21, 2009)

What he said. I need a toke after reading thru that.


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## Tact (Nov 21, 2009)

I think you got confused with what I initially wrote, I shouldn't have said the square footage of the entire space, that is more relevant to ventilation. Also I do not have two lights, but one, with an MH bulb and an HPS bulb, that I would switch depending on the phase.

However, I do not plan on using the light across the entire grow area, like I said in order to reach the lumen requirements I set for: 1) vegetative and 2) flowering, I determined the focused space under the light needed to reach those lumens.

Re-read my post maybe? By keeping the plants under the allotted area I described I would be getting the required lumens I am shooting for... I am not putting the plants across the entire grow room floor space which is indeed 48 feet.

*Again: The most critical point of my post is that the plants are not going on the entire floor space of the grow area, the calculated area is the square footage of the CANOPY maximum that would give the required lumens needed. There was also a number of other questions in the thread..*

One other thing, the relative lumens are quickly degenerated in an inverse dispersion fashion, 140,000 is the lumens at 12", or one foot. When that area double to two feet or 24", the lumens are effectively cut in half due to the inverse dispersion. Just something to point out, as lumens are relative in their original number only at one foot, beyond that you need to start doing some math to figure your actual lumens once they reach your desired location.


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## ozzydiodude (Nov 21, 2009)

You will have to inclose the area under the light so that no lumens escape,
Square footage is for figuring light and cubic footage is used for figuring what is needed for ventilation.

The farther away for the plants the more lumens you loose.


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## Tact (Nov 21, 2009)

Taking your example, and checking the math:

*Not that this is at all relevant to my question but:*_ 

140,000 lumens, from a height of 3 feet, over a 48 square foot area, would be 140,000 / 3x3 / 48 = 324 lumens, 

where as from a height of one foot, the same light over the same 48 square foot area would result in: 140,000 / 1x1 / 48 = 2916 lumens

This is the inverse dispersion, or at least how I understood it, and what prompted me to confirm my question in this forum._

But again, I am not using the the whole 48 square feet floor space, the area designated in my original post is the square footage that the canopy will reside in, not the entire grow room itself.


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## nouvellechef (Nov 21, 2009)

Cubic ftage is more releveant to ventialtion.

Cover walls and ceiling with Panda film, keep it tight

5gal pots, soil or Hydro/DWC/etc. Amount of plants depends on you. Look at my current grow, i have 40 in a 10x12

Those are my thoughts on it.


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## Tact (Nov 21, 2009)

Panda is already on the walls, I will get some on the ceiling though. I am looking at your grow journal now.


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## nouvellechef (Nov 21, 2009)

All i know is i listened to what the peeps said on here, 3k veg, 5k flower. And I had huge dense nugs last time, but not quality genetics. This go around will only make it a 1 grow a year. Mom and dad, grandma/grampa, sis, etc are happy.


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## BBFan (Nov 21, 2009)

Tact said:
			
		

> Taking your example, and checking the math:
> 
> *Not that this is at all relevant to my question but:*
> 
> ...


 
You are correct Tact. Mathmatically speaking, based on the parameters you mention.

Based on the following assumptions:

48 square feet of canopy
A lamp outputting 140,000 lumens
Positioned 3 feet above the canopy

Equals 324 lumens per square foot.


That being said, you are not intending on a 48 square foot canopy. With the proper ventilation you can get the bulb much closer to your canopy. Through the use of panda film as you mention and an effective reflector you will dramatically improve the lumens hitting your plants.

Don't overthink it too much my friend. You should be more concerned with the quality of lumens hitting the plant than with the quantity. If you want to really analyze something, concern yourself more with spectral output.


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## legalize_freedom (Nov 21, 2009)

well said BBfan!


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## Tact (Nov 21, 2009)

BBFan said:
			
		

> You are correct Tact. Mathmatically speaking, based on the parameters you mention.
> 
> Based on the following assumptions:
> 
> ...



Just to reiterate again, the 48 square feet of the room is NOT the same as the canopy square foot.

The canopy size is what I am trying to *calculate*, the canopy size is going to be determined by the math that dictates what the ideal canopy size is, due to lumen dispersion. So I was trying to seek confirmation of my canopy size, which was calculated based on the math above. The 48 square foot of the grow room is really , and truly, irrelevant, 48 square foot, 1000 square feet, the ideal canopy size would be the same if the light and the height of the light remained constant.

So moving on that you confirmed the math, how about the spacing in between each plant for the designated 3'x8' space, how many plants can fit in there in a healthy manner using just topping? I am about to germ seeds for my first grow, 12 in total (10 White Russian, 1 fem. L.A Woman, 1 fem. Lemon Skunk). 

