light positioning for LEDs?

[dustydundee]

So for the last several years, I've been growing with old viparspectra lights, those old style red and blue bulb led lights with the built in fans and everything, cheap lights, probably a 10 year old design by now. They had no dimmer, so I would position them about a foot or a foot and a half above the canopy, and pretty much that was it, and as the plants grew, I'd raise the lights. I grew some great weed with them, but the lights didn't really spread out much light beyond directly underneath them.

Finally decided to upgrade my grow. I'm using a couple 4x4 tents, and I've just invested in a set of Mars Hydro Fc-8000 lights. It was between those and the new AC infinity 4x4 lights, and I chose Mars cause they published better PAR readings then AC for the same price light. Bought them along with an apogee light meter. Feels like I'm 8 generations ahead now. This thing is like the sun.

So here is the general question. With new modern LED lights like these, I can pretty much put the light at the top of my tent, and crank it up, and adjust the dimmer up or down, until I get the right PAR reading on at canopy, even if my canopy is several feet down below. With powerful lights like these, do I still want to position the lights to be about a foot or a foot and a half above my canopy and then set the light to the right setting to hit the canopy, or is it acceptable to position the lights once a few feet above the canopy as long as the right light intensity is measured at the canopy at all times? Meaning, is there any benefit to keeping the light closer to the canopy and dimmed down and to keep moving the lights up with the grow, vs starting them out higher up above the canopy with the brightness cranked higher and letting the plants grow themselves up towards the light without ever physically moving the light and using only the dimmer....., other than the obvious electric costs?

[Ckbrew]

Higher temperatures, wear and tear on the drivers and LEDs, temperature delivered to the plant. It is all a balance. You can use the back of your hand to judge how hot the light is on the plants. Put your hand under the light at a good distance. Feel the heat in the back of your hand. Then slowly raise your hand to the light. At some point the heat will suddenly increase and become uncomfortable. Just below that distance is where I usually set the light. Results will vary, Happy new light.

[Bluey]

I tend to keep mine higher and brighter rather than lower and dimmed.

My reasoning for this is maximum efficacy is at 100%, you get better light spread and penetration and you get a better workspace and more even light across the canopy.

I also mounted my two SF's too close together and to mitigate that I needed height as it was too bright in the middle with them at 30".and full wack!

[golfnrl]

I use HLG lamps. They recommend 24”-30” for veg and about 18” for flower for the lamp I use in the photo tent. I use that as a guide along with the dimmer. And, I use the hand test as well on occasion. I use a lower setting during veg and higher during flower. I just finished a grow with the dimmer never above about 70%. I’ve had this lamp (scorpion Rspec) maxed out during a grow but the heat was difficult to manage. My question is about plant stretching. Doesn’t the higher light promote more distance between nodes and cause the plants to stretch to the light?

[OldManGrower]

It's all a trade off. I have a Spider Farmer ES 3000 and tend to use their recommendations for elevation and intensity. Of course I modify the settings based upon my exact situations but that is my base line.

[Bluey]

There's a couple of Android apps that allow you to use your camera phone to measure PPFD and they do the calculations for DLI. Base line your model to a known height and intensity..calibrate and go from there.

Fine tune your heights from there and keep a close eye on how your plant handles it as always.