Lighting spiral of doubt

G

GreenPeace22

Active Member
Hi RIU!

So, I have a hard time understanding how light REALLY works when growing cannabis. I already have some basic understanding about PPF, PPFD, spectrum, PAR output and all these things, problem is when trying to find the proper sized light for a given space.

Some people think in terms of watts/sq. ft., others on usable PPF at the canopy surface, others say that you should only care to provide 200-400 PPFD for seedling, 400-600 for veg and 600-900 for flower.

IN MY OWN UNDERSTANDING, plants need photons, this means that it is the PPFD that matters, right? So BY MY OWN LOGIC, watts and lumens mean shit for plants. This means that even with a 50 watt bulb, manipulating the distance to achieve the right PPFD for each stage of growth would be enough. But then, why doesn't everyone just run the least watts possible as close as possible to achieve the best PPFD and yield with the minimum electricity?

I'm confused. I'd appreciate if someone can enlighten me and help me understand how so size the proper light for the proper space, with the goal of maximizing (the best possible yield) in that particular space.

Note : I know that light type matters, so any references should be to LED lights, as this are the only lights that I'll be using.
 
Last edited:
pop22

pop22

Well-Known Member
Sizing a light is dependent on the total environment in the grow space, and on your grow style. Your right basically about low and close however that limits your choices to a true SCROG or plant strains that stay short. With a SCROG you've got potential for great yeilds with the right strains, but its a lot of work maintaining a SCROG grow.
But factors in the grow space are: What medium will you use? Soil, hydro, coco? Organic or nutrient fed? CO2 or no CO2? Sea of Green or full size plants? Not all strains are good for SOG and not all strains produce there best as full size plant.

My recommendation is fit your light to your budget then adapt your grow space. Look for efficiency specs for the light you choose and also the ability to dim your light. I build my lights. I go for as much light as I can afford for my space. The nice thing with DIY is you can expand your lighting if needed as budget allows.

My personal choice of lighting is Horticultural Lighting Group's Quantum Board. It efficient, powerful, and reasonably priced. My 4 QB light in my 4'x4' tent is run at 360 watts from the wall, which is about 60% of the boards max. Puts out a lot of light that is spread much more evenly than COB lighting.

and I have a single QB in a small tent, a 2.75' x 2.75' tent. Right now running at 135 watts ( i don't rember the PPF of this board ) and growing this plant she takes up about a 1/3 of the tent and has been under htis light since sprouting.

Female Seeds Chem OG pic1 -6-8-2020.jpg
 
wietefras

wietefras

Well-Known Member
GreenPeace22 said:
This means that even with a 50 watt bulb, manipulating the distance to achieve the right PPFD for each stage of growth would be enough.
Click to expand...
You only manipulate the distance in relation to the surface you want to uniformly light with your lamp. You should not deviate from that distance unless you want to light a larger or smaller area.

Much like a flashlight. Put it closer to a surface for a smaller circle and further away for a larger circle. But once you reach the desired circle size, you should only change the intensity of the flashlight to change the average intensity in the circle.

how so size the proper light for the proper space, with the goal of maximizing (the best possible yield) in that particular space.
Click to expand...
If you start with the PPFD (spot intensity) that you want to achieve then that is the average PPF (total amount of photons emitted per second) divided by surface area you need to look for coming from your lamp.

So say you want an average PPFD of 800umol/s/m2 then you buy a lamp emitting 800umol/s for each m2 you want to light. Perhaps a bit more to compensate for wall losses. Which can differ greatly per type of led light.

If you have a light with say an efficacy of 2.5umol/s/W you'd need a 800umol/s / 2.5umol/s/W = 320W lamp per m2 of surface area.

If you want to compensate for wall losses then you should add some extra light. For instance with a single QB in small tents you lose about 40% of the light on the walls since they need to hang quite high. If instead you use 2 led strips instead of a QB then you can half the hanging height to achieve the same distribution and then you'd lose half the amount of light on the walls. That's why more and more people are using led strips rather than boards.

