That's surprising. Shane (Torpey?) puts a lot of resources into his videos and he reviews products that he doesn't sell so I get the impression that he really is working to "spread the good word". Why wouldn't he take the time to put together a few simple sentences about using his lights? The irony is that the Migro array is a hell of a light in terms of uniformity. It's not a powerhouse light that someone would use with CO2 but that 4 panel array is superb for a 2' x 4' growing autos in a non-CO2 environment.
Good question and there's no "right answer".
There are two factors at play. One is the amount of light falloff as the distance to the target increases from the source. The other has to do with how the light is designed to illuminate the canopy ("uniformity"). Check out the lights at ppfdcharts.com and you'll see that at very close distances, the PPFD is higher than it is when the hang heights higher but the uniformity is lower. The change in height vs the change in uniformity follows a curve. I've mapped out the curve for my Growcraft X3 and 16" is a good height for my set up.
Uniformity is on the Y (vertical) axis and hang height (in inches) is on the X axis. A 16" hang height works well for my tent but I've ended up at about 14" because of plant height vs tent height.
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The Migro has a much steeper curve meaning that it's more uniform at lower power levels and the KS 3000, as I recall, is as good or better - both are
excellent.
Back to your question 75% or 100% - the higher the light is above the canopy (the greater the hang height), the more uniform the coverage will tend to be. So, why not raise it to 36" and crank up the watts? Well, you might not have a tall enough grow space, you might not want to spend the $$ on the extra electricity, and you might not want to drive your light at 100% because it shortens the lifespan of the light and you've got to deal with the heat from the LED's and the heat from the driver.
For me, I'm running my X3 for two Gorilla Glue autos in late flower. Hang height is about 14" (I haven't measured it in some time) and I'm running the light at 255 watts out of 330 or about 77%. That's giving me a good PPFD reading, the light bars are 95°± and the driver is about 105° meaning that light will last a long, long time.
Long explanation for "I dunno".
Penetration - There's no concept of "penetration" to a light - photons go until they're absorbed. Within a leaf, Bruce Bugbee shows how blue and red photons are absorbed more quickly than green photons but pretty much all of the photons hitting the surface of a leaf are absorbed. Sure you can turn it up to 11 and push enough photons so they penetrate the leaf but pretty soon, you'll damage the leaf.
Key point - there is no usable PAR in the shadow of that leaf. There's measurable PAR in the interior but no usable PAR. Where that comes into play is that the "light compensation point" for cannabis is 63 µmols. This means that if a leaf is getting > 63 µmols, it will photosynthesize at a rate higher than the rate at which it consumes energy from the rest of the plant. In the case of checking the canopy, anything in there that's not 63 µmols is a loss from the perspective of photosynthesis. So "penetration", in terms of a light getting photons through the top leaves and making that leaf a net gain to the plant, in terms of photosynthesis, does not happen.
If there's a gap in the canopy, an opening, photons will continue on their merry way until they're absorbed, either by air, a leaf, or the top of the res. Light intensity falls off very rapidly (the inverse square law) so, even if there's nothing in between the top of the canopy and a leaf 2' below the canopy, the amount of PAR available to that leaf 2' away is very low. My plants have some gaps in them - I can see the light hitting the top of the res so I know there's a hole in the canopy. I'll take some readings tomorrow to see just how drastic the drop off is.
Back to 75% power or 100% power with different hang heights - there are a handful of variables so there's more than one right answer. Uniformity is an issue (uniformity tends to increase as you move the light up), and the reflective walls of a tent help, but, per the info at ppfdcharts, it's a tradeoff between input power and hang height. Moving a light up even a few inches requires a significant change in wattage to get the same amount of photons to the canopy. The "right" answer really depends on what factors are the most important.
