The Birefringent Filter

Optical birefringent filters, which depend for their action on the interference of polarized light, can be designed to transmit very sharp bands (down to a fraction of an angstrom in width). The elementary theory necessary for their design is given.Three forms of wide field filters designed by Lyot are described in detail. A more recently developed split element, wide field filter requires only half as many polarizers as the earlier types, which may be an advantage for some applications.Various methods of adjusting the transmission bands of a birefringent filter, including the use of elements of variable thickness, and phase shifters are discussed. For most purposes the electro-optical phase shifters are probably the most promising. For special purposes, such as spectrophotometry, phase shifters composed of rotating fractional wave plates may be more advantageous. Two such phase shifters and their application, in simple and split element filters are described.A few crystalline materials which have been used or might be used to advantage in birefringent filters are mentioned.Finally, the possibility of using polarizing interferometers in combination with birefringent elements for filters with extremely sharp transmission bands (in the range of hundredths or thousandths of an angstrom) is very briefly discussed.