The brand depends on your location. For US peeps I would say get the Agromax powerveg+ bulb. They have a similar spectrum like a 12% Arcadia bulb. Reptisun should also be available but has only 10% UVB.
I would start using it a few days before your push the flower switch to give them time to get used to it. I would start with 1h in the morning and 1h right before lights off. Far-red end of day treatments are not affected by UVB and 5 mins after lights off is still working.
When they are used to it which takes up to a week you can increase the hours per day slowly. When I see the first upcomming trichomes I would use them 2x 3h per day and increase that by another hour the week. So from ~week 3 six hours and from week 8 you can use it all day long to push them to the limits.
12" is a good distance cuz its a 360° bulb and when you use a 120° reflector they cover exactly a 2' wide area. Center readings are almost twice as high as the intensity on the outer ends of the area for this reason I put the shortest plants in the center and the higher ones on the sides to even it out a bit.
You can also use it earlier for the whole day but IMO its wasted energy. Even if the dose princip is not working properly with the different spectrums of pureUV and reptile bulbs, the calculated dose is pretty high.
That's a test with an Arcadia D3deseet with 12% UVB and 30% UVA, bare bulb and with 3 different kind of reflectors.(pdf below) I like the hammered reflector because it has the best distribution even if the center readings are lower. You see ~150μW/cm² at 12"/30cm, the average is probably only 100μW/cm². To calculate the daily dose in kJ/m² you only need to multiply the total microwatts per day by 0,036. 6h á 100μW/cm² is 600μW x 0,036 thats already a dose of 21,6kJ/m² per day.
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The thing why you can't use this method for the pureUV bulbs is their spectrum goes down to the bottom of the UVB range and hits already UVC. 10nm less in this area means the photons carry 10x more energy and in general one can say the more energy in stored inside the photons the more dangerous they get.
X-rays have so much energy that they just go through you and gamma rays would just burn you your ass off.
Below is the full test report to give you some numbers to work with and I will also add a research paper about UVB and what you can expect. They have use a UV burner and have used an additional UVC filter to remove UVC wavelength. So their calculated doses per day are also not comparable with the PureUV bulbs unless you find a UVC filter coating to paint the bulbs with. There are also two lab tests, same strain, same light and enviromental conditions, one with the other w/o UVB.
UVB won and they have only used a dose of 13,4kJ/m². With 6h per day you are already on the high side, with 12h the dose would be 43,6 fucking kJ/m². Mexico city, which is near the equator AND 2000m above zero recieves for instance such a high dose per day and the intensity at noon can reach 450μW/cm²!
If you do it this way, make them used to it and increase slowly, keep ~10-12" distance and so on I guarantee you 100% success! It works and the easiest way to prove it is to split your tent in 2 areas and use only on one area additional UVB.
Same genetic on both sides! I've done the same and with the 2nd run I've added it already to the whole tent and I will never look back. If you have a good rosin press with consitent results you can make a test by yourself and weight the outcome. Pretty sure the UVB weed yields better!
Edit:
Reptile or powerveg+ bulbs are much better for UVB beginners because even if you burn the plants a little you wont lose your whole crop. Agromax bulbs are so strong that even one day with too much can cause a flowering stop and you end up with extremely potent weed but only half as much yield as usual.
Ahh, and make sure the reflector has a metallic surface like hammered or polished aluminum. Paint, plastics, glas and all that swallows UVB completely. The best would be a wing design to get the most even distribution. I've also build so called super spreader(know from the adjust a wing) to reduce the center hotspot by throwing a part of the light back into the reflector to distribute it evenly. But nevertheless I've burned my plants on the 1st run with two PureUV bulbs
although I was extremely careful. They are unduly simply too strong for 16-18" distance even with the spreaders. So stay with the powerveg+ bulbs, they are strong enough and you get the same results. It only takes more energy.