Advancements in the geometrical measurements of the birefringent filter's crystal plates and two-dimensional measurements of Doppler velocity in the solar atmosphere

independently developed by Ohmanin 1938.2A Lyot filter comprises a sequence of elements (stages), which consists of birefringent plates, each twice as thick as itspredecessor installed between polarizers. A thick plate produces a channel spectrum of many thin bands closely spaced,whereas a thin plate produces a channel spectrum of fewer thick bands, with a larger spacing between them.