Height/Power to canopy for Samsung Led’s

Can someone tell me where to begin to figure out how high to hang the lights and at what power.



I’ve just been guessing and experimenting and that’s probably not a good thing for a first gro

I’ve been going by the general 30w a sq ft and hang up between 12”-18”

Running samsung 561c strips
Area is 8 sq ft (2x4 tent)

Thoughts?

"I’ve been going by the general 30w a sq ft and hang up between 12”-18” is spot on. Looking at the photo it does look like your strips are higher than 18inches, but pics can look that way.

You really should stop making different threads asking similar things. You have a diary so would be best to keep it all in one place.

I see the friendly ones are out today

I posted this question in the Led specific forum just for this question. I really don’t think it’s unreasonable to ask here as I’m not getting advice in my journal.

On a positive vibe :
For those in the US happy Labor Day!

Height and power are related by the inverse square law. The closer you get the less power is needed. There are a couple ways to tell if your plants aren't getting enough light. If they are getting long and spindly and "reaching" for the light for one. Getting yourself a LUX or PAR meter is another. I find a LUX meter is cheap and good enough. It is easy enough to get an approximate conversion to PPFD from LUX.
https://www.420magazine.com/community/threads/ppfd-graphs-and-analysis.424174/
there are PPFD threads here as well, just the first i found.

Dont worry about how high the light should be, look at the plant and see if the leaves are cupping. In your photo they are cupping very nicely. It looks like you have enough light, by lowering the lights at this point, you will loose spread and probably have larger top nugs but less on the smaller nugs. Your distance is fine.

I think post #23 is good for those not familiar with lighting metrics and the math.
https://www.rollitup.org/t/slow-yellowish-growth-under-vero-29-3000k-3500k.962065/page-2

I believe @Randomblame later saw evidence that the cheap meters read low buy about 10% ?

1212ham said:

I believe @Randomblame later saw evidence that the cheap meters read low buy about 10% ?

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That may be accurate, or the conversion factor to PPFD is a bit off, my meter reads about 35000 lux during flower and if I crank the lights up higher than that I start running into issues. Even so at 10% that only puts me at 38500, if I set it to 45000-50000 I would flat out kill my plants. Still it's a good reference tool at a fraction of the price of a PAR meter..

canadian1969 said:

That may be accurate, or the conversion factor to PPFD is a bit off, my meter reads about 35000 lux during flower and if I crank the lights up higher than that I start running into issues. Even so at 10% that only puts me at 38500, if I set it to 45000-50000 I would flat out kill my plants. Still it's a good reference tool at a fraction of the price of a PAR meter..

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What size is your space?

canadian1969 said:

Height and power are related by the inverse square law.

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That is a common misconception. That is only the case when you do not have reflective walls and hang the light in an otherwise dark room.

In a normal grow the light does not have the chance to spread further than the walls of the grow. So it cannot grow at a square rate and therefore the light will not diminish at that rate either.

You have wall losses in a grow with walls. Which is a reasonably linear relation. You lose about 1% to 2% for every inch of height. Although that depends on the size of the grow and the reflectivity of the walls. In a 2'x2' white poly tent you can lose 40% of the light even at 18".

The height of a led fixture is determined by the desired uniformity of light on the plants.

The higher you hang the light the better the uniformity, but you also start to lose more and more light on the walls. Every bit of lit up wall means you lose light there. So you also want to hang the light as low as possible, but then you get hot spots. Directly under the strip the light will be too intense and in between there won't be enough.

With lots of measurements and simulations I have come up with a rule of thumb. For single row strips I would use the distance between the led strips as the height to use. If you go higher you are just wasting light on the walls and you should be dimming the drivers instead.

So if you have 14 strips distributed over 4' then I'd go for about 3.5" height. Although that would probably become annoyingly close, but still you could go that close and still have uniform light on the plants.

wietefras said:

That is a common misconception. That is only the case when you do not have reflective walls and hang the light in an otherwise dark room.

In a normal grow the light does not have the chance to spread further than the walls of the grow. So it cannot grow at a square rate and therefore the light will not diminish at that rate either.

You have wall losses in a grow with walls. Which is a reasonably linear relation. You lose about 1% to 2% for every inch of height. Although that depends on the size of the grow and the reflectivity of the walls. In a 2'x2' white poly tent you can lose 40% of the light even at 18".

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So just as a quick experiment, adjusting the zippers in the tent door to only allow the probe in, which I taped to a dowel, my lux meter reads the following with my gen 1 EB strips (16) 1" apart, running at 100 watts (veg at the moment):

40" from light (about 4" off tent floor, just above a shit tonne of cups with new sprouts, so no floor reflection):
16500 LUX
20" from light (half way):
27000 LUX
4" from light (uniformity isn't really an issue as the strips are so close together):
58000LUX

linear, yes, cant believe I didn't do this before, lol

So what I am hearing (and would make sense) is that without the walls, (which provide reflection, scattering and absorption) that would make the losses even more pronounced and conform to the inverse square law. For the sake of simplicity, the higher the light the less intense the light will be, still holds as an axiom. As for uniformity, I dont see how that would be a problem with strips unless they were too close aka too far apart for the height, as to create gaps. Correct?

edit: on uniformity, obviously there would be a sweet spot electrically meaning you would want to get the lights as close as possible while maintaining uniformity as to use the minimum amount of electricity, in my case that would work as I can dim my rig to 10% or roughly 20 watts, but with the new drivers only dimming to 50% that would have to be taken into consideration.

canadian1969 said:

So what I am hearing (and would make sense) is that without the walls, (which provide reflection, scattering and absorption) that would make the losses even more pronounced and conform to the inverse square law.

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Yes exactly, when the light is allowed to spread it will spread over an area which increases by the square of the distance. With (reflective) walls it cannot spread like that.