I will look into spectrum, I got the bulbs from BGHydro.com with my light, I just know the HPS is 140k lumen, the MH 110k lumen, both different brands. I was under the impression MH is more ideal for veg stage, and HPS more ideal for flower, due to blue (MH) / red (hps) and how MJ reacts to those spectrums depending on the phase? No idea really, this is why I post.

Thanks for the responses thus far.


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## leafminer (Nov 21, 2009)

"Inverse dispersion" - sorry, but you don't seem to understand what is going on here. The vertical distance doesn't come into it. Quite simply, the amount of lumens is the total lamp output divided by the area illuminated. 
Photons of a given frequency have the same energy whether they are 1mm or 100 light years from the source. That's a fundamental law of physics and accounts for the fact that starlight reaches us from incredible distances.

What I see far too often here on MP is grow room photos where I can see lamps being used without reflectors! That's about the dumbest thing you can do. IMO intelligent use of reflector boards can keep almost all the lumens on the plants, and those reflectors should be as efficient as possible, which means Mylar. I use reflector boards of 1" white polystyrene faced with Mylar. Some are single boards and I have one of three joined with duct tape like a Chinese screen. It is quick and easy for me to reconfigure the grow room area according to how many plants I have. For instance right now (following selection of fems) I only have two plants in the grow room and they are completely walled with Mylar-faced reflectors to keep almost all the energy on them.


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## leafminer (Nov 21, 2009)

Tact, to answer your question as to how many can fit: 
Six plants. 24 sq.ft, 4 sq ft each. My plants usually are about 2ft x 2ft going into flower. Hell, each fan leaf alone is at least 9" across not including the 3" stems on them. You could probably fit 8 plants in but they would be crowding each other and yield probably would be about the same overall.
So, using 5KL/ft. we get 120K lumens. So your HPS is good for that. Your MH also, just about. I am currently running about 9,800 lumens/sq.ft on mine.They love it.

Edit: to get my fig. of 4 sq ft per plant I am assuming a square canopy. If it were a circle it would be 3.14 sq ft, but in practice you can't use the little missiing corner parts, so I use 4 sq ft. That's for my 3ft tall plants. (Height when starting flower)
 Of course if you are growing miniature 1 ft plants then it will be different.


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## Tact (Nov 21, 2009)

_Maybe I am wrong about a number of things here in regards to light/physics, but if I got my ideal canopy size of *(3'x5') right(?)*, that is all I was looking for._



			
				leafminer said:
			
		

> Quite simply, the amount of lumens is the total lamp output divided by the area illuminated.



That is exactly what I am calculating...

The area illuminated is *increased* as the light is *raised* in height, this disperses the same amount of light over a *greater* area. This calls for a division of the new area, by the (yes constant 140k lumens) output. The lumens do not change, you are correct, but the *area* that the light is hitting has changed.

And as far as not having a hood, I am not sure what you are talking about I have one of the biggest hoods I could find the Magnum XXL . Your comments about starlight has nothing to do with what I am saying. Quite simply, the output lumens remains constant, but if the area the lumens are illuminating goes beyond 1 foot under the light you need to plug that into the calculation, its not about height, and distance travel, as it is about the light being dispersed over a greater illumination area. When that number goes beyond 1 foot in height, you use the formula to multiple out the increased area, then divide it by the lumen output to see what the lumen spread over the new area becomes, this is the dispersion of light. You need to recalculate what that new* area under the light is going to truly receive in lumens as the area has increased without the lumen output increasing, this doesn't mean you can't *see* the light, or that the lumens have become less then 140k, simply they are more spread out! Otherwise everywhere on earth would receive the same amount of lumen output as the equator, and that is not the case is it, it might be sunny in Ecuador as well as the Canada, but Ecuador is most likely getting more lumens, no? 

If my math in regards to the ideal canopy size is correct, though understanding of light dispersion is flawed, then that is fine by me. I am using this information to grow weed in my basement, not put the Hubble replacement in orbit.

Six plants being the ideal amount works perfectly then, assuming I get lucky and have a 40-60% female ratio, that would give me 5-7 female plants. So that's nice.


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## nouvellechef (Nov 21, 2009)

Prob talking about my pic, I don't use hoods. Sounds like you have got your questions answered. Good luck and keep pics updated.


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## BBFan (Nov 21, 2009)

leafminer said:
			
		

> "Inverse dispersion" - sorry, but you don't seem to understand what is going on here. The vertical distance doesn't come into it. Quite simply, the amount of lumens is the total lamp output divided by the area illuminated.
> Photons of a given frequency have the same energy whether they are 1mm or 100 light years from the source. That's a fundamental law of physics and accounts for the fact that starlight reaches us from incredible distances.


 
Hey Leafminer! I always appreciate your approach to things and you and I agree on many subjects.

