PPFD vs higher watt question

What is actually producing the higher yield with an LED? Is it the actual watts at the wall or the PPFD?

For example, If I wanted a total PPFD at the canopy to be 1000 and could achieve that at 450 watts of power. Would I get the same or higher yield @ 600 watts of power with the light higher up and the same 1000 PPFD at the canopy?

Reason I ask is because I have a heat issue and I can achieve the same 1000 PPFD with the light turned down to 450 and cooler temps. But am I losing yield because the watts are lower?

So, can you achieve the same yield @ 450 watts and 1000 PPFD 12 inches from canopy as a 600 watt 20 inches from canopy and the same 1000 PPFD?

Samwiseman420 said:

What is actually producing the higher yield with an LED? Is it the actual watts at the wall or the PPFD?

For example, If I wanted a total PPFD at the canopy to be 1000 and could achieve that at 450 watts of power. Would I get the same or higher yield @ 600 watts of power with the light higher up and the same 1000 PPFD at the canopy?

Reason I ask is because I have a heat issue and I can achieve the same 1000 PPFD with the light turned down to 450 and cooler temps. But am I losing yield because the watts are lower?

So, can you achieve the same yield @ 450 watts and 1000 PPFD 12 inches from canopy as a 600 watt 20 inches from canopy and the same 1000 PPFD?

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Watts are irrelevant (other than heat), light intensity (ppfd) is what matters.

Intensity falls off with the square of distance; e.g double the distance results in 1/4 the intensity.

For the same light, 600w at 20" is not equivalent to 450w at 12", closer to 50%.

its not just the light intensity its also cost savings vs weight of the flowers, for instance 4x600 watt lamps in a co2 enriched room cooled by a mini split costs in this example $1000/month in electricity to run. yields 40 ounces per run. vs 2x650 watt leds in the same co2 enriched room cooled by a minisplit costs $220/month to run if the lights are at 100%. yields 64 ounces per run. thats an electricity savings of 80% and an increase in weight by 60%. this is also the reason why marijuana is so cheap today. not to mention economy of scale in commercial operations

Which light is it, and are you measuring PPFD, or going by the manufacturer's specs?

When you turn down the power, you also turn down both the vertical penetration and the horizontal spread, so you're bound to loose yield and potency in doing so.

Billy the Mountain said:

Intensity falls off with the square of distance

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This is only true for a point source.
HPS lamps with huge reflective surfaces were never a point source and LED fixtures are not even comparable with a point source.

Samwiseman420 said:

can you achieve the same yield @ 450 watts 12 inches from canopy as a 600 watt 20 inches from canopy

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No.
The latter will have more vertical penetration and yield more.

PPFD is the same no matter what height or wattage. 700 ppfd at 12 inches is the same as 700 ppfd at 18 inches,

Samwiseman420 said:

What is actually producing the higher yield with an LED? Is it the actual watts at the wall or the PPFD?

For example, If I wanted a total PPFD at the canopy to be 1000 and could achieve that at 450 watts of power. Would I get the same or higher yield @ 600 watts of power with the light higher up and the same 1000 PPFD at the canopy?

Reason I ask is because I have a heat issue and I can achieve the same 1000 PPFD with the light turned down to 450 and cooler temps. But am I losing yield because the watts are lower?

So, can you achieve the same yield @ 450 watts and 1000 PPFD 12 inches from canopy as a 600 watt 20 inches from canopy and the same 1000 PPFD?

Click to expand...

The color of light shapes a plant; the quantity of photons drives growth.

Watts are input. There is a correlation between watts draw at the wall by the fixture and PPFD but there are a lot of variables so it's not a 1:1 relationship.

Output from a grow light is referred to as PPFD and is measured in micormols per meter square per second (I use "µmol").

The inverse square law is a means of describing how light levels drop of from a point source so it does not apply to grow lights. A very cost effective way to find out how much light is on your plants is to spend $32 to get a Uni-T Bluetooth light meter. I've attached document that will help convert lux to PPFD.

I like the attitude about 1kµmol. I drank the Bugbee Koolaid when I started growing in 2021 and haven't looked back. Most of his videos are very informative and he and Shane@Migro just released another interview video. I'd recommend watching a few other Bugbee videos first but what is so striking about the new one with Shane is that Shane still hasn't got his head (nor redesigned his lights) around the idea that cannabis can thrive at 1kµmol in ambient CO2.

