Lumens at Canopy (doing the maths).

Hi,

Long time lurker - in fact this might be my 1st post.

Anyway - I've gone virt and and love it, but it changes things, i wanted to understand the new relationship of light to the circular/spherical canopy area.

How do i reference the creator of these images?
They are from this Sticky by Hundred Gram Oz


So take a 400w bulb with 50,000 lumens (presuming measured at source by manufacture for rating purposes).

Take the equation for the surface area of a sphere: 4 x Pi x R^2

Surely then the lumens at a given point on the spherical canopy is = Total lumens/[4 x Pi x R^2]

I have read that a plant should get between 2000-3000 lumens, then I needed to know this formula/calculation to understand the distance I could have a plant from the light.

It took me quite a while to get my head around this info. Maybe I'm totally wrong or maybe it is because virt changes the area calculation and the use of reflectors for Horiz further changes things, but lets get to the math to make my point.

At 1 foot from the bulb (in any/all direction) we get:
250w = 28,000/[4 x 3.14 x 1^2] = 2230 lumens.
400w = 50,000/[12.56] = 3980 lumens.
600w = 90,000/[12.56] = 7100 lumens.

At 1.5 feet from the bulb we get:
250w = 28,000/[4 x 3.14 x 1.5^2] = 990
400w = 50,000/[28.26] = 1770
600w = 90,000/[28.26] = 3185

At 2 feet from the bulb we get:
250w = 28,000/[4 x 3.14 x 2^2] = 560
400w = 50,000/[50.24] = 995
600w = 90,000/[50.24] = 1790

So my point or realization is that what i had been using for understanding distance is no longer correct - even in a sticky here in the virt forum.


I now think that these figures must have come from tests with a reflector. I cannot see any other way the lumen output of a bulb can be higher at 4 inches from the bulb than the rated output of the bulb.

So take the math and do your own measurements and calculations. Stay above 2000 lumens at your max distance and i think everything will be ok.

I'm open to being corrected.

I cannot see any other way the lumen output of a bulb can be higher at 4 inches from the bulb than the rated output of the bulb.
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Click to expand...

You are correct it cannot.

But if you look at the second line on the chart it clearly states.

Foot/candles received from varying distance.

And your conversion is for foot/lumens from varying distance THIS is where YOUR discrepancy lies.

If you convert your lumen ratio per foot, to the candle ratio per foot ,I believe you will find that they are on par with the chart you see above.

Example. At 2 feet from the bulb we get: 400w = 50,000/[50.24] = 995 lumens( this is your calculation) or nearly 4000 candles( taken from the above chart).

lumens are calculated over m2
Candles are calculated over ft

Also your calculation is based on your light source being isotropic.

Which It isn't, You would first have to find out how much light is occluded by the lamps base, this can be up to 120 degrees then you would have to base your calculations on this measurement first.

BoB

You lost me with the spherical canopy. A bulb does not output equally in 3 dimensions.

i have found that some strains will tolerate closer distances while others will not (bleaching).
i don't think there is an exact science to growing at the moment.
for example... in most cases you will get more yield from more plants situated further from the light than less plants closer to it. 24 plants circling a lamp will give you more yield (most cases) than 12 even if the lesser amount is closer to the light source. if you get creative with a multi-level grow, then the yield will grow even more.
the best vertical grows always had large plant numbers creating an even canopy around the bulb. you may try to veg longer, but imo it is not as good as larger plant numbers.

so... think about it....
do these charts even make sense in your situation?
it's basically trial and error with a specific strain that will give you the best results. i have some bubba kush that is about 10 inches from a bare 1000w bulb. i have never had harder, tighter nugs before. but i do think if i spread out the circle, i could of added more plants and achieved larger yields with a minimal loss of "hardness."

i think it was said before.... canopy management is probably the most important variable with it comes to light. moving your plants to-and-away from the bulb is something no chart can really replicate. once you find your sweet-spot where the buds are hard and do not bleach/dry-out from the proximity to the bulb, you can add or subtract plants/veg time for an even canopy that will yield the best.

Charts are nice to look at... but they're not gonna grow your plants.

Read, read, and read some more... And then hang your lamp and put a plant or 64 next to it and apply what you've read. Some of it is bound to stick... The rest you can disregard.

Taking into consideration the increased square footage of the vertical setup in the chart... Shouldn't we recalculate the necessary watts per square foot? A 5x5 foot grow tent will thus have 58.75 square feet and at 50 watts per square foot will require 2937.5 watts to optimize light/plant... On all the vertical threads folks are running 600 - 1200 watts in such spaces with adequate results. Should afoaf put 2 1000 watters with a cool tube in the 5x5 or will that be overkill?

I would do just 600's or 750's BUT no cool tube......not how I roll...

Good luck but like Bobble say's, calculations vs reality are MILES apart...that's been my experience at least.

Very informative many thx

I've been pondering this this topic for a long time. In the last few weeks I've been doing some experiments with a cheap lux meter (yes the scale is dumb for grow lights, but if we are only comparing HPS output to other HPS output the scale doesn't matter).

I will put together a thread showing my method and giving all my data in a week or three, but for now here is a sneak peek. Each line represents the distance you get a given lux value from a bulb (each line is a different bulb ballast combination).