The division of light into two components (an "ordinary" and an "extraordinary ray" ), found in materials which have two different indices of refraction in different directions (i.e., when light entering certain transparent materials, such as calcite, splits into two beams which travel at different speeds). Birefringence is also known as double refraction. Crystals possessing birefringence include hexagonal (such as calcite), tetragonal, and trigonal crystal classes exhibit birefringence, and are known as uniaxial. Orthorhombic, monoclinic, triclinic exhibit three indices of refraction. They are therefore trirefringent and are known as biaxial. Birefringent prisms include the Nicol prism, Glan-Foucault prism, Glan-Thompson prism, and Wollaston prism.

Another material, an excellent birefringence optical crystal undoped YVO4 is developed newly. It has very good transmission in a wide wavelength range from visible to infrared, large index of refractivity and birefringence difference. Compared with other important birefringence crystals, YVO4 has higher hardness, better fabrication property, water insoluble and man-made than calcite (CaCO3 single crystal); easier to get large, high quality crystal and lower cost than rutile (TiO2 single crystal). Those outstanding properties make YVO4 very important birefringence optical material and widely used in opto-electronic research, development and industry. For example, the optical communication system needs a huge quantity devices of undoped YVO4, such as fiber optical isolators, circulators, beam displacers, Glan polarizers and other polarizing devices.