Lower Lights vs. Dimmer Switch?

[ExNavyInSTL]

My question is really for clarity to make sure the terms "Lower (or raise) your light" vs just use the dimmer switch are mostly synonymous.

Why I ask the question:

When I look at advice on the forums, I often see someone tell the person to raise the light if there is light stress. Then I might see someone advise someone to lower the lights if a seedling is getting too tall.

My guess is that this is advice that has been passed down for years before LED lights (with dimmer switches). Those phrases just became part of the vernacular with online advice.

To add to that guess, this advice was given before folks were closely tracking PPFD and more DLI (before tools and apps make it easy to know more exactly what the proton count is barreling down on the leaves).

So with this question set up, am I correct that this previous (and current) advice is saying the same (or similar) to someone using the dimmer switch to increase or decrease light intensity?

Of course, if the dimmer is already maxed out or at the dimmest already, then moving the light makes sense.

I ask all this because I think I am right with my assumptions. Mainly, because I like having my light way up high and out of the way and then dialing in the PPFD or DLI to what is required in the moment. With the light up at the top of the tent then any heat created is no longer an issue. Some might say, more importantly, keep it up high to quit banging your head on the light.

I have the luxury of mounting up high because I have plenty of watts to work with, so I understand if someone is running lower wattage lights has to bring them in closer or further away to solve a problem.

Another reason I ask this question is to make there is no special benefit to have your lights closer to the canopy for the plants to flourish. In other words, the closeness is something that is important for healthier photosyntesis (assuming the PPFD is dialed in properly with the dimmer).

I've used to many words, but I close it this way (tell me if I am wrong):

Lower your light = raise your light's intensity by turning up your dimmer switch.
Raise your light = lower your light's intensity by turning down your dimmer switch.

 

[bluter]

i run my entire grows between 18 - 24 inches and use the dimmer to regulate intensity.

there's a difference though. i run old school cobs with a fixed 3000k/3500k mix. it's a definite flower bias. in veg i supplement with a couple hours of burple 5050 smds targeting specific spectrum, at either end of the day cycle.

when using a dimmer my kelvin doesn't change on the main rig. most "full spec" led lights are built as a 'one light works for all stages' basis. this simply isn't true.

using a dimmer on a full spec led rig will drop some parts of the spectrum out more than others. it's simply the way emitters work. everything is only present when the rig is full up, and this is how they are measured. it's not how they usually get used, which is a bit misleading.

 

[SmokingWings]

My guess is that this is advice that has been passed down for years before LED lights (with dimmer switches).

Your question mentioned earlier was interesting. While reading what you wrote and the replies up to now I kept reminding myself that not all the lights on the market have dimmer switches. Maybe a couple years down the road.

Something that occurred to me. If the intensity of the light is reduced by using a dimmer switch how much effect does that have on light penetration into the canopy? Some sort of question when wondering how much the light penetration changes if a regular LED light is raised up higher away from the top of the canopy.

 

[Joe Brewster]

I prefer to keep my lights as close as possible for penetration. I run four old style Cree cobs and the newer Samsung mini Cob strip lighting on a 4x4 frame
Samsung are dimmable but very rarely dim them because my set up is on chains and pulleys and easy to adjust
I find if I do turn it down, I will forget to turn it up later
also, better at keeping the sativa dom plants from stretching too much

 

[bluter]

If the intensity of the light is reduced by using a dimmer switch how much effect does that have on light penetration into the canopy? Some sort of question when wondering how much the light penetration changes if a regular LED light is raised up higher away from the top of the canopy.

both intensity and penetration are reduced in both cases. the same as hid or other source.

 

[Roy Growin]

Higher intensity from further away is more effective
The inverse square law applies to density of radiation, not volume so essentially it's still more light

 

[ExNavyInSTL]

Thanks for all the quick replies.

I totally understand folks NOT using LEDs having to raise and lower lights.

And I get the concerns for penetration.

My thinking was if I can get 40 DLI (if appropriate) at 3-feet above the canopy with the dimmer opened up; then it must be the same if I lower the light to 1-foot away and dim down to 40 DLI at the canopy top.

If I understand @Roy Growin correctly, then I am on the right track by keeping my LED higher. Maybe keeping the light higher also allows more light to bounce around against the reflective interior of the tent, which could help with penetration too.

