LED lights, ppfd, height and intensity - how to dial in ppfd?

[budjamesbud]

i have a question related to height and intensity.
i know the plant needs different levels of ppfd at different stages of growth.

i'm wondering, do growers use a combination of height and intensity to get the reccomended ppfd?

for example, seedlings need a ppfd of around 250 in week 1. my viparspectra xs1500 pro says that to hang at a height of 11" and have at 25% intensity to get a 250ppfd.
i also read that doubling the height will lower ppfd to 25%.... does that mean if i had the height at 22" and 100% intensity the ppfd would be 250?
what would be the difference/ benefit to chosing one over the other.

my second question is on adjusting height and intensity to dial in ppfd.
lets say in week 3/4 ppfd is reccomended to be about 350ppfd
looking at my chart 50% dimmable is about 500ppfd. a little above the 350.
if i look at my chart it looks like raising my light from 11" to 14" would be about a 20% reduction in ppfd.
so would i set the dimmable at 50% and raise to 14" would knock 20% off the 500ppfd and effectively give the plants about 400ppfd?
am i calculating this corrctly?

i've read that for seedlings to keep 18-24" away.... but, if my ppfd map says 250ppd at 11", why would i move it higher?

below is what i've quoted. i would like any feedback and how growers might use a combination of the height and intensity to dial in ppfd.

"The ideal amount of PPFD for plants depends on their stage of development. For quick reference, here are the recommended PPFD ranges for plants:

• Seedling: 100 - 300 μmol/m2/s
• Vegetative: 400 - 600 μmol/m2/s
• Flowering: 800 - 1,000 μmol/m2/s

Take a look at the estimated numbers at different heights on your manufacturer’s PPFD maps. You could add these scores and divide them by the number of ranges you collected to get an average PPFD.
Here's a general LED hanging height guide for common dimmable LED grow lights for the first time:

• Hang your LED grow lights at 18" - 24" at 25 percent intensity for seedlings.
• Bring your LEDs to about 18" during vegetation and set the dimmer to 50 percent.
• Keep your LEDs at 18" and turn them to full intensity."

 

[LoveGrowingIt]

i'm wondering, do growers use a combination of height and intensity to get the recommended ppfd?

Yes. The two are certainly interrelated, and I've pondered the question many times. In most cases, it's best to follow the light manufacturer's recommendations until one learns how to "read the plants" regarding how well they're responding to the light. They don't all respond the same. So, I don't use a light meter. I just raise the intensity until I see the plants complaining, then I lower it. It's basically a clinical-titration method. It is important not to let the crown of the plant get too close to the light, though. For most LEDs, I'd say 18 inches is a good rule-of-thumb minimum distance.

I'm not exactly the sciency type, but I've found this to be helpful when thinking about distance and intensity.

 

[ArtfulCodger]

This might sound argumentative. I don't intend it that way. Your plants don't care what your light meter says. We're not trying to "dial in ppfd." We're trying to dial in plant health. We see so many new growers here burning their plants as they follow a PAR recipe. Plants are wonderful light meters on their own. Leaves that are flat, level, and green are happy with the light level. Leaves that show edge curl, taco-ing, canoe-ing, twisting, and any color other than green are probably getting too much light. Leaves that are still praying an hour before lights-out are ready for more light.

Your plants' genetics, your system, your environment, and your skill all come into play determining how much light is the right amount at a given stage. Take any PAR recipe with a big grain of salt.

All that said, here's what's been successful for me. There are lots of other approaches that work. In seedling, I run a single two-foot 6500K fluorescent T5, six inches above the top of the plants. I move the light up as the plants grow. That lasts a month, more or less. In my veg tent, I run a four-foot, eight bulb 6500K fluorescent T5 at 30 inches above the plant tops. I like T5s in veg...they're gentle and seem to make happy plants. In flower, I run LEDs at the top of my tents, adjusting the intensity as the plants ask/allow. I'll take a PAR reading from time to time, but it's to make a note for what a particular cultivar wanted in a particular stage/situation. I don't make the plants accept an arbitrary PAR level.

I'll hop off my soap box now. This isn't a majority opinion in 2023, I realize.

 

[budjamesbud]

This might sound argumentative. I don't intend it that way. Your plants don't care what your light meter says. We're not trying to "dial in ppfd." We're trying to dial in plant health. We see so many new growers here burning their plants as they follow a PAR recipe. Plants are wonderful light meters on their own. Leaves that are flat, level, and green are happy with the light level. Leaves that show edge curl, taco-ing, canoe-ing, twisting, and any color other than green are probably getting too much light. Leaves that are still praying an hour before lights-out are ready for more light.

