When a light beam propagates through a birefringent medium, it experiences different changes of optical phase for the ordinary and extraordinary polarization components. The difference in those phase shifts is called retardance. It can be specified in radians (rad) or in degrees (°), alternatively in terms of wavelengths; a retardance of <$\lambda$>, for example, corresponds to <$2 \pi$> or 360°.
In optics, a retardance is often introduced by inserting a waveplate – for example, a <$\pi$> retardance with a <$\lambda /2$> plate. With a Babinet–Soleil compensator or a Berek compensator, one can obtain an adjustable amount of retardance. Generally, optical elements for introducing a controlled amount of optical retardance are called retarders.
More complicated effects occur as a result of thermally induced birefringence, for example in a laser crystal, where both the amount of retardance and the axes directions generally depend on the position within the beam profile.
See also: waveplates
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