Of course, running the lights wide open cost more money for electricity.

Thanks again!

 

[Fenderbender]

Benefits of keeping the light high with lots of power is better spread all around and more even intensity.. the closer you get to leds the wilder the numbers get if you move an inch and then you get out from under the board and numbers plummet.. so if you can get the light cone as large as possible by hanging high yet still have enough power to hit the numbers you want that would indeed be best.
Of course might be wasteful.. bunch of smaller lights closer to the plants of lesser wattage could be more efficient.

 

[Delps8]

Thanks for all the quick replies.

I totally understand folks NOT using LEDs having to raise and lower lights.


And I get the concerns for penetration.

My thinking was if I can get 40 DLI (if appropriate) at 3-feet above the canopy with the dimmer opened up; then it must be the same if I lower the light to 1-foot away and dim down to 40 DLI at the canopy top.

If I understand @Roy Growin correctly, then I am on the right track by keeping my LED higher. Maybe keeping the light higher also allows more light to bounce around against the reflective interior of the tent, which could help with penetration too.

Of course, running the lights wide open cost more money for electricity.

Thanks again!

The more you increase the hang height, the more uniform your light cast will be and the lower your PPFD will be. The higher the light, the more light will reflect off of the walls of the grow area but, understanding how quickly photons attenuate, it takes a lot of power to get good light on the canopy.

This is an excellent resource to understand uniformity.

"penetration" - photons strike the top of a leaf and penetrate the leaf but almost all photons (95±%) are absorbed by the leaf which means that there is no usable PAR below a leaf. In short, no light penetrates the canopy. If light is hitting areas below the canopy, either there's a hole in the canopy or…you need a bigger canopy.

"Lower your light" vs "raise your light" - I read those to be instructions on how to achieve a desired outcome rather than specifying the the desired outcome, which would be "Increase/decrease DLI" or "Increase/decrease PPFD".

Re. a DLI of 40 - that's pretty much the max for photoperiods in flower but cannabis will thrive at DLI's 50% higher than that. The reason I bring that up is because researchers, Bugbee being one, has shown that there is a linear relationship between DLI and plant quality, crop yield, and crop quality, as long as the other factors in the grow environment are "dress right dress".

 

[ExNavyInSTL]

@Delps8 Awesome info.

I have watched so many videos of Bruce Bugbee. He and Harley (nutes) saturated my brain the first few months when I got started last year.

Another Youtuber (Welcome to the Grow Tent) mentioned in one of his beginner videos to get your lights to the top of tent. He didn't say it (that I remember), but I assumed he thought we would know to dial it in for PPFD and DLI using a dimmer.

Another is where I got the suggested PPFD and DLI goals for 20/4 Autos (below):

---

AUTO FLOWER LED LIGHT INTENSITY
Recommended LED PPF/PAR Values / DLI Based on 20 hrs. of light.​

Seedlings 150 - 200 umol / DLI 10.80 - 14.4 moles per day
Veg up to 9" 450 umol / DLI 32.40 moles per day
Early bloom 500 umol / DLI 36.00 moles per day
Mid bloom 650 umol / DLI 46.80 moles per day
Late bloom 750 umol / DLI 54.00 moles per day
Ripen 400 umol / DLI 28.80 moles per day
*** Use extreme caution going 700 umol or
higher @ 20 hrs/day this could cause leaf burn**

---

With this grow, I intend to go 18/6 (instead of 20/4) with my Auto Flowers. So, I'll just backward engineer the numbers based on the target DLI because it stays the same whether 20, 18, or 12.

For instance (18/6):
Seedling: ---------- (DLI 10.80) ------- 167 PPFD
Veg up to 9": ----- (DLI 32.40) ------ 500 PPFD
Early bloom: ----- (DLI 36) ----------- 556 PPFD
Mid bloom: ------- (DLI 46.80) ------- 722 PPFD
Late bloom: ------ (DLI 54) ------------ 834 PPFD
Ripen: -------------- (DLI 28.80) -------- 445 PPFD

I'm curious just how much the numbers go up for:

12/12 during a photoperiod flowering period:
Early bloom: ------ (DLI 36) ------------ 834 PPFD
Mid bloom: ------- (DLI 46.80) -------- 1084 PPFD
Late bloom: ------ (DLI 54) ------------- 1250 PPFD
Ripen: -------------- (DLI 28.80) -------- 667 PPFD

By the way, I just use my iPhone for light readings using the App called:

Photone - Grow Light Meter​

The more you increase the hang height, the more uniform your light cast will be and the lower your PPFD will be. The higher the light, the more light will reflect off of the walls of the grow area but, understanding how quickly photons attenuate, it takes a lot of power to get good light on the canopy.

