I posted this in another thread, but take a look at this:
http://cpl.usu.edu/files/publication...b__2576523.pdf
Table 1 has the comparison of warm, neutral, cool white vs single color diodes. For a given amount of Photosynthetic Photon Flux (PPF, you can think it as the foot-candle equivalent for plants, not for human eyes), R+B grow light has more usable spectrum for plants (YPF row). Warm whites similarly has a higher YPF than cool white. However, the 3 white LEDs do pretty well against R+B LEDs (Fig 8A gives you the weight gain per leaf area). Note that warm white seems to give the lower growth than cool white, but interpretation isn't so straight forward. Fig 8A is per unit leaf area, and bluer light (cool white) makes smaller leaves (Fig 7). So the plants under warm white may be gaining similar weight as the plants under cool white, but the leaf size may be bigger (and possibly thinner leaves) under warm whites. I would probably go with warmer white (e.g. 2700K).
This is the data which convinced me that white LEDs aren't so bad. One thing we don't know from this data is how different diodes produce different amount of PPF per watt.
Remember, as David pointed out in the other thread, we have been growing OK with florescent lights (which has only 3 to a few sharp peaks in the emission spectrum).
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