Light power vs distance

OK, I have a 20”x36” tent with 2- 100w. LED Quantum panels. I’ve found that @10” of headroom is well tolerated by my plants in late flower. I’ve been thinking about upgrading to a 300w. But won’t I have to leave more headroom (12-18”) which essentially gives a similar Daily Light Interval?

Hey @6stringT sounds like a good move. Try a bar type light that will provide a better spread and allow more headroom for your tent.
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Or a better choice but might need more headroom but I’ve used a 4000 at 12” during late flower.
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Thanks @LateNightGardner. But the question is, does the system I have come close to say, a 300w LED Q panel? Given that my tent is only 23”x36”.

22”x36” AND running at @10” headroom!

One QB260 or two QB135’s would do well in that space. It’s a far better light than the Mars Hydro.

Look at Horticulture Lighting Group as they are producing the top of the line currently.

Light distance/intensity is based on the inverse square law. Roughly; double the distance and 1/4 the light reaches your target.

  • Specifically, an inverse square law says that intensity equals the inverse of the square of the distance from the source.*

I’d have to say no. There lies the problem you’ll have with a quantum board. The ‘hot spot’ will be to powerful and possible burn your flower.
I was suggesting you look at an led bar type fixture. Same chips as your quantum board but they are spread out and provide a more even distribution of light across your canopy.

Typical 100w quantum board light map…

MARS HYDRO TSW 2000 Led Grow Light 300 Watt 4x4ft Coverage Full Spectrum Growing Lamps for Indoor Plants Dimmable Daisy Chain Seeding Veg Bloom Light for Hydroponics Greenhouse Indoor LED Grow

Or a bar led Samsung LM301B and OSRAM Red LED

More efficient spread over canopy. Your looking for ~1000PPFD. Or your wasting light unless you add CO2.

Thanks @Myfriendis410! Does someone know the answer to the question?? Will 2-100w ATREUM Q panels run at @10” give similar DLI to 1-300w run at @12-15” in such a small space?? I’m an old man on SS so I’ll get the answer BEFORE I spend the $.

Here’s the specifics on the ATREUM Q 100w quantum board.
ATREUM HYDRA-1000 100W LED Grow Light Full Spectrum - Samsung LM301B LED Diodes, Meanwell XLG Driver, Daisy Chain Dimmable Grow Lamp for Indoor Plants Veg Flower Hydroponic Greenhouse 2x2ft Coverage

I’d say YES, your good with those panels given your using a 20”x36” tent.

OK, thanks again @LateNightGardner. It looks as though I’m possibly getting @1000 micro moll/… at 10” at least in the center. I’ve topped the plants and pulled the 4or5 main colas out to the outside to make the canopy flat. The colas in the middle are getting the most powerful light and beginning to catch up with the main ones on the outside.
I started with 3 plants but removed one that was a good little Lemon auto to live in a black dark bag for a few days till chop time. The other 2 are photos still growing bud shoulders . Here is a pic from before the Lemon was removed.

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They are looking healthy to me @6stringT. You should have close to 1000 PPFD over most of the middle 3sq ft with overlap.
My only suggestion would be to get more air circulating lower from bottom to top of your canopy. That will move more CO2 through your plant and get more Oxygen to your roots via the fabric pots your using. Let the hot air up top rise and get pulled out with your exhaust fan with intake air from the bottom.

Thanks again@LateNightGardner. I’ll try moving the fan down. The RH is Low now 25-29%. I’m running a humidifier outside the tent with the zipper flap draped over it a bit.
With 3 plants in there it was a bit crowded making it hard to move the fan around. I’m thinking that near the vent flap at the bottom of the tent may be a good place to move it. I made a mistake and left the tent open with loads of bright hot lights on all night after @ 3-4weeks of flower. A toking genius move! They all looked real bad when I looked in late the next day. The temp was 89F. but I seem to have pulled them through. No hermies or naners. Thanks again LNG!

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They look good to me @6stringT !!
I’ve been fighting a heat wave in here but in TX. Just got a break in the heat and now I’m balancing out the cold. I’ve been using a small floor fan that fits perfectly in my 8” floor vent on my small tent. I’ve had this fan for 4 years now and it just keep ticking.
Honeywell HT-904 TurboForce Tabletop Air Circulator Fan, Small, White – Quiet Personal Fan for Home or Office, 3 Speeds and 90 Degree Pivoting Head

A lot of people misunderstand this, but your dli shouldn’t change much with light height. It may increase your ppfd reading directly below center of your light, but there should be an inverse to other parts of your grow space that keeps your ppfd average similar. You have to remember that ppfd is ppf per square meter, and changing the height of your light doesn’t put any more or less light into your space. Assuming light output is the same, your theoretical ppfd is the same.

There are some exceptions, like rate of reflection on different surfaces or if your light was so high that walls are eating up photons that will never make the canopy.

Thanks @dbrn32 for your input, got it. But headroom? I’ve topped and spread out, tying down the main larger colas to the outside. So at 8-10” headroom, the center, where shorter colas live, seem to be getting more intense light ppfd. I’m really trying to justify not spending the money on a new light, while getting the most out of the 2 100w Q panels on 2 plants in a 20x36” tent. Your input is welcome and respected. I love learning about this endeavor. Thanks again!

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Inverse square law? Dang friend I might have to dust off a physics book.

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I consider distance from the canopy a temp issue while PPFD is a rheostat adjustment issue.

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The headroom isn’t a concern of mine. I just wanted to make sure that you’re calculating dli properly if trying to use dli method. The dli of your space needs to be calculated on the ppfd average of your space.

Hey @LateNightGardner, I just found out that my 8-10” height may not be optimal. With the Hydra 1000, when moved from 12” up to 14” the readings drop only slightly in the middle of the affected area BUT, the outside corners actually increase. So, as I remember, at @ 14” the center area declines to only the mid 800s +, while the corners and periphery, pick up to the mid 6-700s. Since I spread the main colas out to the outside corners, I was loosing some energy where I really needed it. Great stuff to know. So up they go to @14” .

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They :eyes: good from the added attention. Keep pushing it @6stringT