Changing distance to plants vs dimming

am about to switch to led, been reading threads for monthes and still have a question :

for lights adjustments ( small plant, full veg, bloom ) people seem to play more often with distance to plants than with dimming ( maybe I'm wrong ? ) despite most lights seeming dimmable ( maybe I'm wrong again ? ).

Is there a reason for this ? or am I wrong on my assumptions ?

Thanks and have a nice day !

Playing with distance is a waste of electricity. You are wasting the light on your walls to reduce the light on the plants. Bad idea, but perhaps people do this because it's easier.

Personally I leave the lights at the correct height for optimal uniformity and dim them if needed.

Thanks @wietefras ! nice datalogger

The only reason to play with distance is if your light isn't dimmable.. Otherwise do as stated above..

Thanks @HydoDan !

Use both.

HydoDan said:

The only reason to play with distance is if your light isn't dimmable.. Otherwise do as stated above..

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Shouldn't you also take into account the light spread if you have it really close and dim it you might not get enough light to the outer plants

I fall in the other camp. Here is a quote from a different thread, and sums up why I choose to raise rather than dim.

Greengenes707 said:

Dimming lowers PPFD(effects photosynthetic rate) as well as total energy(effects metabolism) in the grow area.

When you dim, you provide less over all photons, but maintain the same light/reflection pattern, and capture/incident angles. This shows as lower numbers on the quantum meter. And is ok to compare them 1:1, as all the variables except power were constant.

When you raise the light while maintain full PPF, you don't really lose the energy. You also don't lose the same amount of photons over the crop that is shown on your single spot meter. When the lights are raised, the light pattern is changed as is incident angles. It is impossible to show and compare with out a 3D map showing and recording every bit of light that is received by the canopy from all angles.

Click to expand...

I get faster growth, and hardier growth even though the peak PPFD are equal.
Both ways grow just fine. And variables will influence the better choice per situation.

Danny88 said:

Shouldn't you also take into account the light spread if you have it really close and dim it you might not get enough light to the outer plants

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It was more about been to far than to close : I read stuff like 3' away, seems a lot !

Danny88 said:

Shouldn't you also take into account the light spread if you have it really close and dim it you might not get enough light to the outer plants

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This. You should have the light source at a height that covers the surface of your canopy and then dim down to the desired PPFD.

Photon Flinger said:

This. You should have the light source at a height that covers the surface of your canopy and then dim down to the desired PPFD.

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Ah righ yes I get you now

Hi @Greengenes707,

thanks for your input ! so 3' to start is ok ?

Thanks @Photon Flinger !

That's what I was planning to do.

Then came @Greengenes707 !

drgroove said:

Hi @Greengenes707,
thanks for your input ! so 3' to start is ok ?

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With out any info on the fixture, it seems higher than necessary. My goal is to hit/maintain a peak PPFD figure. If PPFD is what you want when it's at 3' then ya, it's fine. I know not everyone has a meter handy so it's not as simple as just hit "x" ppfd for some people. So a general rule of thumb to start at, has always been 24' for most fixtures and builds. And most all plants end up being comfortable closer, but not to many respond poorly at 24" or even higher.
More common is people to crunch lights down on top and seeing negative effects, rather than starting too far away and creeping down.

Thanks @Greengenes707 !

Greengenes707 said:

I fall in the other camp. Here is a quote from a different thread, and sums up why I choose to raise rather than dim.

When you raise the light while maintain full PPF, you don't really lose the energy.

Click to expand...

You do lose quite a lot of light. Do a lux or PPFD matrix and see how much the average drops.

Malocan did this already and he measured a 20% loss of light by moving the lights from 12" to 24":
31.5cm / 12"


60cm / 24"


Although it looks like his fixture was too small for the space. If the COBs would go all the way to the sides, the uniformity would already be fine at 12". Then the wall losses after moving the fixture up would be even greater though.

12" is probably optimal for most builds (without reflectors).

Thanks @wietefras !

Nobody has mentioned it yet but I raise and lower my light at certain times of the year simply because I need the heat output of it to keep my temps in check.

wietefras said:

You do lose quite a lot of light. Do a lux or PPFD matrix and see how much the average drops.

Malocan did this already and he measured a 20% loss of light by moving the lights from 12" to 24":
31.5cm / 12"
View attachment 3962012

60cm / 24"
View attachment 3962014

Although it looks like his fixture was too small for the space. If the COBs would go all the way to the sides, the uniformity would already be fine at 12". Then the wall losses after moving the fixture up would be even greater though.
12" is probably optimal for most builds (without reflectors).

Click to expand...

Did you read the quote? Here it is again...

Greengenes707 said:

Dimming lowers PPFD(effects photosynthetic rate) as well as total energy(effects metabolism) in the grow area.

When you dim, you provide less over all photons, but maintain the same light/reflection pattern, and capture/incident angles. This shows as lower numbers on the quantum meter. And is ok to compare them 1:1, as all the variables except power were constant.

When you raise the light while maintain full PPF, you don't really lose the energy. You also don't lose the same amount of photons over the crop that is shown on your single spot meter. When the lights are raised, the light pattern is changed as is incident angles. It is impossible to show and compare with out a 3D map showing and recording every bit of light that is received by the canopy from all angles.

Click to expand...

I fully understand and said it will change on the spot measurement readings. But without a 3d model of the canopy topography and incident angles of the light source it's not the full story of the entire crop.

If you want PPFD charts at different heights, I got plenty for you. But again, doesn't show the whole picture of full power raised dimmed at the same height.



I have personally grown both ways. And prefer full and raised to hit a lower PPFD. I simple get more growth faster, and is hardier.
I am also a proponent of not dimming or backing off that much anyways.
In the natural environment, a cannabis plants receives the most intense light and most DLI during the peak of veg and beginning of flower. Then decreases as flowering progresses. And even in early veg/spring, get very high ppfd levels, with a slightly lower DLI via the day's durations.

One more side by side to do, unfortunatly I'm far from being able to do that.

good point @Danielson999 !