Quote Originally Posted by RackitMan
As you are the most knowledgable here, I have two more questions if I may.

1. I can find nothing in the science that suggests a 6 or 8 to 1 red/blue ratio. Is this just attempting to copy HPS/MH spectra or are there real studies that support this? On one advanced cannabis board (engineers not growers), the conclusions was a blue to red ratio of 1.5 to 1 is best, while the absorption graphs look like it should be closer to the inverse, but nothing as extreme as all of the popular LED lights.

2. As you stated, and I have come to the same theoretical conclusion, that it's the photosynthetic photon flux density that is most important.

Let's assume these mfg numbers are accurate for this discussion. If I have a white LED that outputs 100 lumens/watt and has a higher total PPFD than either a 75 lumen/watt red or blue LED, (sorry no spreadsheet) why would I not go with all white LEDs? The spectra is something roughly like 40% blue, 10% green, 20% yellow and 30% red. (Don't hold me to those numbers.)

Any light you can shed (no pun) on these topics would be greatly appreciated.

Thanks!
1. There is nothing to support the current red-dominant ratios being made, and in fact several other studies show that greater amounts of blue are needed for proper flowering and seed production and bulk fruiting. At best they emulate the light from an HPS (though they never hit the photon flux density the HPS outputs in the same peaks.) Most MH lamps will outperform typical LED panels with such crappy balance. As for which spectral peaks matter, that's a really, REALLY dependent question. It depends upon the plant species, sometimes varying wildly between individual strains. In most plants that are green/yellow in color, They'll tend to use the red and blue peaks, green and yellow tend to inhibit the growth process except for trace amounts. The plant is still absorbing *SOME* of that light, but the majority is reflected back to us. On the other hand, you take something like the 'Wandering Jew' or 'Creeping Jesus' plant, which is predominantly purple/blue in color, is typically found in shaded areas, so mostly green and yellow light filter through to it, and it takes maximum advantage of this. This plant will actually PREFER an abundance of green and yellow light, and unless one tailored an LED panel to accomodate for this, the best bet for this would be the HPS lamp.

Assuming the 100lux/w figure, I wouldn't go with the white LEDs mainly because they burn out faster than any other pure-color LED - phosphors are used to shift the light into other visible wavelengths. Added to that, again, depending upon the plant you're growing, some of that light will act as an inhibitor. Of course, given that particular type of LED doing such lux at such low green/yellow output, you'd be better off finding the single-color versions of that same bin diode and using that instead, as you'll be pumping maximum usable power to the plant that way. A lux figure won't usually help you, but if you've got that plus the output chart, you can figure out what's rolling from where, as you weigh the lux figure to the green wavelengths, and then you can see the relative higher outputs for the other usable colors. This is how I determined usability of non-LED growing lamps when no PPFD figures are available (Maybe two bulb makers on the entire plant for HID/Fluorescent actually include that figure.)