Colored glass filters are relatively robust but should be kept free of fingerprints while in use or storage. Clean with lens tissue as you would eyeglasses and handle around edges. Final cleaning with alcohol on a Q-tip cotton swab followed by wiping with lens tissue will remove most fluorescent materials that might be present. Colored glass filters should be very stable unless used directly in high-intensity light. These filters should be inspected visually for unevenness in color about every 6 months.
Interference filters are usually sealed around the edges and can generally be easily handled. They can be cleaned with the Q-tip and lens tissue method provided that no cleaning liquid is allowed to reach the edges, where there may be access into the dielectric layers. They should never be immersed in any liquids. However, the edges of some dichroic filters may not be sealed, leaving the dielectric coating exposed. These filters are extremely fragile. Never directly touch the dielectric material, and do not use any liquids to attempt to clean the filter because the dielectric material may be released from the substrate. If you cannot store dichroic filters in a support that keeps the surface untouched, keep them protected between two sheets of lens paper in a solid box. Interference filters will degrade in high-intensity light but may also show changes if, for example, high humidity penetrates the dielectric layers. The best way to confirm the quality of an interference filter is to obtain a transmission spectrum of the filter for comparison against the original spectrum. Also, examine the filter under a low-power microscope to look for pinhole defects in the dielectric material. Under low-humidity, low-light conditions in which the filter is free of contact with "the outside world," interference filters can last more than 5 to 10 years.
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