This is an excellent resource to understand uniformity.

"penetration" - photons strike the top of a leaf and penetrate the leaf but almost all photons (95±%) are absorbed by the leaf which means that there is no usable PAR below a leaf. In short, no light penetrates the canopy. If light is hitting areas below the canopy, either there's a hole in the canopy or…you need a bigger canopy.

"Lower your light" vs "raise your light" - I read those to be instructions on how to achieve a desired outcome rather than specifying the the desired outcome, which would be "Increase/decrease DLI" or "Increase/decrease PPFD".

Re. a DLI of 40 - that's pretty much the max for photoperiods in flower but cannabis will thrive at DLI's 50% higher than that. The reason I bring that up is because researchers, Bugbee being one, has shown that there is a linear relationship between DLI and plant quality, crop yield, and crop quality, as long as the other factors in the grow environment are "dress right dress".

 

[Jaz]

am I correct that this previous (and current) advice is saying the same (or similar) to someone using the dimmer switch to increase or decrease light intensity?

No you are not correct, let's take this 6" hang vs a 36" hang both at 100%.

^ Look at those hot spots vs cool spots. It's about double the intensity from hottest to coolest.

As you can see above, the further away the light is, the more even the distribution is across the full area, no hot spots with the physical raising of the light.

Going by your logic, if you use the dimmer in the 6" hang, you just lower the intensities of each square area.
There will still be hot spots and cool spots but at lower intensities.

Whereas if you raise the light, the intensity is distributed more evenly across the full area, so it is definitely not the same as using the dimmer dial.

I hope this helps with your understanding.

Good question btw.

j

 

[Delps8]

@Delps8 Awesome info.

I have watched so many videos of Bruce Bugbee. He and Harley (nutes) saturated my brain the first few months when I got started last year.

Another Youtuber (Welcome to the Grow Tent) mentioned in one of his beginner videos to get your lights to the top of tent. He didn't say it (that I remember), but I assumed he thought we would know to dial it in for PPFD and DLI using a dimmer.

I've also spent a great deal of time watching Migro videos. He is where I got the suggested PPFD and DLI goals for 20/4 Autos (below):

---

AUTO FLOWER LED LIGHT INTENSITY
Recommended LED PPF/PAR Values / DLI Based on 20 hrs. of light.​

Seedlings 150 - 200 umol / DLI 10.80 - 14.4 moles per day
Veg up to 9" 450 umol / DLI 32.40 moles per day
Early bloom 500 umol / DLI 36.00 moles per day
Mid bloom 650 umol / DLI 46.80 moles per day
Late bloom 750 umol / DLI 54.00 moles per day
Ripen 400 umol / DLI 28.80 moles per day
*** Use extreme caution going 700 umol or higher @ 20 hrs/day this could cause leaf burn**

---

With this grow, I intend to go 18/6 (instead of 20/4) with my Auto Flowers. So, I'll just backward engineer the numbers based on the target DLI because it stays the same whether 20, 18, or 12.

For instance (18/6):
Seedling: ---------- (DLI 10.80) ------- 167 PPFD
Veg up to 9": ----- (DLI 32.40) ------ 500 PPFD
Early bloom: ----- (DLI 36) ----------- 556 PPFD
Mid bloom: ------- (DLI 46.80) ------- 722 PPFD
Late bloom: ------ (DLI 54) ------------ 834 PPFD
Ripen: -------------- (DLI 28.80) -------- 445 PPFD

The calculator I used for these conversions is located here:
Daily Light Integral (DLI) Calculator

I'm curious just how much the numbers go up for:

12/12 during a photoperiod flowering period:
Early bloom: ------ (DLI 36) ------------ 834 PPFD
Mid bloom: ------- (DLI 46.80) -------- 1084 PPFD
Late bloom: ------ (DLI 54) ------------- 1250 PPFD
Ripen: -------------- (DLI 28.80) -------- 667 PPFD

I was surprised by how low his recommendations are for DLI values. In the same way that the folks at growlightmeter.com have zero research for their recommendations, I haven't seen or heard of any research that Shane uses to support his recommendations. However, he has geared his products to support those light levels so I'll give him marks for consistency.