So for a 2'x2' you'd need 0.36m2 * 320W = 115W for the aforementioned average of 800umol/s per m2. For a QB add 40% which makes 160W. With led strips you'd lose 20% of the light on the walls so you'd need 120% * 115W = 140W.

With a 4'x4' you have relatively only half the wall area lit up compared to a 2'x2. So then you'd need 115W * 4 = 460W as the base. Plus 20% for four QB's which makes 552W. Or plus 10% for 8 led strips which makes 506W.

You can compensate for average desired PPFD if different from 800umol/s/m2 and for efficacy if different from 2.5umol/s/W.

It's not an exact number though. Anywhere between 600 and 1000 umol/s/m2 PPFD would be fine in flower. So a bit more or less light would be fine too.
 
G

GreenPeace22

Active Member
pop22 said:
Sizing a light is dependent on the total environment in the grow space, and on your grow style. Your right basically about low and close however that limits your choices to a true SCROG or plant strains that stay short. With a SCROG you've got potential for great yeilds with the right strains, but its a lot of work maintaining a SCROG grow.
But factors in the grow space are: What medium will you use? Soil, hydro, coco? Organic or nutrient fed? CO2 or no CO2? Sea of Green or full size plants? Not all strains are good for SOG and not all strains produce there best as full size plant.

My recommendation is fit your light to your budget then adapt your grow space. Look for efficiency specs for the light you choose and also the ability to dim your light. I build my lights. I go for as much light as I can afford for my space. The nice thing with DIY is you can expand your lighting if needed as budget allows.

My personal choice of lighting is Horticultural Lighting Group's Quantum Board. It efficient, powerful, and reasonably priced. My 4 QB light in my 4'x4' tent is run at 360 watts from the wall, which is about 60% of the boards max. Puts out a lot of light that is spread much more evenly than COB lighting.

and I have a single QB in a small tent, a 2.75' x 2.75' tent. Right now running at 135 watts ( i don't rember the PPF of this board ) and growing this plant she takes up about a 1/3 of the tent and has been under htis light since sprouting.

View attachment 4591036
Click to expand...
Thank you for your time!
Right now, I am also using a HLG QB 135W Kit Rspec in my 2x2 grow space that I'm currently running. It can pull up to 165 true watts from the wall. I'm thinking about upgrading to a little bigger space, say 2x3 or 2x4, and I know that this will demand more light, so I'm trying to figure how much more, and what light (like brand and model) will be the best investment.
 
TreeFarmerCharlie

TreeFarmerCharlie

Well-Known Member
GreenPeace22 said:
Thank you for your time!
Right now, I am also using a HLG QB 135W Kit Rspec in my 2x2 grow space that I'm currently running. It can pull up to 165 true watts from the wall. I'm thinking about upgrading to a little bigger space, say 2x3 or 2x4, and I know that this will demand more light, so I'm trying to figure how much more, and what light (like brand and model) will be the best investment.
Click to expand...
The easiest way to do this would be to just go with a 2x4 and add another of the same light. That way you have the same lighting in the whole tent.
 
G

GreenPeace22

Active Member
TreeFarmerCharlie said:
The reason why many growers, myself included, recommend watts/sqft for specific LEDs is because those numbers work out to suitable PPF for those specific types of LEDs and it’s a lot easier for most people to understand.
Click to expand...
So what would be your recommendation of watts/sqft for a LED grow light? I want the sweetspot, where the yield would be the most decent for the electrical bill. No wasted energy, only what's needed..
 
TreeFarmerCharlie

TreeFarmerCharlie

Well-Known Member
GreenPeace22 said:
So what would be your recommendation of watts/sqft for a LED grow light? I want the sweetspot, where the yield would be the most decent for the electrical bill. No wasted energy, only what's needed..
Click to expand...
If you look at most of HLGs sqft recommendations for their lights they come out to about 30W/sqft. The "go to" recommendation of most LED growers is 30W - 35W which is why you'll hear a lot of people recommend 32W/sqft for quantum boards.
 
