The effects of Solarization
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. The only way to bring back the richness of color is to install a replacement filter.
The graph shows typical solarization curves for different brands of short wave filters. Hoya filters have the slowest solarization rate, a degradation of only 15% after 250 hours as compared to Schott which degrades to 60% in the same time.
While solarization of SW filters is a well-known phenomenon, it is not generally recognized that the same process occurs in the glass tubing used to make short wave UV lamps. It too will solarize in the same way as filters, although at a slower rate. Other factors also affect the UV light output of a short wave lamp, notably the dark rings which form near the filaments. Again, the only way to restore the light intensity is to install a replacement lamp.
Corning no longer makes filters, they sold their formulation to Kopp Glass Co.