I'm a major noob in the growing field, but I've been doing my share of reading on it for the past nine years, somewhat before I failed my first grow attempt. And, the way I see it, there's something off here.
A mixed spectrum is better to plants, but it has nothing to do with the amount of UVB. With red light, plants increase photosynthesis rate and stem growth accelerates, that's why it makes for sturdy plants and fat buds. Blue light is important to regulate stomatas, which are pores on leaves and stems that promote CO2, O2 and water vapor exchange for photosynthesis, breathing and temperature regulation. With proper regulation, the plant can better produce its sugars and other extras such as THC, thus increasing bud quality.
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This should be deep enough to explain it while shallow enough not to bend minds. They mention 6% blue in the PAR spectrum is enough, btw, more of it only adds to stem length.)
What about UVB? Well, anybody who's been out in the sun more than they should know UVB radiation burns pretty bad. UVA does too, but to a much lesser degree. It happens UVB will burn plant tissue too, and cannabis came up with great sunscreen which incidentally makes us trippy and/or sleepy. More UVB, more protection needed, more stoned heads. And, as far as I know, 6500K fluoros do not emit more UVB than 2700K. If they do, it must be a marginal difference. That's why they make special UVB bulbs for reptiles, or people would just put their iguanas under 6500K CFLs. Of course, as everything in life there's an optimum for UVB and too much of it will burn the plants irrespective of how much resin it's coated in.
I guess that's it, feel free to disagree or bash me if I'm totally wrong. Or to endorse this info!