LED lights -- dimming vs lowering

[Death The Cultivator]

Hi! I'm running an HLG 225 QB in a 2x2x7 tent and was wondering what the thoughts were on dimming your LED's vs raising and lowering them? What are the benefits of each? What are the drawbacks? I can dim my lights and stay at within an appropriate lux level, so technically i could have them 12" from the plant without burning it. There have been studies done that say if you dim LED's then you're changing the light spectrum and not getting the full effect, but HLG conducted a study on their lights and said unless you dim it to 10%, there is no real difference in spectrum.
So. My question is... should i have my light running at 90% at 40" from my plant (im aiming for giving her 18k lux), OR.. should i dim my lights to 40% and bring them to 30" from my plant?

Imo dimming ruins the manufacturer ppfd measurements among other things where altering the light height or light schedule would be more effective.

 

[briefbriefs]

Imo dimming ruins the manufacturer ppfd measurements among other things where altering the light height or light schedule would be more effective.

thank you for your input! i really appreciate it. HLG has a blog post that claims their light spectrum remains unchanged if you dim their lights (along with a few charts) but didnt say anything about the ppfd being affected or not. If your experience is any indicator, that is definitely an issue worth considering when dimming an LED board.

 

[Death The Cultivator]

thank you for your input! i really appreciate it. HLG has a blog post that claims their light spectrum remains unchanged if you dim their lights (along with a few charts) but didnt say anything about the ppfd being affected or not. If your experience is any indicator, that is definitely an issue worth considering when dimming an LED board.

Efficiency can be an issue with dimming as well. Cannabis is a high DLI plant thriving at up to approximately 65ish so unless you have a high PPFD requiring c02 I don't see the need to dim them.

https://smartgrow.systems/sgs-original-research-on-the-importance-of-daily-light-integral-dli/

PAR meters aren't affordable yet but much more accurate than using lumens. Hopefully they become as important in the grow room as EC and pH meters

 

[parsing_trees]

damn that really sucks =\ but hey...its only through mistakes that we learn. And you dont need a par meter! Get a lux meter. I found one on amazon for $30. The recommended lux levels for LEDs are:
Seedling = 5k - 15k
Establishment (as they get their first true fan leaves) = 20k-25k
Veg = 25k - 30k
Early flower/Stretch = 35k
Flower = 45.5k-63.5k

Recommended by whom? I did the math with DLI, lux levels, PPFD, etc., and came to the conclusion that anything recommending the above probably has photos (and 12 hours of light per day) in mind, rather than autos (18 to 24/day). I also tried it and about 60k at the top cola burned the helllllllll out of one of my plants. I don't know what better lux targets are for autos, but I've started to see light stress any time I go above about 30k at 20/4 or 24/0. (Whether or not my lights are too close.) If those levels have worked for any of you that would be good to know, but in that case I really wonder what must be doing so wrong.

 

[Simplicio]

Assuming the same lux reading at the top of the canopy, plants are better off further away using full power without dimming than they are with dimmed light closer.
There are at least 2 reasons:
1. More light will reflect off the walls and hit lower plant parts.
2. There will be less drop-off in intensity for leaves further from the light. Although the inverse square law doesn't strictly apply to our lights, it does to some degree.

If lights are dimmed, 2 ft away, then applying the inverse square law to leaves 1 ft below shows that they get (1/2)^2 = 1/4 as much light as the top.
Instead, if lights are 3 ft away, then leaves 1 ft below the top get (2/3)^2 = 4/9 as much light, which is almost half instead of 1/4.

 

[briefbriefs]

Recommended by whom? I did the math with DLI, lux levels, PPFD, etc., and came to the conclusion that anything recommending the above probably has photos (and 12 hours of light per day) in mind, rather than autos (18 to 24/day). I also tried it and about 60k at the top cola burned the helllllllll out of one of my plants. I don't know what better lux targets are for autos, but I've started to see light stress any time I go above about 30k at 20/4 or 24/0. (Whether or not my lights are too close.) If those levels have worked for any of you that would be good to know, but in that case I really wonder what must be doing so wrong.

interesting! I’m curious to see the effects I find once I hit 30k...I’m also running 20/4 and I’ve noticed bits and pieces of what you said. The lux levels were recommended both by a grower on the 420mag forums (master grower) and by another grower who does mostly autos. I’ll post my findings as I discover. This is a mystery worth solving.

 

[briefbriefs]

Assuming the same lux reading at the top of the canopy, plants are better off further away using full power without dimming than they are with dimmed light closer.
There are at least 2 reasons:
1. More light will reflect off the walls and hit lower plant parts.
2. There will be less drop-off in intensity for leaves further from the light. Although the inverse square law doesn't strictly apply to our lights, it does to some degree.

If lights are dimmed, 2 ft away, then applying the inverse square law to leaves 1 ft below shows that they get (1/2)^2 = 1/4 as much light as the top.
Instead, if lights are 3 ft away, then leaves 1 ft below the top get (2/3)^2 = 4/9 as much light, which is almost half instead of 1/4.

I hadn’t considered the square law. This is a VERY good argument for having the lights at full brightness but a greater distance, especially because I’m using it in such a small (& tall) reflective tent, I can really capitalize on using the reflected light for the lower budsites.

 

[briefbriefs]

PAR meters aren't affordable yet but much more accurate than using lumens. Hopefully they become as important in the grow room as EC and pH meters

You’re absolutely right about that. I currently can’t justify the cost of a par meter so I make do with a Lux meter but agree with you and do hope to own one some day.

 

[tip]

I completely understand the need for heat but I think you should be dehumidifying instead of humidifying. You won't be happy with your results at 67% RH. You need to get that down to the 40's, preferably low 40's by harvest. I like to be under 60% by week two.

 

[parsing_trees]

interesting! I’m curious to see the effects I find once I hit 30k...I’m also running 20/4 and I’ve noticed bits and pieces of what you said.

The specific symptom was yellowing between the leaf veins, which looked a lot like a magnesium deficiency, but it was limited to the top parts of the canopy closest to the light; mag deficiencies usually start from the lower/older growth and progress upward. Also, adding extra magnesium (in the form of dyna-gro mag pro) didn't help at all. Keeping the lights a bit further and under 35k total lux made it go away.

What Simplicio said above about the inverse square law makes a lot of sense, I'll try keeping the lights on full but further away next time.