A type of interference filter used for studying the Sun’s corona; also known as a Lyot filter after its inventor, B. Lyot. It uses a stack of double-refracting (i.e. birefringent) quartz crystals alternated with polarizing materials. Light passing through the crystals is separated into two beams, mutually polarized at right angles to each other. The crystals also rotate the direction of polarization of the beams, the extent depending on the wavelength of the light. By blocking successively narrow wavelength ranges using the polarizing material it is possible to achieve a filter which transmits a band as narrow as 0.4 nm, using a filter some 40 mm thick. These filters are costly and are rarely encountered.