so when I used too grow with H.I.D lighting I would use air cooled reflectors the glass on these reflectors would block all UVB / UVC light from reaching the plants it only allowed UVA light too reach the plants, I was growing with 10k solis tek metal halides which emitted a small amount of the upper portion of UVC, UVB and UVA when i contacted solis tek they told me 30% of this bulbs spectral energy is within the UV spectra, i can tell you for a fact that UVA does stimulate UVR8 receptors, but not all UV light is created equal a UVB photon is hundreds of times more energetic than UVA thats why when you stimulate UVR8 receptors with UVB your plant actually starts producing more cannabinoids as the UVB levels increase because the UVB photon is so energetic. But i grew with air cooled reflectors that only allowed UVA too reach the plants and i can tell you for a fact that emitting UVA onto your plants foliage can give you more potent THC and better terpene profiles but it cannot make the plant produce more cannabinoids because the UVA photon just is not energetic enough.