1000 is usually around the point where the plant gets light bleaching.of course co2 and genetics allsoo take part.
What you should look at is dli because those 1000ppfd have to work for 12h.
Wattage really doesnt matter

Samwiseman420 said:

What is actually producing the higher yield with an LED? Is it the actual watts at the wall or the PPFD?

For example, If I wanted a total PPFD at the canopy to be 1000 and could achieve that at 450 watts of power. Would I get the same or higher yield @ 600 watts of power with the light higher up and the same 1000 PPFD at the canopy?

Reason I ask is because I have a heat issue and I can achieve the same 1000 PPFD with the light turned down to 450 and cooler temps. But am I losing yield because the watts are lower?

So, can you achieve the same yield @ 450 watts and 1000 PPFD 12 inches from canopy as a 600 watt 20 inches from canopy and the same 1000 PPFD?

Click to expand...

If light across your cannopy remain the same then yield should remain the same. Thing is that youre probably not getting the exact same reading across your cannopy thru those 450w, you may get spot measurements the same but likely not the same in the entire cannopy.

Light is allways weaker on the ends and less power makes a smaller radius meaning middle will be 1000 but corner 450.with 600w corner will be 750

Just out of interest what temp you hitting at full power?

coreywebster said:

Just out of interest what temp you hitting at full power?

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Upper 80's @ 600 watts. I can bring the temp down to around 85 by going down to 500 watts. I'm trying to bring my lights closer @ lower watts/heat to retain the same PPFD and not lose yield.

I'm in a 4 x 8 tent with two Diablo's in it.

Samwiseman420 said:

Upper 80's @ 600 watts. I can bring the temp down to around 85 by going down to 500 watts. I'm trying to bring my lights closer @ lower watts/heat to retain the same PPFD and not lose yield.

I'm in a 4 x 8 tent with two Diablo's in it.

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85 is the highest I've seen any recommendation for grow temp, regardless of VPD. I temper, pun intended, my temperatures to keep them between 80 and 85. In fact, I just got an alert that my grow hit 85 so I went to the tent and opened the flaps a bit.

My rationale?

In the Chandra paper, which looks at cannabis growth for varying temps and varying PPFD's, temps higher than 85 caused a drop in net photosynthesis.



The other data point is that Mitch Westmoreland, has stated in a video about hemp, that they saw a 50% reduction in cannabanoids. I just hit YouTube and search for "Mitch Westmoreland hemp" and this is a link to a short where he talks about high temps. He goes into more detail in one of his full length (9 minute?) videos.

Last week Shane and Dr. B. released an hour long± YT video and Bugbee alluded to the work that Westmoreland was doing (he's a PhD student working with Bugbee) and said that temperature is more important than light and that they would be publlishing a paper shortly on that topic. Those are his exact words, please understand, but he was putting huge emphasis on temperature over light which really got my attention because I've always thought of Bugbee as the light and nutrition guy but their obervations re. temperature should be really interesting.

During the interview, the discussion was to go with lower temps "toward the end". He didn't go into detail about what that actually means but, as I interpreted it, the idea was to keep temps down in late/very late flower. The video covers other topics, including recommendations for PPFD/DLI. As usual, worth listening to.

Samwiseman420 said:

Upper 80's @ 600 watts. I can bring the temp down to around 85 by going down to 500 watts. I'm trying to bring my lights closer @ lower watts/heat to retain the same PPFD and not lose yield.

I'm in a 4 x 8 tent with two Diablo's in it.

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Led plants do fine on 80-84F, its actually the recommended temps for high intensity. Missing infra red heat from HID makes the plants a bit colder. Read up on vpd if you havent allready.

One thing that's ambiguous in the Chandra paper is that he doesn't specify if the temps cited are leaf surface temps or ambient air temps. I assume air temp by default since I think he'd specify if it was leaf temp. Also, I'm assuming that an HID light was used for the experiment.

Modern LED lights with their minimal UV output, don't directly heat the leaves like an HID, so the "optimal" ambient temp will be different.

