As I said, irradiance data is not enough to know the absolute emission of the lamps, that is which we want to know.

The 0.5W/m2 is a direct conversion from 50uW/cm2. There is 10000cm2 on a m2, so its 500000uW/m2=500mW/m2 (1mW=1000uW).

In general, for experiments Im interested in both irradiances obtained at top plans and the absolute emission, because it allows to calculate the average dosage along all the grow. At 12" instead of 18" irradiance below the buld for sure was over 0.5W/m2, which is a high dosage, specially when used along all the photoperiod (in Nature, UV irradiance drops a lot at sunrise and sunset, the high peak irradiance is just around the noon hours). But if we think on the average dosage of UVB along all the grow, I think that was way lower than those 0.5W/m2 (perhaps 1W/m2). But as it was provided for 18 and 12h a day, UVB dosage along the day probably was significative enough to notice the effect.

But anyway, impossible to say any accurate figure. So lets let it as a significant UVB dosage, but for sure not excessive, give or take in line with outdoor irradiance per day on tropical areas.

Thank for the info, oldmac, I suspected an higher humidity during flowering. But 40% is right.

As for the increase in gross matter, its a conflictive topic. Ive seen too studies showing a role of UVB stopping, inhibiting or delaying growth, but usually those effects appear at high irradiances. Sure that one of the most known effects of UVB is to promote thicker leaves (heavier) and compact plants (say, like extreme sun adpatation), but that it results on a overall increase on weight depends of other factors aswell (compact plants with thicker leaves increase production on crowded spaces and/or lit at high irradiances). Especially when combined with strong green-yellow spectrums, that are way better used by thick leaves, while thin leaves uses better red/blue dominant spectrums.

So of course that is possible to increase overall weight when adding UVB, but for my research (and no practice, so put a grain of salt), it just happen of very specific conditions. Anyway, in your case it was the case, and as the general setup can be considered as very typical, we can conclude that many indoors growers could benefit of it.

But I think its going to be required to check if happen the same when using LED lighting rich on red and blue instead of HPS as main lighting (not for crowded spaces, almost everybody grows so indoors). Not only because the spectrum, but because the average irradiance, in general with LED lighting average irradiance is way lower than when using HIDs, and both factors plays against the benefit of thicker leaves.