The light saturation point for cannabis is 800 to 1000 µmols and cannabis can be grown under 24 hours of light. My past three grows were autos and I ran them using 18/6. As far as the plant is concerned, there's no need for that. My current grow, germinated 9/1/22, was going to be 24/0 but I checked my electricity bill and discovered that my rate per KwH was 61¢ between 1700 and 2000 versus 34¢, so I went to a 20 hour day with the lights off between 1730 and 2030. That'll save me $250± per grow so I made the change. Were the cost of electricity level, I'd run 24/0.

Re light cycles - there's nothing that cannabis does during lights out that it doesn't do with the lights on. The "dark cycle" is a complete misnomer, the real name being the Calvin cycle.

Re. DLI's - Bugbee's most popular video and the paper by Chandra illustrate that the curve for net photosynthesis (Pn) starts to roll off when PPFD > 500µmols. I assumed that Pn was a proxy for yield and accepted that, even though it's not a linear relationship, the electricity bill would be higher but at least I'd be getting more weed. That turned out to be close but not quite on target.

The key point is that we're not harvesting photosynthesis, we're harvesting (well I'm harvesting) cannabis flower and the good news is that, while Pn is not linear, it turns out that plant yield, crop yield, and crop quality are linear meaning that the more light you give cannabis, you get more growth from the plant and the ratio of flower to inflorescence increases. Refer to the Bugbee video about indoor hemp production - it's extremely informative about the relationship between DLI and yield, as well as a host of other issue. There's other research that confirms Bugbee - I can find the cites if you're interested.

I'm a believer in "big light" but I ran "regular" levels for a couple of grows and, sure enough, everything turned out fine. About halfway through the third grow, after watching a DeBacco video for the Nth time, I decided to "turn it up to 11". Check the grow journals at the links and you'll see how things turned out. My light levels for my current grow are below. The DLI's are for the average values from the two plants. The only time I've seen any evidence of too much light is when some of the PPFD values were well into the 950 - 1050 µmols range. At 850 to about 950, this strain is doing quite well.

Again, running autos at a DLI of 40 moles or gradually ramping up light levels will get a very acceptable crop but there's no particular reason to limit your yield by sticking to those light levels. Per Bugbee, a lot of his clients are leaving a lot of money on the table by not giving their plants high light" and, as long as the rest of the grow is squared away, cannabis will click right along at DLI's > 60 mols.

and a family portrait:

 

[ExNavyInSTL]

I think you may have misunderstood my premise for my personal philosophy about hanging lights. I always thought hanging it high was the best way.

Here is the paragraph where I made this known:

"I ask all this because I think I am right with my assumptions. Mainly, because I like having my light way up high and out of the way and then dialing in the PPFD or DLI to what is required in the moment. With the light up at the top of the tent then any heat created is no longer an issue. Some might say, more importantly, keep it up high to quit banging your head on the light."

However, I agree with you. My hypotheticals were definitely off.

No you are not correct, let's take this 6" hang vs a 36" hang both at 100%.

^ Look at those hot spots vs cool spots. It's about double the intensity from hottest to coolest.

As you can see above, the further away the light is, the more even the distribution is across the full area, no hot spots with the physical raising of the light.

Going by your logic, if you use the dimmer in the 6" hang, you just lower the intensities of each square area.
There will still be hot spots and cool spots but at lower intensities.

Whereas if you raise the light, the intensity is distributed more evenly across the full area, so it is definitely not the same as using the dimmer dial.

I hope this helps with your understanding.

Good question btw.

j

 

[bluter]

Higher intensity from further away is more effective
The inverse square law applies to density of radiation, not volume so essentially it's still more light

check in to how a led photon acts as opposed to actual sunlight or one produced by hid. it's messed up. i always thought it was all the same, i was really wrong.

the throw on a decent led - distance traveled / penetration / is crazy, but it is super directional, and has sharp drop off.

 

[Fenderbender]

check in to how a led photon acts as opposed to actual sunlight or one produced by hid. it's messed up. i always thought it was all the same, i was really wrong.

the throw on a decent led - distance traveled / penetration / is crazy, but it is super directional, and has sharp drop off.