Your plants' genetics, your system, your environment, and your skill all come into play determining how much light is the right amount at a given stage. Take any PAR recipe with a big grain of salt.

All that said, here's what's been successful for me. There are lots of other approaches that work. In seedling, I run a single two-foot 6500K fluorescent T5, six inches above the top of the plants. I move the light up as the plants grow. That lasts a month, more or less. In my veg tent, I run a four-foot, eight bulb 6500K fluorescent T5 at 30 inches above the plant tops. I like T5s in veg...they're gentle and seem to make happy plants. In flower, I run LEDs at the top of my tents, adjusting the intensity as the plants ask/allow. I'll take a PAR reading from time to time, but it's to make a note for what a particular cultivar wanted in a particular stage/situation. I don't make the plants accept an arbitrary PAR level.

I'll hop off my soap box now. This isn't a majority opinion in 2023, I realize.

i appreciate your comments!
this is what i'm looking for
i think i'll start with the reccomended settings and then watch for the signs you mention and adjust accordingly

thanks!!!

 

[LoveGrowingIt]

Plants are wonderful light meters on their own.

There's no better method than reading the plant, but that's a learned skill. That's why I advise starting with the manufacturer's recommendations. Understanding how these plants grow gets easier with every grow—and the advice of the helpful folks here.

 

[ArtfulCodger]

There's no better method than reading the plant, but that's a learned skill. That's why I advise starting with the manufacturer's recommendations. Understanding how these plants grow gets easier with every grow—and the advice of the helpful folks here.

It's always looked to me like the manufacturers' directions tend to recommend hanging too low. You can't smoke a PAR map. Well, you probably can, but I doubt you'd do it twice.

 

[Pilted]

Reading the plant is the way to do it, but I think some of you long time growers forget what it's like to be new. When just starting out reading the plant is very difficult and a ppfd meter can help you understand the basics behind light intensity quickly and easily. After you gain this understanding it makes reading the light intensity and plant health much easier. Or at least it did for me.

There is an app called photone it's free for Android and a paid app on iPhone. It uses the selfie camera on your phone to read ppfd. Use this to get an idea of where you should be at but always watch the plant for signs, she will tell you everything you need to know. But the app is helpful to get an idea of the ppfd, which in turns shows you distance, light intensity, and gives you a good ballpark to work with.

End of the day do not rely on the ppfd meter rely on the plants health.

 

[budjamesbud]

Reading the plant is the way to do it, but I think some of you long time growers forget what it's like to be new. When just starting out reading the plant is very difficult and a ppfd meter can help you understand the basics behind light intensity quickly and easily. After you gain this understanding it makes reading the light intensity and plant health much easier. Or at least it did for me.

There is an app called photone it's free for Android and a paid app on iPhone. It uses the selfie camera on your phone to read ppfd. Use this to get an idea of where you should be at but always watch the plant for signs, she will tell you everything you need to know. But the app is helpful to get an idea of the ppfd, which in turns shows you distance, light intensity, and gives you a good ballpark to work with.

End of the day do not rely on the ppfd meter rely on the plants health.

cool, thanks for letting me know about the app.
i think i have an android tablet somewhere at home....?
i'll see if ican getthe app installed for free and try it out

 

[imgrowing]

Plants are wonderful light meters on their own. Leaves that are flat, level, and green are happy with the light level. Leaves that show edge curl, taco-ing, canoe-ing, twisting, and any color other than green are probably getting too much light. Leaves that are still praying an hour before lights-out are ready for more light.

so if the leaves are praying they need more light and if flat they're good?
i thought it was the other way around, i,m just a newbie though

 

[ArtfulCodger]

so if the leaves are praying they need more light and if flat they're good?
i thought it was the other way around, i,m just a newbie though

Flat and level = good. Praying = more, please.

 

[LoveGrowingIt]

It's always looked to me like the manufacturers' directions tend to recommend hanging too low. You can't smoke a PAR map. Well, you probably can, but I doubt you'd do it twice.

Well... I did say "starting" with the manufacturer's recommendations. It's a lot cheaper than buying a meter.

Flat and level = good. Praying = more, please.

Praying can also mean too much light, especially if the leaves are curling like a canoe.

 

[quirk]

During veg I keep the light farther away and increase intensity to reach desired PPFD and in flower I will lower with a decrease in intensity to achieve the correct parameters in flower.. This works well with my somewhat even canopies..
Don't forget the ever critical DLI.