I have this in my threads, but think you will find it most enlightening, even though written in 5/2012
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Originally Posted by Phaeton View Post
The spectrum of light the plant uses efficiently changes with the intensity.
At low light levels red is used most then blue, just like the chlorophyl charts.
As the light gets brighter more blue is used, and more. The break point is reached about 50% of max leaf capacity then green starts coming on. As the intensity continues up red and blue remain steady but green use continues to grow until at maximum it is almost half of all the light energy being used.
This is one reason why, even with the rather shitty, lopsided spectrum produced by HPS, one can still get good results with them:
I'm glad we've finally gotten some good studies on green light over the past several years, as has been mentioned previously (link) by a few of us.
While 'every lumen (or rather, PPFD) is sacred', I'm of the camp that would prefer a higher level of (adjustable) full-intensity, multi-spectrum (i.e. 'white') light incorporated into the main fixture, for that very reason.
And with the recent increases in the efficiency of neutral whites, there's no reason why you can't get perfectly good results with just a two-channel, adjustable led fixture (neutral white, and red), supplementing with the aforementioned only as needed.
For reference, here are the LUXEON (Rebel) Neutral White and CREE (XP-E) Whites - relative spectral distribution:
This image has been resized. Click this bar to view the full image. The original image is sized 801x530.
Now, let's look at all of them superimposed over the PRC (can't get this chart to C/P)
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Originally Posted by Phaeton View Post
UVB and near infrared are used for non chlorophyll purposes on healthy well lit plants.
Es verdad, amigo. And the best, most economical sources of IR for most growers are still simple, good ol' halogen lights, and Reptisun fluoros for UV-B, at present. (the dangers - and potential litigious nightmare - of incorporating expensive UV-B LEDs into a fixture notwithstanding)
Besides, without complete, independent control of the latter two (i.e. independent of the activity and photoperiod of the 'main' LED fixture), the grower's control over their desired photomorphological changes becomes rather tenuous at best.
If one is going to go that route, then both UV- and high-intensity, blue-mediated light damage (since that is what it is) should be adjustable - both in intensity, as well as photoperiodicity and duration.
As the higher-energy end of the spectrum isn't really a 'finishing/maturing' as much as it is a degradation (i.e. blue and UV pass through clear trichomes just fine; it's only when they become cloudy that they show any significant absorption of that energy, and quickly turn from cloudy to amber - at which point one should watch 'em like a hawk to keep your product from degrading too soon and ruining the desired effect), it should always be incorporated judiciously at first, and in small doses - until the effect on that particular cut is well-established, after which it can then be predicted with a 'fair' level of accuracy.
(note: neutral white I called 'normal white' here for some reason. Wonder what I was smokin' at the time...<whistles>)
As one can see, the CREE Neutral White (I call it 'Goldilocks', because it's almost 'just right' ) has a RSPD that still allows nearly ~25% of its total power in the blue range (and plants only really 'need' ~8-10%), and more that 1/3 of which (i.e. the area under the curve) is over ~580nm or so (which has a Photosynthetic RS of over 90%!) - which is much better than even your typical 'Enhanced HPS'.
Couple that with strong white light (green-response chlorophyll extending throughout and deep into leaf structures, with a net effect at or near that of the (mostly) surface-level blue and reds), which also takes care of most of the ~660nm+ you actually need for photomorphogenesis - and you can get by with 630nm reds just fine.
(i.e. 630nm red is ~95% of the PSR of 660nm, AND they currently still have ~20-30% greater radiometric efficiency - as well as being cheaper than the deep reds - so there's more 'bang for the buck'):