I see penetration coming from the directionality.. LEDs shine mostly straight but with HPS or so it goes out in all directions and bounces around in all directions getting more light in under the top canopy at angles or through reflections whereas with LED there is very much less energy available at an angle to the lights so there's also less to bounce off the walls or like in a multiple lamp setup a plant is going to get less side lighting from other lamps in the setup with certain type of leds.

 

[SmokingWings]

both intensity and penetration are reduced in both cases. the same as hid or other source.

Pretty much what I was thinking.

Brought up the topic since so many growers become concerned about the light penetration, especially those that are new to growing under lights. Something else to think about when staring at the plants.

 

[bluter]

I see penetration coming from the directionality.. LEDs shine mostly straight but with HPS or so it goes out in all directions and bounces around in all directions getting more light in under the top canopy at angles or through reflections whereas with LED there is very much less energy available at an angle to the lights so there's also less to bounce off the walls or like in a multiple lamp setup a plant is going to get less side lighting from other lamps in the setup with certain type of leds.

pretty much what we learned. hid was great at single source, like the sun, and i would agree that it had better bounce.

led on the other hand is best at direct source. 'corn cob' led lights are intended to replace commercial high bay lighting, and are built with many emitters in a 180 degree pattern, in order to splay the light out in a similar fashion to standard hid. otherwise high mounted led tends to create pools of light directly under the fixture, with sharp drop off at the edges, similar to a spotlight.


edit : when i set up a space with led i like to use wall to wall lighting. i want most of the ceiling covered by the rig.

 

[Jaz]

I was surprised by how low his recommendations are for DLI values.

He lives in Ireland where it's not legal to grow cannabis; it's why you see houseplants under his lights.

This DLI calculator has a calc. feature specifically for cannabis, one for houseplants and one for fruit and veg.

Photone Plant Lighting Calculator

With the light up at the top of the tent then any heat created is no longer an issue.

Unless you want a bit of warmth in there.

During winter I keep the driver in the cupboard down close to the pots, when the weather warms up I keep the driver outside. I also adjust the light hours schedule to the weather/season; off during the day when outside is warm, on during the night when it's cold to save turning on a heater.

j

 

[Delps8]

He lives in Ireland where it's not legal to grow cannabis; it's why you see houseplants under his lights.

The lat and long of his location in one of his YT videos placed him in/near Dublin, IIRC.

Possession and/or use of cannabis in is illegal in the United States.

I'm not clear on what that has to do with recommended DLI for cannabis.

This DLI calculator has a calc. feature specifically for cannabis, one for houseplants and one for fruit and veg.

Photone Plant Lighting Calculator

I saw that and it even puts in a silly little "calculating" graphic. Ugh.

The calculator doesn't differentiate between photoperiods and auto flowers though other parts of the site do. You'll also note that there are no citations for the recommended values and no mention of research.

I recommend that site for info re. grow lighting basics but, lacking citations for their recommendations, I look at that site, as I have learned over the 35 years that I've been using the Internet and its predecessors, "it's just dots on the screen".

 

[Fenderbender]

pretty much what we learned. hid was great at single source, like the sun, and i would agree that it had better bounce.

led on the other hand is best at direct source. 'corn cob' led lights are intended to replace commercial high bay lighting, and are built with many emitters in a 180 degree pattern, in order to splay the light out in a similar fashion to standard hid. otherwise high mounted led tends to create pools of light directly under the fixture, with sharp drop off at the edges, similar to a spotlight.

Most professional installations I see the leds way up high unless it's for shoots or so, but then of course the whole ceiling is plastered with them.

I was thinking about bounce though as now I've scrogged wall to wall with a LED and had very poor penetration as I had a bunch of albino buds underneath not being able to do photosynthesis versus the most well round buds I ever grew was in a 1x1m tent where I only had one plant in the middle with a HPS above it, there all the buds were very well formed all over & under the base of the bud was really fat and then a big top grew out of it, now my tops are fat but it gets slimmer down the branch as the canopy gets very dense.
So you kinda don't want to use the entire space but a gap wide enough for light to bounce sideways into and underneath the plant and the distance of the wall just in the sweetspot for a boosting effect of the lights.