Studies of narrow-band interference filters have shown that two mechanisms exist that cause drift to shorter wavelengths. One is dependent on the thermal history of the filter, and the other depends on the radiation history. The present paper presents experimental results on thermal effects; and it is shown that by a proper bake cycle, ZnS-cryolite filters are stable for years if stored at less than 38°C.
Alan M. Title, Thomas P. Pope, and John P. Andelin, Jr., "Drift in Interference Filters. Part 1.," Appl. Opt. 13, 2675-2679 (1974)