Short-wave filters in ultraviolet light assemblies degrade over time through a process called solarization. This reduces the amount of UV light that can pass through the filter. While there's no visible change in the filtered light, there is an ongoing reduction over time in the UV intensity produced by your light.
From an article in SPIE-The International Society for Optical Engineering, volume 70, "Properties and Characteristics of Optical Glass" by Donald Newsome Fluorescent Mineral Society, comparison testing for solarization of short-wave ultraviolet-transmitting, visible-absorbing filters were conducted.
In all, over 30 samples were tested from five different companies:
Corning Glass Works (now Kopp) with their #9863 filter.
Hoya Optics with their U-325C filter
Raytech Industries with their Color Blaze filter.
Schott Glass Technology Inc. with their UG 5 filter.
UVP Inc. with their UV G filter.
In the test results, Hoya Optics Inc. showed the overall lowest rate of solarization in the experiment. The average of the six U-325C filters also had the highest short-wave ultraviolet transmission at 253.7 nanometers during the entire duration of the test.
Using analysis generating software we re-plotted the test results for the first 100 hours of testing for five of the samples (one from each company).