Clarification of Lumens rating and Inverse Square Law

[kaeden]

Hey everybody.

Just want to say that so far I have loved being here and learning so much! I've settled on the design components and elements for my grow which I hope will be rather awesome and I am currently in the planning stages.

What I wish to enquire about is the specifics about the Lumens that a bulb is rated at. I have recieved conflicting information that it is the rating of the bulb 1ft from source (Footcandles?) or that it is the base Lumens produced by the bulb at source. Can someone please clarify this for me? I have searched and as I said found too much conflicting information.

Second of all, I am fairly certain but just wish to clarify. The amount of Lumens being received at the source is equal to

Lumens / (4 x Pie x Distance from source ^2)

If someone could just confirm that for me, thank you. Tedious I understand and I am fairly certain I am correct but my uncertainty about the Lumens rating on bulbs has me concerned.

Thanks, Kaeden

 

[jm420]

plants dont see lumens people do

 

[jm420]

The right spectrum and watts per sq foot are more important <IMO

 

[kaeden]

Not that I am not grateful but you didn't answer the question.
I refrained from mentioning this but due to the small space of my proposed grow I am going to be using CFLs. Also what you said is only in part true, plants will absorb the photonic energy emitted by a source providing it is of the right spectrum as you said. However it has also been said that this is limited at ~7500 Lumens.
So plants see light, lumens is merely a measure of that light. All I would like is clarifying answers for the questions I asked above.
Regards, K

 

[Birdo]

For your first question, I have no idea what the industry standards are when it comes to measuring lumens. My advice would be to call a bulb company and ask how they measure the lumens of their bulbs.

Here's something from Jorge's book (I know, I know, not the most comprehensive/detailed/accurate gardening book in the world, but it does have some good stuff concerning electricity that is absent from most grow books) that might help. This is probably just a general guide, I imagine individual bulbs may vary slightly. The following is all measured at a distance of a foot.

1000 watt HPS-140000 lumens
1000 watt MH-115000
600 watt HPS-90000
400 watt HPS-50000
400 watt MH-40000

I'm no expert but my basic understanding is that the inverse square law means that light intensity doubles for every six inches the bulb is to the canopy.

I know from experience the closer the light, the bigger/better the plants. I think light intensity is just as important as watt per square foot.

 

[Birdo]

Sorry, I was typing a reply when you posted that last bit there, I didn't see it...What is the size of the space you plan on using? I would suggest trying to find a space large enough to accomodate a 250 HPS, in my experience CFL's weren't the best.

 

[kaeden]

This is why I didn't originally post the information pertaining to my bulb type. I can assure you that CFLs are more than sufficient for what I need to do and a HPS/MH would generate far too much heat and require too much space. This is for a Micro Perpetual SOG.

Anyway your understanding of the Inverse square law if wrong, sorry. Light intensity halves it doesn't double, other wise the further we took our hand away from a bulb the hotter it would get? XD

Watt per square foot seems to be a useless measurement seeing as bulbs can drain as many watts as they wish but still only utilise a portion of them in converting them to photonic energy.

Thank you for at least answering my first question in part though.

 

[Birdo]

light intensity doubles for every six inches the bulb is to the canopy.

My bad bro, I should have put the word closer after six inches...I thought it was easily inferred I was refering to the intensity of the light doubling by being closer, as I would have used the word "from" the light to describe it being away from the canopy. ex

light intensity doubles for every six inches the bulb is FROM the canopy.

I apologize for the idiosyncratic nature of my speech, I understand how that could be confusing, but for you to imply that I don't understand in the terms you did

other wise the further we took our hand away from a bulb the hotter it would get? XD

is somewhat insulting to me. Even if you did misinterpret what I had originally said about the inverse square law, when you read

I know from experience the closer the light, the bigger/better the plants. I think light intensity is just as important as watt per square foot.

it should have been pretty clear that I understand that light gets brighter the closer it is to something.

So, we both agree light intensity double for every six inches it is closer to something, and halves for every six inches it is farther away from something.

I also understand what your saying when it comes to watts being a useless measure, when you're say, comparing incandescent bulbs to HIDs, but given that most of us are in the HID paradigm, it is nice to have standards to describe to each other the ideal ratios for watts/sq ft/lumens.

You say watts per square foot is useless...jm420 says lumens are useless. Both are as equally important as the other, as is the spectrum.

I think light intensity is just as important as watt per square foot.

Ex. If we take an air cooled 1000w HID and put it over a 5X4 space at a height of 3 feet off the canopy we will have 50 watts per square foot at 15000 lumens (the goal being at least 10000). This will give a person a decent harvest.

Now take the same 1000 watt light, and put it over a 1 X 1 or a 10 X 10 garden, and neither one will yield anything like the first example. Take that original 5 X 4 garden and lift the light 10 feet off the canopy and then see what happens. Or use 1000 watts of incandescent or fluorescent or LED light. I realize these examples are extreme and unrealistic, but I feel they demonstrate that it's the balance of the ratios of space and light intensity that optimize a garden.

I won't try to dissuade you from using CFL's, I have seen some alright plants grown using them, but the yield is quite small compared to the big lights. When I suggested the 250 HPS I forgot to put air cooled, that might be something you want to look into, those things are life savers.

I suggest using a few 100 watt CFLs, a trip to the indoor gardening center might be needed for that. I know from experience that Home Depot sells a nice 45 watt daylight CFL that works well.

I recently finished up experimenting using 180 watts of LED light with 135 watts of those CFL's I mentioned. I would only do it again if space/heat was a factor so I can respect your decision to go down that route if that is the case for you.

One other thing, if you are using CFLs the inverse square law won't matter much, you will want your bulbs with a few inches of the plants, certainly no more than 6.

All I would like is clarifying answers for the questions I asked above.

Sorry bra, I don't mean to knock your thread off course, I'll stay out of it after this. Sorry if I came off like a prick, no disrespect intended, I admire you for what your trying to do, and I hope everything works out well for you. Peace, B.

 

[rick shaw]

Here's the formula for actual lumens reaching your plants. Intensity = light output divided by distance squared. I=L/D-2.The output of a 400 watt HID is 50,000 lumens @ 1 foot, 2 feet away 12,500 lumens, 3 feet= 5,555 lumens, 4 feet= 3,571 lumens. Watt's per square foot of canopy is another way aim for at least 80 watts per square foot,With A/C hoods you can park them right on top.