I've been running my tent at 30-32c (leaf temps 27-29c) with 850ppm CO2; they seem to thrive in those conditions.

Billy the Mountain said:

One thing that's ambiguous in the Chandra paper is that he doesn't specify if the temps cited are leaf surface temps or ambient air temps. I assume air temp by default since I think he'd specify if it was leaf temp. Also, I'm assuming that an HID light was used for the experiment.

Modern LED lights with their minimal UV output, don't directly heat the leaves like an HID, so the "optimal" ambient temp will be different.

I've been running my tent at 30-32c (leaf temps 27-29c) with 850ppm CO2; they seem to thrive in those conditions.

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Sounds like you're right in the sweet spot there.

The light source is discussed on page 300 - "Different PPFD were provided with the help of an artificial light source (Model LI-6400-02; light emitting silicon diode; LI-COR), fixed on the top of the leaf chamber and were recorded with the help of quantum sensor kept in range of 660-675 nm, mounted at the leaf level."

LST vs ambient - I would assume ambient unless otherwise specified. LST is used in VPD and perhaps captured at a data element but I've never seen any indication that "growing temps" involved LST.

Delps8 said:

Sounds like you're right in the sweet spot there.

The light source is discussed on page 300 - "Different PPFD were provided with the help of an artificial light source (Model LI-6400-02; light emitting silicon diode; LI-COR), fixed on the top of the leaf chamber and were recorded with the help of quantum sensor kept in range of 660-675 nm, mounted at the leaf level."

LST vs ambient - I would assume ambient unless otherwise specified. LST is used in VPD and perhaps captured at a data element but I've never seen any indication that "growing temps" involved LST.

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Thanks!, I missed the light info.

LST vs. ambient is largely irrelevant if he was also using LEDs.

I'm surprised LEDs were already being used for research purposes in 2008.

Billy the Mountain said:

Thanks!, I missed the light info.

LST vs. ambient is largely irrelevant if he was also using LEDs.

I'm surprised LEDs were already being used for research purposes in 2008.

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Check, check, check.

It's more than "a light", as it turns out. But, never fear, you can get one used on eBay!

One item of note - Chandra discusses "net photosynthesis" and, for about a year after I first read the paper, I was OK with that. The curve for netP starts to rolloff at a pretty low PPFD and yet Bugbee and others were/are insistent on having PPFD's well over 1kµmol. "It just didn't set right!"

If netP rolled off so low, why all the fuss about DLI's?

Check out the attached paper. The key phrase - "However, the yield results of this trial demonstrated cannabis’ immense plasticity for exploiting the incident lighting environment by efficiently increasing marketable biomass up to extremely high—for indoor production—LIs (Figure 7A)."

Apparently, one of the pointy-headed people wanted to answer that same question. "plasticity" is a great way to describe it. The rhetorical question - if net P is rolling off, why does crop yield and crop quality increase?

Those two papers were "milestones" in how I light my grows. Even after I'd read them, multiple times, and had read very similar data from other sources, it took me a couple of grows to "turn it up to 11".

Delps8 said:

Sounds like you're right in the sweet spot there.

The light source is discussed on page 300 - "Different PPFD were provided with the help of an artificial light source (Model LI-6400-02; light emitting silicon diode; LI-COR), fixed on the top of the leaf chamber and were recorded with the help of quantum sensor kept in range of 660-675 nm, mounted at the leaf level."

LST vs ambient - I would assume ambient unless otherwise specified. LST is used in VPD and perhaps captured at a data element but I've never seen any indication that "growing temps" involved LST.

Click to expand...

Pretty sure Li-cor refers to the ppfd meter, not the light source.

Edit: Sorry take that back, seems like its some kinda lighting system. Though li-cor also do ppfd meters afaik

Samwiseman420 said:

Upper 80's @ 600 watts. I can bring the temp down to around 85 by going down to 500 watts. I'm trying to bring my lights closer @ lower watts/heat to retain the same PPFD and not lose yield.

I'm in a 4 x 8 tent with two Diablo's in it.

Click to expand...

My summer grows often go above 90°F when lights are on, and the plants have never had any issues at those temps.

The best fix is to increase air extraction and circulation and/or use AC but not the cheapest.