The general method of making an evaporated multiple-layer interference filter from two dielectrics is described for any desired wave-length range. The proper thickness of each layer to give maximum reflection in the desired region and at the desired angle of incidence is first computed, and next the visible reflectivity of such a film at normal incidence. This visible reflectivity is used as the gauge to determine the proper thicknesses of the films and the point at which the evaporation of each layer should be stopped. Practical difficulties encountered in making these filters are also discussed with various ways of surmounting them.
Several examples of special filters for particular purposes are reviewed in detail, including infra-red and polarizing filters.
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