A filter is a kind of optical filter in the shape of a piece. Usually its transmittance (reflectivity) is related to the wavelength, and some filters’ transmittance (reflectivity) is related to polarization or spatial distribution. The birefringent filter is composed of birefringent crystals and uses the interference of polarized light to obtain monochromatic light. It is mainly used for monochromatic light detection of sunlight, and is also an ideal element for tuning, spectral resolution and line width compression.
