CobKits
Well-Known Member
thats awesome i cant wait to see their results, ive been out of the loop a bit lately
Intro to led - feedback
- Thread starter Palomar
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CobKits
Well-Known Member
my par meter at 18" from a 9 cob array n a tent says otherwise, its very uniform.
your case only holds water if people are growing right up to their cobs, which they arent in most cases. at appropriate distance there is zero difference between a cob rig and a board rig in "penetration" and "shading". both act as the "wide light source" you cited on the last page.
nfhiggs
Well-Known Member
That's at 18". How's the uniformity at 8"?
But they are growing right up close to their STRIPS - which is the strips advantage. They have similar light levels at 18" and below (if running the same wattage and efficacy) but allow growth right up within a few inches of the strips.
CobKits
Well-Known Member
it matters not as theres no plants there
yes thats an advantage of strips, being able to get closer
however being that close to any distributed light source will give you tremendous difference in par once your distance drops
if you are 18" from your light source, going from 18 to 24" (6", 25% farther from the light) gives a much more uniform PAR from top to bottom of canopy than going from 3" to 9" (6", 300% farther from the light)
just because you can grow close to a light source doesnt mean you should.
same reason that people who use T5s still keep a distance from the lights, even though you could technically grow right up to them
while i try to keep canopy at the same distance sometimes it goes wild and ive had colas grow up to 6" directly under a 75W cob to no ill effects. its not a 1000W bulb so the "problens" reported are minimal
again you cant get ideal PAR ("similar light levels") at 18" and at the same time as ideal PAR right up against the strips. if you have 700-1000 PPFD at 18" youd be well over 2000 PPFD right up against the strips - you cant have both, physics is against you.
the only way to get more uniform par from top of canopy to the bottom is to increase distance of light source from the canopy, so that the ratio of d1 and d2 are as close to 1 as possible (d1=distance from light source to top of canopy, d2=distance from light source to bottom of canopy)like an HID at 36-48", or as a "perfect" example, the sun where d1=93M miles and d2 = (93M miles+8 feet), d1/f2= 1 and allows you to have an 8 foot tall plant with same intensity of light at bottom of plant as the top.
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Aolelon
Well-Known Member
My plants atm are 12" from my 360w qb/fstrip build with no I'll effects.
nfhiggs
Well-Known Member
No, the physics is with me - do some ray tracing - and don't forget reflective walls are part of the equation. Its all about overlap and reflection. And 6" or even 3" is not "right up against" the strips. The point is that at 18" you have all of the light sources contributing photons to a given spot. At a 6" distance you have, for the most part, only the diodes that are within a 6" radius contributing to a given point. The closer you get, the fewer diodes are "seen" by a given point. Graying Geek showed this in his measurements. He showed less than a 50% drop in light intensity from 4 inches to 24 inches, if I recall correctly, it went from something like 50k lux to 33k lux..
OK, follow me on this. One big, bright ass cob at 300W - aside from the efficiency loss, why does no one use that? What's the PPFD at 24" and 48" in a 3x3 reflective tent?
OK, now break that 300W up into four 75W cobs - better right? Ask yourself why. What's the PPFD at 24" and 48" for a 3x3, assuming equal spacing?
Now break that up into twelve 25W cobs - what does that get you? Again what would you see at 24" and 48" in a reflective 3x3?
Assuming equal efficacy of the above scenarios, the total PPF output of each one is the same, correct? so PPFD levels (once you get to uniformity) in the same size tent are going to be the same are they not?
Do you not see that it is simply a mathematical progression towards a theoretical ideal? Strips with lots of diodes is just several more steps down that progression from multiple COBs.
That is what conventional wisdom tells us, yes - BUT, you are forgetting that the grow tent is an enclosed, reflective space.
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skoomd
Well-Known Member
All solid, good info.
Here's a picture from an older grow of mine where I had diodes nearly touching the plants. No burning at all, not even close lol. This clearly shows that when you get super close to spread out diodes, it wont burn because only the diodes near each part of the plant hit that part of the plant.
And yeah, my samsung diodes (single row 22" strips @ 32w each) are only 1200-1500PPFD once you get less than 1 or 2 inches away from them. But at 10" the PPFD is about 750 and at 24" the ppfd is 660. At 36", it is 500.
CobKits
Well-Known Member
long read and i feel were on the same team but both taking extreme positions for the sake of argument
but link this id like to see it, and ill respond
ive got a 9-cob rig in a 3x3 tha ti can take some measurements with, an throw some boards in and do the same
i would say that my 9 cob rig (they are about 12" center to center) would give the exact same measurements at 24" and 48" as a board/strip rig of equal efficacy. once your distance of measurement is at or greater than cob spacing the light is uniform and its the exact same principle of overlap/incidental/combined light we are measuring
the only advantage of strips is close up uniformity really (if you are using it). and then when you are stuffing colas in the 6-12" zone below your lights the measurement you took in an empty tent of 'uniform par' 18" down is irrelevant because you just shaded it regardless of how many points of light you have
i mean i get your points but these strips arent some magical unicorn that gives you a perfectly lit canopy 24" deep at uniform levels. otherwise people would be doubling their yields with them on every grow. remember the entire discussion arose when skoomd claimed that cobs were all problematic contrary to the results of almost everybody whos ever used them. yes you can burn a plant by getting it too close to a light. thats HID vs T8 101, but thats grower fundamentals not the lights fault.
but link this id like to see it, and ill respond
ive got a 9-cob rig in a 3x3 tha ti can take some measurements with, an throw some boards in and do the same
as one aside your breaking 300W cobs into 75W cobs into 25W cobs makes sense but at some points you are in diminishing returns. the difference between 4 cobs and 12 cobs in a 3x3 isnt as apparent as you think there would be little to no difference in average par at 24 and 48". 24" might have a little bit more variation, thats about all. neither strips nor boards are going to magically throw more light deeper in an empty tent. theres still only so many photons to work with
i would say that my 9 cob rig (they are about 12" center to center) would give the exact same measurements at 24" and 48" as a board/strip rig of equal efficacy. once your distance of measurement is at or greater than cob spacing the light is uniform and its the exact same principle of overlap/incidental/combined light we are measuring
the only advantage of strips is close up uniformity really (if you are using it). and then when you are stuffing colas in the 6-12" zone below your lights the measurement you took in an empty tent of 'uniform par' 18" down is irrelevant because you just shaded it regardless of how many points of light you have
i mean i get your points but these strips arent some magical unicorn that gives you a perfectly lit canopy 24" deep at uniform levels. otherwise people would be doubling their yields with them on every grow. remember the entire discussion arose when skoomd claimed that cobs were all problematic contrary to the results of almost everybody whos ever used them. yes you can burn a plant by getting it too close to a light. thats HID vs T8 101, but thats grower fundamentals not the lights fault.
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skoomd
Well-Known Member
Take readings in the canopy. The PPFD might look good on a flat surface, but in the canopy is where the difference between cobs/qbs/strips becomes very apparent.
4 cobs in a 3x3 and 12 cobs in a 3x3 is a fairly big difference really. Even 12 cobs in a 3x3 tent, you couldnt go nearly as close to them without burning as you could with even 6-10 strips. Like i said earlier, no matter how many cobs you have (unless you were doing maybe like 8 per square foot lol), they will act as little spot lights over your canopy. Whereas running 75-100 little >0.3w diodes spread out per square foot only acts like little spotlights at 1 or 2" away.
I can take readings and compare yours to my 10 strip build at 24". But I already know the outcome lol.
This is 20x cobs in a 4x4 tent, 32 watts per square foot. Taken at 24" distance. The PPFD is 596. This is nearly the same cobs per square feet as 12x in a 3x3 (1.25 for this, 1.33 for 12 in a 3x3).
The uniformity is good, BUT......
Compared to my 10 strips in a 3x3 tent, taken at 18" distance, it's just not nearly as uniform. The corners are pretty similar, but the sides and center readings of my strips are much more uniform than the COBs. And my PPFD at 12" is 720 versus 695 PPFD at 18". The small change is because of the huge goldilocks zone. 3% difference between 720 and 695.
For comparison, here's that COB build at 12" distance. 744 PPFD, which is 25% higher than 596 PPFD at 24".
And my strips at 12". The corners on both builds are again similar, and my left/right sides are actually not as good as the COBs but my top/bottom sides are.
The big difference is the center readings. Overall, mine is still much more uniform.
CobKits
Well-Known Member
to get a true measure of shading and "penetration" ill make some checkerboard shades that take out 10%, 20% etc of the light and measure that way.
this arbitrary my canopy vs your canopy is useless anecdotal data tbh
will look at your data later
its a little apples vs oranges tho.not sure of how far the cob measurements go, is that 3x3 grid, or are the corners 2" from the side of the 4x4 tent?
i will say that first cob rig measurement at 24" looks fantastic
and ill bet your strips at 24" would look damn near identical
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skoomd
Well-Known Member
Yeah it's anecdotal. But a good piece of data regardless.
As for how his light was built, it is about 6-8" from all sides of the tent. So 40x40 - 42x42". Mine is 3-6" from all sides. 24x30".
Defintiely not a perfect comparison, but good for some fun data.
The PPFD uniformity using the lowest corner of my tent vs. the highest middle reading = 0.37 @ 12" and 0.61 @ 18".
The PPFD uniformity using the lowest corner of his tent vs the highest middle reading = 0.39 @ 12" and 0.54 @ 24".
So my ppfd uniformity at 18" is 13% better than his at 24".
And if we factor in how much more even my strips are in the middle at both heights, the overall PPFD uniformity is much better at 18" than the cobs are at 24".
CobKits
Well-Known Member
i meant the measurements, is that your grid in electrical tape on the bottom?
and if so how can we compare the strips that only measure out to a 3x3 square vs the cob readings all the way to corners of the 4x4? the deep corner measurements will *definitely* lower uniformity
and if so how can we compare the strips that only measure out to a 3x3 square vs the cob readings all the way to corners of the 4x4? the deep corner measurements will *definitely* lower uniformity
skoomd
Well-Known Member
Not my cob build, but yes it is. And I have a grid to put into my 3x3 for PPFD measurements too. But not that many spots lol.
It's comparable because both lights are similar in size for the space they are in, and use similar numbers of cobs per square feet. Mine is 1.11 strips per square foot, that cob build is 1.25 cobs per square foot. And similar watts/square foot.
I'll have the 24" PPFD map for my 3x3 strip build a bit later, lights are off though for another 5 hours.
nfhiggs
Well-Known Member
LOL - yeah I do tend to do that sometimes.
If you're referring to Graying Geeks measurements I'll try to dig it up
Yes, exactly - but its the area between 24" and 0" that is primarily affected (by distributing the light source). That zone becomes more homogenous - while the 24" and 48" distances are relatively unaffected, the point at which uniformity is achieved moves upward towards the lights with each reduction.
You are correct in that there is a point of diminishing returns. Once you get to less than 6" spacing between light sources, the gains become relatively small. The thing about doing this with COB's is that it gets astronomically expensive to go below one per square foot. Even one per square foot can get pretty pricy. I tend toward favoring strips because they are economical in this regard while still being stupid efficient.