UVR8 is a protein found in many plants and animals, including cannabis indica and cannabis sativa. The protein is essentially a messenger that send information to the host plant, but only when it is exposed to UVB radiation at 285nm. Virtually no fluorescent tubes produce UVB in the 280nm range due to the limitations of the glass. The Flower Power is the sole exception, seeing it uses a special glass designed specifically for transparency across the entire UVB spectrum.
While the protein is sensitive to most the UVB spectrum, 280nm-320nm, it is most sensitive in the higher energy portion of the spectrum, around 285nm, meaning it can be triggered using the least amount of UV radiation if a sources produces sufficient light at 285nm.
In all plants, this causes a plant stress message to be sent to the host plant, which can react according to its own DNA. In the case of cannabis, the plant reacts by producing more trichomes and more THC. This is because THC is the "sunblock" for the plant's buds, which are really just nurseries for the plants seeds. As UVA and UVB can damage the DNA of unprotected seeds, the plant has evolved this protective mechanism over millennia, and it appears that the only purpose for THC is to protect the potential seeds from damage.
Once triggered via the UVR8 protein, the plant will shift resources from growth to protection. This means that once a plant is being exposed to UVB @ 285nm during the flowering season (when nights are at least 12 hours long) it will grow less tall and instead produce more THC. When outside, the grower has no control over the UVB but indoor growers can use lamps like the Flower Power to deliver more UVB than the plant would normally get outside, thus keeping the plant in virtual "protect mode", and continuously producing THC at the maximum level it's genetics will allow.
Documented cases range from 15% to 33% increase in the total THC and with total THC values approaching 35% using high levels of UVB.
It should be noted that using UVC will not achieve this for a number of reasons; The wavelength is too small to trigger the protein, the energy level is prone to simply damage the plant without triggering a response, and UVC does not penetrate as deeply as UVB or UVA, so the plant again doesn't receive a signal and is simply damaged.
UVA's role is different and being studied by Solacure and a number of cannabis growing experts. Please note that Solacure does not cultivate cannabis nor sponsor said growth and depends on 3rd parties for grow testing.