Drift in Interference Filters 2: Radiation Effects

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Abstract
Studies of peak transmission drift in narrow-band interference filters have shown that there exist two mechanisms that cause drift toward shorter wavelengths. One is dependent on the thermal history of the filter and is discussed in Part 1 of this paper. The other is dependent on the exposure of the filter to radiation. For ZnS–cryolite filters of the design [(HL)4H8(LH)4L]3L−1, it is experimentally demonstrated that the filters are most sensitive to radiation in a 100-Å band centered at approximately 3900 Å. The drift rate in the focal plane of an f/20 solar image is approximately 3 Å/100 h of exposure. Further, it is also shown by model calculations that the observed radiation-induced drift is consistent with the hypothesis that the optical thickness of ZnS decreases in proportion to the radiant energy absorbed.

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