However, in this instance, Tact is indeed correct. The Inverse-Square Law does indeed apply in indoor gardening as far as luminance is concerned. Essentially, the further your light, the less the intensity.



			
				Tact said:
			
		

> The canopy size is what I am trying to *calculate*, the canopy size is going to be determined by the math that dictates what the ideal canopy size is, due to lumen dispersion. So I was trying to seek confirmation of my canopy size, which was calculated based on the math above. *The 48 square foot of the grow room is really , and truly, irrelevant, 48 square foot*, 1000 square feet, the ideal canopy size would be the same if the light and the height of the light remained constant.


 
Tact-

I get that the 48 square feet is irrelevant- but it is the number *you* used in your calculation.

Canopy size is absolutely, completely, 100% dependent upon...........

wait for it......

A whole lot of things! 

You can assume a certain germination percentage. You can assume a certain female/male ratio. You can assume minimum stress factors that may impact production. Experience will be your guide.

You have to determine what your canopy size will or can be- you are in control here.  You need to determine what your objectives are. You need to ask yourself these basic questions:

Are you looking for maximum yield in a specific area?

Are you looking for the shortest production time?

You can do a SOG or SCROG- both proven high producers.  You can veg for 12 weeks and train, train, train.

Good luck to you my friend.  Happy Growing!


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## dirtyolsouth (Nov 21, 2009)

HI bud,

Plant the seeds and keep the reflective walls as close as you can to your plants and the rest will take care of itself.  Go thru a grow to see what happens rather than try to guess what's going to happen from a theoretical computing approach.  All that really matters is getting max light to as much bud in your canopy as you can.  For a 1K HPS you can see on the light distance chart you have about 36" before you get out of the ideal penetration and max lumen range of your bulb.  

Since this is your first grow you may be more interested in flowering them with less veg time so that you get some buds and are able to choose a pheno or phenos for future grows.  But if you're patient you'd do well to veg close to 2 months and go for larger plants and more yield...  Decisions, decisions, decisions...

Peace!


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## DonJones (Nov 21, 2009)

Tact,

Hi man.  I see you got your room together.

On your light question, dirtyolsouth is correct; trust your instincts and learn from your attempts what works for you and don't worry about the rocket scientist approach.  I've seen some phenomenal grows where the grower seemed to defy every conventional wisdom and on the other hand I've seen some dismal failures where the grower did everything by the book.

Take your light, set it 18" from the floor, mark the edges of the bright spot, you'll be able to see what I mean, and that gives you the MAXIMUM canopy size and location regardless of the math.  That is the easiest and most dependable way that I know to figure it out.  Actually you might want to stay slightly smaller than the footprint because that will be the outer edge and won't allow for any angular issues to the outer edge of the canopy. 

If you really want to screw everything up from a math basis, go to Home Depot and buy four 3' warm white,(about 3500 to 3700 Kelvin temperature color) T5 floros, mount them horizontal around the outside of your canopy a little above the top of your pots with the reflectors pointed slightly above horizontal so you get the additional red spectrum light into the lower part of your canopy.  It is way more effective than adding additional light over head or even adding the reflective inner wall.  If you can find an economical source of HO (high output) T5 that is even better because they put put about twice as much light for the same sized tube.

Then you will have a lot less need to be fighting the inner wall thing.


Now on the question of the reflective inner short wall, remember you need AIR FLOW through the entire canopy at least as much as you need light penetration.  So keep your inner wall far enough away that you have GOOD air flow.  If you can get an oscillating fan set up in one corner of the grow area that will blow across the active canopy and somewhat gently cause the plants to move like in a breeze, that is probably the best.  If you really want to optimize the effect of the fan move it from one corner to next every day.

This is hard to do unless your reflective inner wall is no higher than your light and preferably lower.  Actually the real purpose of the inner barrier is NOT to capture the light as much as it is to redirect it onto the bottom buds.  So as long as you don't see significantly dimmer light at the bottom you are alright.  The way one old timer put it was, "Make sure that you can see distinct shadows on your hand all the way down, not just generalized dim light."

Depending upon the particular plant and how dense the canopy is, you may see light starvation in the lower parts of your plants without the inner wall, but I'm personally of the belief that your center plant should be about 2' from the wall.  That lets you get the indirect light on the bottom and still lets the air flow through the canopy.

Remember, we are growing weeds that have grown wild in most of the world for eons.  They are very hardy and you have to really screw up to seriously hurt them.

I believe strongly in KISS -- Keep it simple Stupid-- and that is NOT to say you are stupid, just that we have a tendency to analyze things to death instead of just getting a basic understand and then doing it.


Give us some pictures of what you are doing man.  It sounds like you're doing very well.

Good smoking!


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## Tact (Nov 22, 2009)

Roger that.

Resolved, thanks guys.


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