COB distance

Megalomando said:

I started the thread awhile back but still haven't found the answer yet.

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i think its been discussed a few times in this thread thats because there is no universal answer

plant size, root health, temp, humidity, growth stage, genetics, air circulation, optics/wall reflectance, nute strength, soil vs hydro, bennies, etc.make every single garden unique

most cobs without reflectors will put out about 500 ppfd at 12" at 50W. 500 ppfd might be too much for 24 hour vegging, and it might not be enough for a flowering setup.

you need to experiment with your setup. distance is your friend esp in a tent, as it makes the light more uniform across the canopy

If you have bleaching only in the upper area, but the level below it does not have bleaching, then the logical step is to increase the lamp by at least this distance. Saying between the floors are 4-6" and current heights is 1', raise your lamp to 1,5-2' and the whitening should stop!

Buy a simple lux meter from .bay or ... zon (12-15 $) and do a few readings. 50-60k lux is near the maximum without additional CO².
But already high nutrient levels required and probably CalMag necessary. If you stay under 50,000lx, you should not get any problems with bleaching.

75 w cree cxb 3590's colas 3-4" away, no bleaching only a few scorched fans from nearly touching the cobs. 50w cxb3590s 10" away no bleaching. (My environment inside the tent is spotthefuck on tho).

Randomblame said:

If you have bleaching only in the upper area, but the level below it does not have bleaching, then the logical step is to increase the lamp by at least this distance. Saying between the floors are 4-6" and current heights is 1', raise your lamp to 1,5-2' and the whitening should stop!

Buy a simple lux meter from .bay or ... zon (12-15 $) and do a few readings. 50-60k lux is near the maximum without additional CO².
But already high nutrient levels required and probably CalMag necessary. If you stay under 50,000lx, you should not get any problems with bleaching.

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Sorry, been away and just got back. I tried getting some photos but none came out well with the lighting. But it definitely seems like the LEDs are burning the leaves. I see perfect green leaves just under bright yellow ones where the upper leaves prevented direct light. I had no idea the COBs would put out so much light as to damage the plant.

So taking your suggestion of a Lux meter which I never had considered, I find quite a few of them. Reading on light meters and it sounds like a PAR meter would be better for LED but I have no idea, this is all new. So what would be my better choice, a PAR or a LUX meter? And any suggestions on which would be a reliable one that won't break the bank? I'd pay up to $100 or so for the right tool but if what I need costs $30, that's more better.

Any suggestions to a particular meter?

Thanks!

Megalomando said:

I started the thread awhile back but still haven't found the answer yet. I am running 3 heatsinks, each has 4 CXB3590 I'm growing in a 4x4x7 enclosure with tips at least 1' away from any part of the plant. Even when keeping the power down to 150W per heatsink I am finding bleaching and browning leaves.

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The height of the fixture is determined by the desired light distribution. You don't hang your heights higher to get less light on the plants, but to prevent hot spots exactly under the light.

I'm guessing you have these GPLed or Todoledgow fixtures with the COBs about 30cm apart. Then the height should be fine around 30cm to give an adequate light distribution.

If not then you are supplying the plants with too much light. Then don;t hang the lights higher to waste more light on the walls, but dim them. 600W is a bit much perhaps for a 4'x4' tent, but healthy plants should be fine. If your plants are still suffering then you just have to make doe with what you ended up with. They will not recover in flowering since they won't develop much.

Next run try to keep the plants in a better shape while vegging and then see how much light they can take. For this round I'd suggest to dim it down again to around 450W and keep it there for a while. If that's still to much perhaps dim it back even further.

Is your climate ok? Not too dry, too hot or too windy? Those conditions can also make it more difficult for the leaves to cope with the light.

The fixtures are home made, COBs are CXB3590. 4 COBs are on one heatsink for 200W total. In Veg, the COBs are all 5000K and in Bloom, they're placed alternating as 3000K - 3500K - 3000K - 3500K on each fixture. In veg they have a 5000K dedicated similar source at the lowest power & with no light damage; nice green leaves everywhere.

If there is too much light in the tent then I have to reduce that. Each driver is adjustable and while I can connect an in-line watt meter to tell the power, the idea mentioned to measure the light at the tips makes sense. I would rather have the light just right than too much or too little. Humidity is around 60, temp is around 85, air circulates through the tent and feeding is twice a week.

I've done a good job keeping the plant non-stressed in feeding/temp/humidity, but excess lighting does seem to be the issue at this time. So I do want to work on the light energy and need to know with LED lighting, what is a worthy meter to get and then, what is the optimal light reading/number of the light at the tips?

One meter shows LUX and another shows PAR. If PAR is better with LED, then what value of PAR?
If LUX, is 50K my goal at the tips?

Ah ok. If you have 12 COBs in that 4x4 then they would be about 30cm apart. If you keep the height between COBs and plants the same as the spacing between the COBs then the distribution should be good.

Probably the total amount of light is the issue. Your lights are probably producing around 1400umol/s over a 1.44m2 surface. That's 1000umol/s/m2 PPFD. You'd need to deduct wall losses, but normally people go for 700 to 800umol/s/m2 (before wall losses). If you dim to 150W, you should be in that region.

If you divide the lux values by 70 you should get a decent indication of the PPFD. So yes, 50k should be fine. Or with the PAR meter something between 700 and 800umol/s/m2 on average.

The peak values can be a bit higher though.

Still if your plants can;t deal with what you are giving them, perhaps go the other way around and measure the intensity where the leaves still look good. Then you can go with that as the maximum on the tips.

Excellent ideas. Any suggestion of a decent (accurate) PAR meter? I can just buy one but I don't know if one brand/model is reliable & accurate.

Thanks!

Personally I use a lux meter. I feel it's really not that important to be slightly more accurate and a good PARmeter (ie significantly more accurate than using a lux meter) costs hundreds of euro's. If not a thousand euro.

wietefras said:

If you divide the lux values by 70 you should get a decent indication of the PPFD. So yes, 50k should be fine. Or with the PAR meter something between 700 and 800umol/s/m2 on average.

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Hi, it's the first time I hear this.
Could you please argue this theory? does it apply to any type of LED (COB, QB) and brand?
best regards

rocho said:

Hi, it's the first time I hear this.
Could you please argue this theory? does it apply to any type of LED (COB, QB) and brand?
best regards

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There are several threads discussing this topic here in the led forum section. Alesh gave us a whole explanation of how to convert between curves (SPD curve vs lumen curve) and how to calculate the conversion factors.

The factor is different for each spectrum of light unfortunately. Although it usually doesn't matter that much, so 70 is a decent compromise for a quick indication. Especially for the same CRI and color temperature there will hardly be any difference.

Still you can calculate it from the charts and there are plenty of these conversion factors posted. Some people use a multiplier and others (like me use a divider): See this thread with some numbers: https://www.rollitup.org/t/par-multiplier-thread.928907/#post-13196094

16 3590's in a 45"x45" area here. One way I keep from "bleaching" since my canopy isn't necessarily even due to growing several strains is to tie the offending cola down or rotate the plant a bit to get it out from under direct light. An inch or two to the side usually does it. That's all I have to add as others have pretty much covered it.