If you want to cover a 4 x 4 area you need 2x 4ft bulbs at 24in. Start them out at 15 minutes every hour for 8 hours, and build up to 2-4 hours. If the bulbs are closer, reduce the exposure time. Main thing is to look for leaf stress, and then cut the lights back so its not stressed.
Trick is to give them all they can stand, and not damage them. And considering you have 600w at the wall, you about the same as a 1000w HID if you have the very best diodes. 100w HID, or equal, and 2x 4ft bulbs are whats recommended for a 4 x 4 area. You can easuily fill a 4 x 4 area with 3 plants if they are genetically capable, and you veg them long enough.
I run them starting from seedlings-flowering, not just the last few weeks. Ive seen them get burned doing that.
Theres also a technique of running them for a certain amount of time, turning tem off, and keep doing that for x amount of hours. But Ive not persued that aspect.
Generally in nature the sun is strongest from 10am-4pm, so 6 hours seems right to me to keep it simple.
Also more Sativa dominant plants will possibly react more, as they have evolved with high levels of UVA/B. Plants closer to the Equator.
Also high altitude., has higher UVA/B levels
from what little I know, UVA creates more physical trichomes, and UVB changes the chemical profile.
One could increase physical amounts of visiual trichomes, and still not change the profile to be more psychoactive. Activating the UVR8 gene, and also supplying UVA works together.
Also 1 question to ask is??
Does marijuana have the RUV8 Gene??? Simple question. Right??? Answer is Yes. Many plants have it. Tomatoes, flowers, veggies. Fruits
So it only makes logical sense to activate the gene as the sun would do, if one can get a light that can trigger that gene.
The proved way back in the 1973-74 in Skylab that the chemical profile of marijuana can be altered by using different light wave-lenghts.
Weed was one of, if not the first plant they tried to grow in space, and when the budded it, they flashed it with extremely strong levels of XENON, just for milliseconds, burning the buds, and they said it changed the chemical profile of the oil to keep it simple ( For Me ).
And to me its a no brainer to supply UVA/B if the plant gets it in nature, and the sun does go down to 280nm outside. I wouldnt go less than that for safety sake though, and kill the plant. 280nm is the line of UAB/UVC.
The article was in High Times
The manufacturer lists Perdue University, and University of Florida, Texas A&M as 3 examples of research labs that use their bulbs. Theyve also been making Non Plant Specific bulbs for Show Pigs, and curing wood since the 1980s. And NO I dont work for them. Satisfied Customer. I wish I did.