You can actually see some inverse square effect when you go really close up to the strips too. When the height is less than needed to achieve uniformity, the light is still spreading. So inverse square will apply somewhat, but you shouldn't go so close anyway.

Indeed you will still have less light when you raise it higher from optimum height, but you'd normally be better of dimming the drivers rather than wasting light on the walls.

The optimal height depends only on the spacing of the light points. Regardless of total light produced. Of course it can be quicker and easier to just lift the lights a bit higher, but I personally try to resist that urge and just dim down a bit.

BTW Your test shows that hanging the lights at 20" instead of 4" (where it probably should be) means about half the light is wasted on the walls. That's a staggering amount of light gone for no reason really. Finding the right height is really not a small thing and very worthwhile to optimize.

@Randomblame

Great stuff everyone, thx for brainstorming. I hope RB can enlighten us. I ordered the lux meter he suggests....should be a great tool.

wietefras said:

BTW Your test shows that hanging the lights at 20" instead of 4" (where it probably should be) means about half the light is wasted on the walls. That's a staggering amount of light gone for no reason really. Finding the right height is really not a small thing and very worthwhile to optimize.

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Couldn't agree more, 4" is a little crazy, lol,. but 6-10" for sure. All depends on how far you can dim your lights and as you say, how far apart the strips are. Electrically it cost me about $3/month total, and I could half that at least, I am totally being lazy. What I had wrong was the application of the inverse square law. So thanks for clearing that up for me.

canadian1969 said:

Couldn't agree more, 4" is a little crazy, lol,.

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Yeah, that's why I tend to go for double row leds. Lower cost, less work and the bigger spacing (and therefore higher optimal height) isn't an issue since I don't hang it closer than 6"- 10" anyway.

About +-10 inches is perfect as the angle the light casts fills about a third of one of my 2x4 trays. This allows me to equal the lumens per square foot (I know its a bit of a useless parameter, but it is what I had as a starting point) of an HID system with only 3 double row strips per tray

SMT69 said:

@Randomblame

Great stuff everyone, thx for brainstorming. I hope RB can enlighten us. I ordered the lux meter he suggests....should be a great tool.

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I'm not entirely convinced that table of lux values is actually correct. I noticed my seedlings do tend to stretch when I give them that little light. Also in early grow they develop a lot better when I whack them with the full amount of light (same as they get in late grow).

I'm wondering if I should use lower light intensities in late grow compared to flower. I actually use the same intensity for those too. It doesn't make sense to dim when the plants can take it, but I do wonder if the drop in DLI when going from late flower (18 hours light) to flower (12 hour light) would go smoother ie, use something like 500umol/s/m2 in late grow and then boost to 750umol/s/m2 when flower starts to keep the DLI the same going from 18 hours to 12 hours light. Not entirely sure DLI is that translatable though. Higher (and lower) intensities also have their effect on the plants

SMT69 said:

...should be a great tool.

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The undisputed most beneficial tool, I have acquired during my entire DIY LED grow light endeavor, is my "Dr. Meter" brand LUX device, coming in with an initial investment of a staggering $28.50 USD......

wietefras said:

I'm not entirely convinced that table of lux values is actually correct. I noticed my seedlings do tend to stretch when I give them that little light. Also in early grow they develop a lot better when I whack them with the full amount of light (same as they get in late grow).

I'm wondering if I should use lower light intensities in late grow compared to flower. I actually use the same intensity for those too. It doesn't make sense to dim when the plants can take it, but I do wonder if the drop in DLI when going from late flower (18 hours light) to flower (12 hour light) would go smoother ie, use something like 500umol/s/m2 in late grow and then boost to 750umol/s/m2 when flower starts to keep the DLI the same going from 18 hours to 12 hours light. Not entirely sure DLI is that translatable though. Higher (and lower) intensities also have their effect on the plants

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Well, those are the numbers that I stick to. I should say that I use a coolwhite / red mix for germination that only hangs 4" above the jiffies and the high blue content prevents them from stretch. After around a week when they have the 1st. pair of leafes I transplant the yiffy tabs in soil and put them in the 3ft² veg corner with 3000°K and they get early veg intensity.

In the last week only a few new flowers form but the existing flowers mature and getting fat. Back then I dimmed my HPS too in the last week without any losses and I still do the same with LED. Just give it a try, mate.

I took over a lot from the good old HPS times. My 600w HPS ballast was also dimmable from 360w, 440w, 600w and 660w and I finished the vegging with 440w and then continued with 600w to keep the same DLI to soften the transition.
I always try to avoid stress in flowering.
Of course, one could get better results with using more light. I have to pay very much attention to my power consumption as well and always try to take only as much as absolutely needed.
One can use my table and I guarantee good success but if you want you can take more. It's yours..

These three yiffies are from yesterday, BlueDream, SuperSilverHase and ColumbianGold. Expect them to break thru in one or two days. You can see the blue-reddish tint on the pics. I'll upload a few more when plants have a few nodes but I can already say that there will be no stretch.

Randomblame said:

Well, those are the numbers that I stick to. I should say that I use a coolwhite / red mix for germination that only hangs 4" above the jiffies and the high blue content prevents them from stretch.

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See, you mentioning low height makes me "worry" a little. Does that mean it might be more than 15.000lux on the seedlings? At close distances (ie lower than the distance between the strips), inverse square does play a role and intensity can change quite substantially with the light an inch higher or lower.

I did get somewhat stretchy seedlings using a led fixture instead of (much higher Kelvin) fluorescent tubes. Although in the end their stems turned out fine too. So i'm not even sure a bit of stretch in the seedlings matters that much really. Apart from the plants starting out somewhat taller obviously.

Just before harvest with a 2.5cm/1" 2 euro coin for scale: