Lowrider72
Well-Known Member
Kriptastic girls you have there Sir.
Loving the show, Im like a friendly stalker....just like lookin at plants...
hehe.
Loving the show, Im like a friendly stalker....just like lookin at plants...
hehe.
Sponsored Grow: KIND LED K5 XL1000 With Ace Seeds In High Brix Soil
- Thread starter Mr. Krip
- Start date
Evenin to ya Krip and friends. May i suggest putting your new light right at the top of your tent. You will have less messin with it while it will provide it's biggest 'footprint' and encourage even growth throughout the tent. Just looking at the companies recommendations,..if that light is as powerful as they say, then crank it up to full power , put it at the top and leave it alone. Just a thought...cheers eh.
- Thread starter
- #123
Evenin to ya Krip and friends. May i suggest putting your new light right at the top of your tent. You will have less messin with it while it will provide it's biggest 'footprint' and encourage even growth throughout the tent. Just looking at the companies recommendations,..if that light is as powerful as they say, then crank it up to full power , put it at the top and leave it alone. Just a thought...cheers eh.
It's a great suggestion IF coverage was the only issue but, as indoor growers, the biggest battle we face is the "inverse square law" which says if you increase the distance from the light source by a factor of X, the amount of light hitting the plant decreases by X-squared (raised to 2nd power). So, double the distance of the light from the plants, and 1/4 the amount of light will reach the plants; at 4x the distance, you get 1/16th the light.
In other words, raising the light will cover more floor space but get less PPFD to the plants.
It's a great suggestion IF coverage was the only issue but, as indoor growers, the biggest battle we face is the "inverse square law" which says if you increase the distance from the light source by a factor of X, the amount of light hitting the plant decreases by X-squared (raised to 2nd power). So, double the distance of the light from the plants, and 1/4 the amount of light will reach the plants; at 4x the distance, you get 1/16th the light.
In other words, raising the light will cover more floor space but get less PPFD to the plants.
I'm not satisfied that this relationship holds true in a reflective tent. Once the light fixture is high enough so that the beam hits the sides of the tent reflections occur. Eventually that reflected light will reach your plants, although it will be diminished. The amount the light is diminished by will be a function of the reflectivity of the tent's material and how often it was reflected before a plant snatches it up. The height of the fixture in this case could be used to further even the light spread over the canopy.
In an open, non-reflective space, I am in full agreement with the "inverse square law."
Oh ya..understood, Krip, i know that, If i was gonna keep moving my lights up all the time to get that 'perfect' spot,..thats all i'de be doing. I guess thats why my GR is well lit , if i could raise them another couple feet, that would be awesome. Anyhow , that light is a beauty and you will do her fine...
- Thread starter
- #126
Well, believe me, it does hold true because it's a LAW of physics!
Let me try to explain (as a non-physicist!)....
If your light is hitting a reflective surface, it has already missed the plant. So, depending on the distance of the reflective surface from the light, some amount of PPFD will hit the reflective surface and be bounced off to either hit the plant or another reflective surface.
However, when the light hits the reflective surface, that surface now becomes the new light source. Each time the light hits a reflective surface instead of the plant, the PPFD is reduced and that surface becomes the new light source. The more times the light is reflected before hitting the plants, the less PPFD will actually reach the plants; so you can never get back to the same PPFD you would have had if the light had hit the plant directly and not been reflected (shortest distance between two points is a straight line
).Does that make sense?
Yes, perfectly ,..that's why its so much better to cover a grow area from many different directions. . For example,..it's way better to cover an area with two 400 W lights , as apposed to one 1000. Using only one light for a grow is already a disadvantage. ....
Well, believe me, it does hold true because it's a LAW of physics!
Let me try to explain (as a non-physicist!)....
If your light is hitting a reflective surface, it has already missed the plant. So, depending on the distance of the reflective surface from the light, some amount of PPFD will hit the reflective surface and be bounced off to either hit the plant or another reflective surface.
However, when the light hits the reflective surface, that surface now becomes the new light source. Each time the light hits a reflective surface instead of the plant, the PPFD is reduced and that surface becomes the new light source. The more times the light is reflected before hitting the plants, the less PPFD will actually reach the plants; so you can never get back to the same PPFD you would have had if the light had hit the plant directly and not been reflected (shortest distance between two points is a straight line
Does that make sense?
Exactly my point. The amount emitted by the fixture remains the same. The amount directly reaching the canopy is diminished. This direct light is supplemented by the reflected light that reaches the canopy. The reflected light plus the direct light that reaches the canopy is less than that emitted by the fixture due to the absorption of the tent walls. I still maintain that this can be used to even the light over the canopy. The only way to prove or disprove this would be to take measurements at various heights. I wish I had a quantum meter, or even a remote reading Lux meter.
This is why I used eight COBs on 1' centers in a 2' X 4' tent.
The amount of light lost on a reflective surface has to be pretty significant. Not that I make a habit of this, lol, but looking at my light and looking at my tent walls doesn’t have the same effect on my eyes. I will take a quick peek occasionally just to make sure all the diodes are functional.
I guess as you raise the light the readings at the edge of the tent would go up....to a certain point, and then they would start to read less there as well. Would that be the sweet spot for a given grow space.? The point where the edge of the space received the maximum amount of light?
I don't know. I'm going to have to measure it when I have the time and materials available. I'd order a Lux meter from Amazon, but I have too much to do before I need to put both of my tents into production.
- Thread starter
- #134
EXACTLY!!!
That depends and not necessarily. As you're raising the lights to cover the outer edges, the center of the tent is getting less PPFD and, again, that inverse square law is a bitch because whatever the increase in distance, the light reaching the plant decreases exponentially.
So, it depends on the light and the grow area, but to take it to an extreme, it's exactly why a single very small light isn't suited to a very large grow area. You really want the lights as close to the plants as possible, without causing negative reaction to the plants, to get the maximum possible PPFD.
- Thread starter
- #135
I agree it will "even" the light - it will just be at a reduced intensity as the distance from the light source to the plants increases.
I’ve been happy with my grows so far.....but I think I need another light!
- Thread starter
- #137
Just to clarify, the reflectivity is key because it takes photons that missed the plant and hopefully reflects some of those back on the plant, even if at a reduced intensity. Better less photons than no photons, at all!
Fanleaf took PPFD measurements and found that the Law doesn't work well with a highly dispersed source. Under Samsung strips the measured loss was closer to half, at twice the distance.
The other thing to consider is that no photons escape the tent, so it's only a matter of how much they get bounced around and decay.
Mostly, headroom will affect stretch. I mount my lights at the very top and move the plants up or down if I have to, but otherwise keep the tops at least 2 feet away.
The other thing to consider is that no photons escape the tent, so it's only a matter of how much they get bounced around and decay.
Mostly, headroom will affect stretch. I mount my lights at the very top and move the plants up or down if I have to, but otherwise keep the tops at least 2 feet away.
- Thread starter
- #139
OK, so I see how you guys are looking at this. Because of the reflectivity, using the inverse square law is not an effective way to gauge PPFD at various distances in a grow tent. I can agree with this BUT I maintain that the inverse square law still applies (again, it's a law!).Fanleaf took PPFD measurements and found that the Law doesn't work well with a highly dispersed source. Under Samsung strips the measured loss was closer to half, at twice the distance.
The other thing to consider is that no photons escape the tent, so it's only a matter of how much they get bounced around and decay.
Mostly, headroom will affect stretch. I mount my lights at the very top and move the plants up or down if I have to, but otherwise keep the tops at least 2 feet away.
The reason why headroom effects stretch is that less PPFD is reaching the plant at greater distances so the plant is trying to make up for this by getting its leaves closer to the lights so it can better photosynthesize.
Another way to look at the Inverse Square Law is that, when you double the distance from the light source, the amount of coverage is 4x the area (which is why the plant gets 1/4th the light).
But, because of the reflectivity, some of the loss is offset so it will measure something greater than 1/4th.
It's both reflectivity and dispersion. You're entirely correct about loss of intensity, and it drops off quickly, just not as fast as you'd think. My calculations showed that headroom under a dispersed source is roughly equal to the productive canopy depth. If the top of the canopy gets 1000-1200 umols, then at twice the depth it should get about 500 umols which is the low end of the productive range.
So if you have one foot of headroom and the top is 1200 umols, a foot deep into the canopy it's down to 500. If you want a deep canopy you need deep headroom.
Kinda counter-intuitive.
So if you have one foot of headroom and the top is 1200 umols, a foot deep into the canopy it's down to 500. If you want a deep canopy you need deep headroom.
Kinda counter-intuitive.
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