G

GreenPeace22

Active Member
sarahJane211 said:
HLG+QBs or Mars Hydro (or clones of).
No other choices to make.

1 QB (120w) covers approx 2' x 2'
Click to expand...
I am currently running 4 smaller plants in a 2x2 using a HLG QB 135W Kit Rspec. Pulls 160 true wats from the wall, but for some reason I have a paranoid feeling that it is not enough. Also, I'm thinking to upgrading to a 2x3 or 2x4.
 
G

GreenPeace22

Active Member
wietefras said:
You only manipulate the distance in relation to the surface you want to uniformly light with your lamp. You should not deviate from that distance unless you want to light a larger or smaller area.

Much like a flashlight. Put it closer to a surface for a smaller circle and further away for a larger circle. But once you reach the desired circle size, you should only change the intensity of the flashlight to change the average intensity in the circle.

If you start with the PPFD (spot intensity) that you want to achieve then that is the average PPF (total amount of photons emitted per second) divided by surface area you need to look for coming from your lamp.

So say you want an average PPFD of 800umol/s/m2 then you buy a lamp emitting 800umol/s for each m2 you want to light. Perhaps a bit more to compensate for wall losses. Which can differ greatly per type of led light.

If you have a light with say an efficacy of 2.5umol/s/W you'd need a 800umol/s / 2.5umol/s/W = 320W lamp per m2 of surface area.

If you want to compensate for wall losses then you should add some extra light. For instance with a single QB in small tents you lose about 40% of the light on the walls since they need to hang quite high. If instead you use 2 led strips instead of a QB then you can half the hanging height to achieve the same distribution and then you'd lose half the amount of light on the walls. That's why more and more people are using led strips rather than boards.

So for a 2'x2' you'd need 0.36m2 * 320W = 115W for the aforementioned average of 800umol/s per m2. For a QB add 40% which makes 160W. With led strips you'd lose 20% of the light on the walls so you'd need 120% * 115W = 140W.

With a 4'x4' you have relatively only half the wall area lit up compared to a 2'x2. So then you'd need 115W * 4 = 460W as the base. Plus 20% for four QB's which makes 552W. Or plus 10% for 8 led strips which makes 506W.

You can compensate for average desired PPFD if different from 800umol/s/m2 and for efficacy if different from 2.5umol/s/W.

It's not an exact number though. Anywhere between 600 and 1000 umol/s/m2 PPFD would be fine in flower. So a bit more or less light would be fine too.
Click to expand...
Thank you for your time, very useful info! Will do more research on this.
Also, I was thinking about LED strips being more efficient, but dunno where to start from. Will do my homework..
 
TreeFarmerCharlie

TreeFarmerCharlie

Well-Known Member
GreenPeace22 said:
Thank you for your time, very useful info! Will do more research on this.
Also, I was thinking about LED strips being more efficient, but dunno where to start from. Will do my homework..
Click to expand...
Start with the thread linked below. That's where I got all the information I needed to build my strip lights.

Samsung F Strip Build

I built this unit yesterday and since I can't show it to anyone personally, I am going to show it off here instead. It was very helpful for me to look at other builds and there is a ton of information on this website so thanks everyone. Overall Dimensions 37" x 24" Strips - 6 x Samsung F-Series...
www.rollitup.org
 
pop22

pop22

Well-Known Member
Your current light gives you 40w/sq.ft. More than enough for a grow without CO2. As someone else said, go to a 2x4 and another HLG kit. Bang for the buck, you can't beat HLG, as they've got it all efficiency, spectrum, massive quantities of light and a fair price. Want a better light? you can spend thousands and get a Fluence to get a couple more points of efficiency, or buy Chinese junk that may or may not have real Samsung chips and an iffy warranty. Nope, you've already got the best!



GreenPeace22 said:
I am currently running 4 smaller plants in a 2x2 using a HLG QB 135W Kit Rspec. Pulls 160 true wats from the wall, but for some reason I have a paranoid feeling that it is not enough. Also, I'm thinking to upgrading to a 2x3 or 2x4.
Click to expand...
 
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