Here is a graph with both the UV values and Solar Radiation from some tempest (brightness is directly derived from solar radiation, or the other way around) . Clearly (I think) the UV values got a calibration applied but the solar radiation did not. As it is the same sensor chip being used you might wonder why? Of course this might be ok, but thinking of it, a probable cause of having wrong values might be the exact position and distance from the sensor to the Teflon measuring window. If that distance is very small the measurement would be very sensitive to even the slightest change in distance. Another cause, for older stations, might be a dirty or less transparent window.
Without further experimentation, I would assume that any correction done to the UV might also be needed for the Solar Radiation/Brightness.
This particular graph was from someone with a Tempest reporting too low temperatures,which might be explained by having too high Solar Radiation reported. If Solar radiation would have gotten the same correction as UV did, the reported temperature should go up (assuming the algorithm for correcting the temperatures in tempest uses the Solar Radiation.
All speculation, but plausible.