Calculating proper lighting…when distances change

[Mozzy]

I’ve been thinking about this for a while , so hopefully this is not a stupid question: Is there a linear relationship or formula for calculating how the known PPFD at a distance, changes when the distance changes?

I’ll use my light as an example, however the question should apply to any light with a tested map I think . I have a @Marshydro SP3000, and there are 3rd party tested numbers out there which helps as a starting point:

I understand that based on these readings, at an average canopy that is 19.5” (50cm) from the light, at 100%, it is getting ~875 PPFD (~980 in the middle/~750 corners). I believe if you turn the dimmer to 50%...you are getting ~438 PPFD (~490 in the middle/~375 corners); and that this dimmer effect is roughly linear across the range.

But what if instead of being ~20” away from the canopy, you are 10” away, or 15”, etc.? Is there a way to manually calculate the PPFD difference from the tested numbers, based on a change in distance from the measured height?

 

[PeaceLove&Buds]

 

[Death The Cultivator]

I’ve been thinking about this for a while , so hopefully this is not a stupid question: Is there a linear relationship or formula for calculating how the known PPFD at a distance, changes when the distance changes?

I’ll use my light as an example, however the question should apply to any light with a tested map I think . I have a @Marshydro SP3000, and there are 3rd party tested numbers out there which helps as a starting point:
View attachment 1632099

I understand that based on these readings, at an average canopy that is 19.5” (50cm) from the light, at 100%, it is getting ~875 PPFD (~980 in the middle/~750 corners). I believe if you turn the dimmer to 50%...you are getting ~438 PPFD (~490 in the middle/~375 corners); and that this dimmer effect is roughly linear across the range.

But what if instead of being ~20” away from the canopy, you are 10” away, or 15”, etc.? Is there a way to manually calculate the PPFD difference from the tested numbers, based on a change in distance from the measured height?

No, you need a par meter if you're going to adjust the height of your lights or should your plants change in height. Everything else is a guess.

 

[Badfinger]

As mentioned above, u will need some sort of tool to measure your DLI or PPFD, otherwise your shooting from the hip.
A lot of us use a phone app called Photone to aid us when adjusting height. This will show u DLI, PPFD, LUX and KELVIN. Its actually a very nice free app.
By the way, I'm doing my very first grow with the SP3000, since down sizing my tent to a 2x4. Seems like a good light so far.
Another handy guide is this chart.

 

[Mozzy]

No, you need a par meter if you're going to adjust the height of your lights or should your plants change in height. Everything else is a guess.

Thanks for confirming DTG. That is unfortunate there isn't any criteria for making an educated guess; I was hoping I had just forgotten a formula from a science class in school or something.

It is also very disappointing that light manufacturers don't just provide PPFD maps at several distances as a standard practice…seems like a very inexpensive way to help customers estimate (especially new ones), unless they are just selling trash and don't want to publicly put out their numbers.

 

[Mozzy]

As mentioned above, u will need some sort of tool to measure your DLI or PPFD, otherwise your shooting from the hip.
A lot of us use a phone app called Photone to aid us when adjusting height. This will show u DLI, PPFD, LUX and KELVIN. Its actually a very nice free app.
By the way, I'm doing my very first grow with the SP3000, since down sizing my tent to a 2x4. Seems like a good light so far.
Another handy guide is this chart.
View attachment 1632293

Thanks Badfinger. I tried that a while ago and wasn't successful with it, either due to my hardware or user error.

Good luck! Do you have a journal going for it? I've definitely been able to grow some flowers with it in that size space.

I've had to detach and/or reattach the driver throughout grows related to heat, and it barely fits through the hole in the back of the tent so it's tedious and cumbersome every time with the dimmer box attached on a length of cord too…hopefully it'll be easier for you to manage.

My biggest struggle with the light early on was what dimmer % and distances to use from seed-to-harvest, and what little advice or recommendations I could find online were either completely wrong, or just not accurate for running autos 18/6, and I've cooked some plants with it as a result. Subsequently, I haven't had to run it higher than ~60-70% max throughout a grow depending on plant heights/conditions, so I assume that'll help with longevity at least.

Also thanks, I came across that chart a little while ago and it has seemed like a good reference. Appreciate it.

 

[Death The Cultivator]

Thanks for confirming DTG. That is unfortunate there isn't any criteria for making an educated guess; I was hoping I had just forgotten a formula from a science class in school or something.

It is also very disappointing that light manufacturers don't just provide PPFD maps at several distances as a standard practice…seems like a very inexpensive way to help customers estimate (especially new ones), unless they are just selling trash and don't want to publicly put out their numbers.

It's not really up to the manufacturers to completely figure your lighting out for you. $160 par meter for spot checking to make adjustments with a dimmable fixture is all you need. I advise against expecting apps to measure light accurately for horticultural purposes, par meter will always be your best best to make sure the charts and your personal adjustments are correct

 

[Badfinger]

Thanks Badfinger. I tried that a while ago and wasn't successful with it, either due to my hardware or user error.

Good luck! Do you have a journal going for it? I've definitely been able to grow some flowers with it in that size space.

I've had to detach and/or reattach the driver throughout grows related to heat, and it barely fits through the hole in the back of the tent so it's tedious and cumbersome every time with the dimmer box attached on a length of cord too…hopefully it'll be easier for you to manage.

My biggest struggle with the light early on was what dimmer % and distances to use from seed-to-harvest, and what little advice or recommendations I could find online were either completely wrong, or just not accurate for running autos 18/6, and I've cooked some plants with it as a result. Subsequently, I haven't had to run it higher than ~60-70% max throughout a grow depending on plant heights/conditions, so I assume that'll help with longevity at least.

Also thanks, I came across that chart a little while ago and it has seemed like a good reference. Appreciate it.

Sorry to hear the app didnt work for you. Since we use the same light, if u ever try it again, let me know, i'll share my settings with you. I'm an android guy, but when my daughter did a side by side comparison with me using her Iphone, our numbers were exactly the same.

 

[Mozzy]

Sorry to hear the app didnt work for you. Since we use the same light, if u ever try it again, let me know, i'll share my settings with you. I'm an android guy, but when my daughter did a side by side comparison with me using her Iphone, our numbers were exactly the same.

Very interesting! Thank you. I really appreciate the offer.

 

[Mozzy]

Found this on another site and wanted to share here in case it helps someone: