Question about LED QB distance

[jemro86]

I have two tents, a 4x4 and a 2x4. The 4x4 has a 480 watt and a 240 watt geeklight (from alibaba, using lm301h, cree red 660s, IR&UV), and the 2x4 has a 240 and a 120. Obviously even at 24"+ running them at full power is just too much, so my question is - is it better to have the boards placed higher and turned up a bit more, or lowered and turn the wattage down? I have a hydrofarm par meter I use to check light levels, but I am curious which would be better, or if there would be any difference.

 

[OldManRiver]

I run mine at about 80% power, at about 28 inches. I run two of these (older version) in a 4x4. https://horticulturelightinggroup.com/collections/kits/products/260w-qb-v2-rspec-led-kit

 

[jemro86]

I run mine at about 80% power, at about 28 inches. I run two of these (older version) in a 4x4. https://horticulturelightinggroup.com/collections/kits/products/260w-qb-v2-rspec-led-kit

During your entire flower period? I basically have 3 of those in my 4x4 tent and 1.5 In my 2x4. I currently have the lights drawing about 500 watts in the 4x4 at about 24-28” and the 2x4 275 watts at about the same height. I had them a bit higher (power) but started seeing light stress and some foxtailing.
But if I was to lower the lights and reduce the watts but still be putting out the same par, then it would be exactly the same as more watts at a higher height, right? Or is there some weird lighting “law” that I don’t know about

 

[JWM2]

You’re right in that lowering the lights and running them at a lower wattage is basically the same as raising them and running at a higher wattage. The difference is cover. How much area the lights cover. I use a light meter to dial in my lights. Here’s the meter.

 

[jemro86]

You’re right in that lowering the lights and running them at a lower wattage is basically the same as raising them and running at a higher wattage. The difference is cover. How much area the lights cover. I use a light meter to dial in my lights. Here’s the meter.



View attachment 1041542

Thanks. I have that meter actually and also the hydro farm par meter (cheapest par meter on the market, not 100% perfect but good enough for me), so I figured it wouldn’t be any different, but wanted to ask all you pros over here :)

 

[OldManRiver]

You’re right in that lowering the lights and running them at a lower wattage is basically the same as raising them and running at a higher wattage. The difference is cover. How much area the lights cover.

More power at greater distance gives less attenuation of the light as it penetrates lower in the canopy.

 

[jemro86]

More power at greater distance gives less attenuation of the light as it penetrates lower in the canopy.

So if I understand you correctly then, you’re saying that let’s say if the light is at 24” and is putting out 800 ppfd at let’s say 250 watts (making numbers up for the watts, just for examples), and at 12” the same 800 ppfd but at 125 watts, that the 24” at 250 would be better?

 

[NorGrower]

^ Yes, more watts = more light = more light penetration.

While it's great that you've understood the concept of PPFD, not everyone does, you have to remember that a PPFD measurement is a spot value. An increased electricity (watt) consumption means that your lamp is putting out more light in total (PPF).
PPFD = spot measurement of light intensity
PPF = total light output (imagine that you'd check PPFD on every single spot across your grow area, then add all the PPFD values together)

I recommend watching this video that explains the different terms, well invested 10 min:

How many watts your lamps should draw will come down to what phase your plant(s) is in. Since you mention 800 PPFD, I'm assuming that you're in mid/late flowering? In that case, you likely have a bushy plant which means you want maximum light penetration.
If you'd have a small plant in early veg, deep light penetration would not be as important as you only have 1 or 2 layers of leaves, i.e. not much to penetrate. Makes sense :)?

 

[Porky1982]

I use HLG 260w Rspec and the dimmer is in a really stupid spot so I just lifted em as high as they go. I have 3 in a 8ftx4ft. When plants are small I use 1 light after about week 3 I put 2 on then 3 when I go to 12/12.
Electricity use is about the same as if I dimmed them and hung them closer.

 

[jemro86]

^ Yes, more watts = more light = more light penetration.

While it's great that you've understood the concept of PPFD, not everyone does, you have to remember that a PPFD measurement is a spot value. An increased electricity (watt) consumption means that your lamp is putting out more light in total (PPF).
PPFD = spot measurement of light intensity
PPF = total light output (imagine that you'd check PPFD on every single spot across your grow area, then add all the PPFD values together)

I recommend watching this video that explains the different terms, well invested 10 min:

How many watts your lamps should draw will come down to what phase your plant(s) is in. Since you mention 800 PPFD, I'm assuming that you're in mid/late flowering? In that case, you likely have a bushy plant which means you want maximum light penetration.
If you'd have a small plant in early veg, deep light penetration would not be as important as you only have 1 or 2 layers of leaves, i.e. not much to penetrate. Makes sense :)?

Hey, I appreciate the time you took for the response. I watched the video, and around the 7+ minute mark (6:51 exact), his exact words were "plants do not care how much electricity in watts that a light uses, how efficient it is, or how much light it produces in total, only the ppfd of the area it is placed in". Which, I would think, would mean that a lower watt at lets say 24" would be the same as raising the watts and raising the light to 36". As long as the PPFD is the same where the plant is, then there isnt a difference?

And yes, I am about to start my 8th week since flip, and my plants are super bushy (thank you scrog :P), so penetration is really important. If I can get better penetration by raising the watts and raising the lights, I would, but I dont wanna waste electricity for no reason.

 

[NorGrower]

Anytime, my friend.
I think what he means by:
"plants do not care how much electricity in watts that a light uses, how efficient it is, or how much light it produces in total, only the ppfd of the area it is placed in"

... is that a poorly performing 300W lamp doesn't have to be better than a well performing 200W lamp. I.e. the number of watts a lamp can potentially consume doesn't indicate how much light (PPF) it will emit. A poorly built "Amazon" light with low efficacy (efficiency) but high draw power (watt) could very well emit less light and as a result lower average PPFD than a low draw power lamp. All watts are not equal. This is why we look at efficacy, sometimes called PPE (Photosynthetic Photon Efficacy) in the unit µmol/J. A "Chinese Amazon light", in lack of a better description can often perform at or below a PPE of 1.0 µmol/J. Comparing to a HLG or any other premium light, which runs at 2.2+ µmol/J. The efficacy rating tells us how efficiently a lamp converts electricity into light.
A 100W lamp with an efficacy of 1.0 µmol/J would emit X quantities of photons.
A 100W lamp with an efficacy of 2.0 µmol/J would emit 2X quantities of photons.
A 200W lamp with an efficacy of 0.5 µmol/J would emit X quantities of photons.
This is argument behind "plants don't care how many watts your lamp consumes" as the light output is determined by both consumed watts and efficacy.

I'd like to circle back to my previous point about light penetration and high watt consumption. We can achieve high PPFD with practically any lamp. Even your iPhone flashlight can produce a few hundred PPFD if the light is close enough to the object. But the coverage and penetration will not be great. To penetrate several layers of leaves you need high PPFD in combination with a lot of light (high PPF / high watt).

In early and mid veg where you have a few layers of leaves you could absolutely dim down the lamp as there's not a lot of layers to penetrate. But as soon as you have layers and layers of leaves and the light needs to get through to the very bottom, you'll do better which high power.

 

[Scrogger2190]

If you have a light meter then you dont have to do all the fancy math. So use your light meter to get the correct intensity, and keep in mind that when you raise the light the footprint will be bigger, but you will have to crank the power up to get the same intensity. So use the light meter to keep intensity consistent, and by raising or lowering the light you expand or shrink the footprint. Make the footprint fit as perfect as you can and that's your "sweet spot" power wise