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. This in turn reduces the intensity of fluorescence seen in your mineral specimens, and your minerals appear faded or dull.
From
an article in
SPIE-The International Society for Optical
Engineering, volume
970, "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 #9863filter.
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).
Hoya filters have the slowest solarization rate, a degradation of only 15% after 250 hours.