Some tests out of link above...ON DECAY OF UVA UVB SPECTRUM OVER TIME.

We were interested to find that the initial output of individual tubes of the same type from the same manufacturer may vary somewhat. Individual tubes then seem to vary slightly in their rate of decay, as well. We have several tubes on long-term measurement and whereas all have decayed somewhere around 15 - 20% over the first three months, some lamps have decayed steadily throughout, whereas others appear to have lost most of this in the first 150 hours and thereafter have decayed more slowly.
We have found considerable variation in the output of older tubes submitted for testing after many months of use. With some brands, tubes that had been in use for a year were found to be emitting as much, or more UVB than others of the same type that were only five or six months old. We do not know whether this is due mainly to a difference in output from the beginning, or to different rates of decay