12/12 Light Dimming?

Now in 12/12 cycle with 1000w equivalent full spec led 5 x 5 area. Do i need to dim to 85% or less for flowering?

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Make/model of the light?

Are you growing autos or photos?

In general, you want as much light as you can give them in flowering. DLI (daily light integral) is a common way of managing light levels.


I go 100% for flower. You can dial it back if need be.


vivosun vs4000, no autoflwrs.

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Good light, but not enough to fully light a 5x5 for flowering. A good rule of thumb for the lights built with Samsung LM301h diodes is ~35 watts per square foot of grow space. At 400 watts I would expect the light to adequately flower 2 plants.

The short answer to the question is that you should run the light at 100%.


With no change in intensity, the moment you flip to 12/12 the total amount of light is reduced. I.e.: going from 18/6 to 12/12 you’ve reduced the light by 1/3.


Considering your information, all these plants, 3 cali dream and 2 skywalker, are very healthy and were bushy until trimmed, big fan leaves, 36 inches tall and main stalk is larger diameter than a quarter, so they seem to be doing well with this light source.

the vs4000 led input wattage is 400, the light output is more like 1000w cmh equivalent and plant tops have grown a 1-2 inches since the 12/12 cycle 1 week ago. so good results so far, stay at 100%


This is not possible unless Vivosun has figured out how to defy the laws of physics. 400 watts is 400 watts and draw from the wall is the metric that counts. It’s simply not possible to make 1000 watts of output with 400 watts of input. The 35 watts metric I mentioned above is for high-efficiency LEDs. CMH is far less efficient and a lot of that wattage goes toward making heat rather than light, hence the disparate comparison.

They are likely to do fairly well during veg. Cannabis is a very light hungry plant. You will get bigger, more dense flowers with more lighting.


What he said: :point_up_2:

Not saying that 400w input to led would expel 1000 watts of output! Im talking about the total lumination! If 400w led input results in only 400w of lumination then why go to an led for anything? Could it be cost of power consumption or amount of light produced? like the walmart brand light led bulbs and ge and others that say on the package they use 9 watts of power but produce 60 watt equivalent incandescent bulb light emission? Is that wrong? This vs4000 is bright as hell without having 1000w MH consumption and the heat, works perfectly and plants are huge.#

The problem is how to compare different quality lights (LED’s) with each other and they’re not all the same. Using ‘watts’ is a poor metric while actual photons is what is important. MH/HPS or CMH puts out roughly 125 lumens per watt with a fair amount of heat. A high end LED fixture puts out over 200 lumens per watt with far less heat generated. “Comparing” LED’s to conventional lights has been going on for years and is confusing to the user.

You need to learn about Photosynthetic Active Radiation, Photosynthetic Photon Flux Density and Daily Light Integral. This is a more technical and accurate representation of how much light is needed.

The reason your plants are doing well under 400 watts of LED’s is plants in veg need about half the light needed to properly flower out your grow space. All growers need to get past the hype and look at actual photons delivered to the grow space. And you don’t have enough.

My gen 1 Samsung LED fixtures (diy) deliver 156 lumens per watt in a flowering spectrum so most of the photons generated are usable by the plant. I typically run 200 watts, per plant to achieve decent yields.

You can argue all you want but physics is physics. Plant biology is plant biology. Getting caught up in the hype and misrepresentation of light manufacturers’ claims will only cause you trouble downstream. Lots of growers here trying to advise you that have already gone down this road.

You could grow two plants to full flower under that light. If that’s what you want then you are in good shape.


I can never pass up the opportunity to run numbers. This is VS4000 PPFD map hung at 18"

The size is a little odd. But each square represent 6 2/3"
I added the values in the Black and Blue lines. The area inside the blue square is roughly 4’x4" more precisely 15 sq ft. The area inside the black square if roughly 3x3. More precisely 7.7 sq ft.
I obtained the average PPFD within the two areas as noted and also calculated its DLI for 12 hours.
The main point is the intensity (PPFD) drop significantly as you move away from the center.

Below is a chart giving recommended DLI for 18 hours and 12 hours of light. Many of us run considerably higher than what is given in this chart.

If the plants are centered to take advantage of the higher PPFD they will probably do OK. If they are pushing out to the walls they will come up short on light (DLI)


Here are specs on that light. I’d hang it at about 16 inches from canopy. Maybe 12 but due to the hot spot on quantum boards clear out the center of canopy under fixture so light is 14-16 inches at center.

It has the Samsung LM301 diodes, Efficiency rating 2.9 μmol/J. Not bad but PPFD not great around edge of fixture.
For future reference: Bar light fixtures spread light a bit more evenly.


Sure isnt. 2 550r 480w BARELY covered my 5x5. Switched to different shape lights. 3 300w. That light you have is about size of a 550 with less horsepower. If I wasnt getting full potential with 2 no way gonna snatch it with 1. Just my opinion. If cant get another…tighten the footprint to 3x3 or 3.5x3.5. Keep plants there and screw the edges for now. Same diodes. Prob not same bin but lm301b samsung.

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Ah, well, I do have an alternate high photon output light source that Im sure will help with the PAR levels, its a free of cost wireless system 93 million miles away and its called a yellow dwarf.

Plants are huge and very full already and still growing, light is at max height as well, i just hope they dont do a growth spurt like corn does and end up reaching the light

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I used to do that when I lived in SoCal. Definitely lowers your electrical costs.


You can do some low stress training(LST) to lower the canopy. Just do some light bending of your tall main branches.
Low stress training is bending but not breaking the stems. You can use lattice or twine of string to bend them about 2 foot away from fixture.
The first 2-4 weeks of flowering your plants will “stretch” setting bud sites. Not a good time to cut or break any branches with bud sites. LST is the best way to amend them now.

Voltage is an input.
Wattage is the energy consumed in the circuitry.
No product has 400WATTS input (no 400 volts, for an LED light).
Forum got the answer right , above.

I agree. two 350Rs would do my 4x4 just fine.
I currently have a Vipar Spectra V1200. The sellers write good words to sell junk.
The V1200 is not close to the HLG350R, except for generating heat and using twice the power.
Watts meter and LUX dialed in for 15-20 inches flowering ladies.
V1200 and HLG 350 almost fill tent space

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