Newbie lighting lumens per square foot question

wow...the blind leading the blind.

surprised that NO ONE here mentioned that lumens actually don't mean anything when growing plants. try to google the definition of a lumen. it's a measurement of visible light to the human eye. plants don't have eyes.... you should be more concerned with PAR value, footcandles (fc) and of course spectrum. plus that chart that someone posted is dogshit and the numbers are WAY off. your plants want 5,000-10,000fc. anything over 14,000fc and you are actually harming your plants and risk light bleaching late in flower.

somepotname said:

On a clear day the sun gives 100,000 lumens per square meter on average at sea level.

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Which is 30,000 lumens per sq/ft....what the thread was talking about....anything over 8000 or so for 12 hours is asking for trouble....heat issues, light bleaching, and deficiency...I run air cooled t5s at about 7000-7500 lumens per sq/ft and see nice results.....just my 2 cents

dont want to double post:


https://www.rollitup.org/indoor-growing/64064-anyone-know-how-many-foot.html

PROF XAVIER said:

Which is 30,000 lumens per sq/ft....what the thread was talking about....anything over 8000 or so for 12 hours is asking for trouble....heat issues, light bleaching, and deficiency...I run air cooled t5s at about 7000-7500 lumens per sq/ft and see nice results.....just my 2 cents

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How do you get t5s to output 7000-7500 per sq/ft?

Disregard my last question. What I would like to know is: I've been crunching numbers. All my lights are 4 ft long t5 6500k bulbs. I have 25 plants in veg as of now. I'm wondering if 4 bulbs at about 3 inches away from the plants would be enough for now?

I have 2 light fixtures over them that are each about 4ft long, and 8 inches wide maybe. Each bulb outputs about 2700 lumens. So all 4 bulbs together output about 10.8k lumens or so, correct? Assume the area is that of the size of light fixtures because they're directly above the plants.


4 ft x 1.5 ft of plants, all in vegetative. I have 4 bulbs on them, each 32 watt, 2750 lumens 6500k temp. Should that be enough? And for how long?

babygro said:

Okay. 53,000 lumens divided by 2 foot squared (53,000/2x2) which is 13,250 lumens at the plant tops.

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thanks for this formula

samljer said:

Actually his math was a little off as well.

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Slipon said:

may add that I keep my HPS 16 inch`s away from my tops ... 18-22 inches to most tops though ..
and I had 6-7K foot candles at my very top .. most tops had around 5K .. bottom growth had around 2-3K

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dont have that device yet. but thanks for the link slipon. i know this is an old thread. im new here and just lurking around
learning many thing from the site before i start my first grow this year.

this is to shed more light on Inverse sqr law..
i tried to compute the lumens output of a hid just like the one you posted.

i noctice from the post
footcandles = lumen/10.75
lumen = fc*10.75
12inch or 1ft distance of 600hps is 92000 lumens or 8.52k footcandles
20inch or 1.66ft distance of 600hps is 55200 lumens or around 5k footcandles based in Slipon Post

Formula i think in inverse sqr law for hid is:
lumens based on distance = HID lumens*(12 inch / distance in inch)
cause in 12 inch or 1ft the rating of hid doesnt drop based on light tester i think

eg1:
HID 400w = 50000
Distance in inch on light test to HID = 12 inch
lumens based on distance = 50000*(12/12)
lumens based on distance = 50000*(1)
lumens based on distance = 50000 lumens on that distance of 1 feet or 12 inch with the area of 1sqd.f or 144sqd.inch illuminated light

eg2: based on siphon light meter
HID 600w = 92000
Distance in inch on light test to HID = 20 inch
lumens based on distance = 92000*(12/20)]
lumens based on distance = 92000*(0.60)
lumens based on distance = 55200 lumens or 5134 fc on that distance of 1.66 feet or 20 inch with the area of 2.77sq.f or 400sq.inch illuminated light

the further the light the less lumens/fc and greater area if illumination[/CODE]


IF my assumptions are right the he uses 600w hps and a reflector hood then this is right
Inverse sqr law doesnt apply on reflected light
we can all estimate the lumens with just a ruler.

this are base on my estimation if reflector is present and play a big part in lumens concentration on a focal point

this is a sheet of estimation