Birefringence occurs when an optical material in the path of a beam of light causes the beam to be split into two polarization components which travel at different velocities. Birefringence is measured as the difference of indices of the refraction of the components within the material. For a light beam that has been split into two components by a material, birefringence is the difference of the indices of refraction of the components within the material. This can also be called double refraction of light.
Birefringence is an intrinsic property of many optical materials, and may also be induced by external forces applied to the material. The induced birefringence may be temporary, as when the material is oscillated, or the birefringence may be residual, as may happen when, for example, the material undergoes thermal stress during production of the material.
Retardation or retardance represents the integrated effect of birefringence acting along the path of a light beam that traverses a sample of the optical material. If the incident light beam is linearly polarized, the two orthogonal components of the polarized light will exit the sample with a phase difference, called the retardance. The fundamental unit of retardance is length, such as nanometers (nm). It is frequently convenient, however, to express retardance in units of phase angle (waves, radians, or degrees), which is proportional to the retardance (nm) divided by the wavelength of the light (nm). An "average" or "normalized" birefringence for a sample is sometimes computed by dividing the measured retardation magnitude by the thickness of the sample.
The two orthogonal, polarized beam components mentioned above are parallel to two orthogonal axes associated with the optical material, which axes are referred to as the "fast axis" and the "slow axis." The fast axis is the axis of the material that aligns with the faster moving component of the polarized light through the sample. Therefore, a complete description of the retardance of a sample along a given optical path requires specifying both the magnitude of the retardance and its relative angular orientation of the fast (or slow) axis of the sample.
Many transparent solids are optically isotropic, meaning that the index of refraction is equal in all directions throughout the crystalline lattice. Examples of isotropic solids are glass, table salt, many polymers, and a wide variety of both organic and inorganic compounds.
Crystals are classified as being either isotropic or anisotropic depending upon their optical behavior and whether or not their axes are equivalent. All isotropic crystals have equivalent axes that interact with light in a similar manner, regardless of the crystal orientation with respect to incident light waves. Light entering an isotropic crystal is refracted at a constant angle and passes through the crystal at a single velocity without being polarized by interaction with the electronic components of the crystalline lattice. This means there will be no birefringence effects. Click here for a demonstration.
It is also important to point out that some materials may be isotropic at some wavelengths of light and exhibit birefringence characteristics at others.
The term anisotropy refers to a non-uniform spatial distribution of properties, which result in different values being obtained when specimens are probed from several directions within the same material. Observed properties are often dependent on the particular probe being employed and often vary depending upon the whether the observed phenomena are based on optical, acoustical, thermal, magnetic, or electrical events. On the other hand, isotropic properties remain symmetrical, regardless of the direction of measurement with each type of probe reporting identical results.
When light enters the optical axis of uniaxial (one type of anisotropic) crystals, it behaves in a manner similar to the interaction with isotropic crystals, and passes through at a single velocity. However, when light enters a non-equivalent axis, it is refracted into two rays each polarized with their vibration directions oriented at right angles to one another, and traveling at different velocities. This phenomenon is birefringence and is exhibited to a greater or lesser degree in all anisotropic crystals. The axis along which light moves the fastest is called the fast axis. Click here for a demonstration.
The residual linear birefringence in an optical component affects its quality, especially when used in polarization related instruments. Hinds Instruments' photoelastic modulation technology can be applied to the measurement of linear birefringence of transparent optical materials and is used in Exicor instrumentation in patented and proprietary ways to produce unprecedented sensitivity.