Retardation – Retardation describes the phase shift between the polarization component projected along the fast axis and the component projected along the slow axis. Retardation is specified in units of degrees, waves, or nanometers. One full wave of retardation is equivalent to 360°, or the number of nanometers at the wavelength of interest. Tolerance on retardation is typically stated in degrees, natural or decimal fractions of a full wave, or nanometers. Examples of typical retardation specifications and tolerances are:
λ/4 ± λ/300
λ/2 ± 0.003λ
λ/2 ± 1°
430nm ± 2nm
The most popular retardation values are λ/4, λ/2, and 1λ, but other values can be useful in certain applications. For example, internal reflection from a prism causes a phase shift between components that may be troublesome; a compensating waveplate can restore the desired polarization.
In Figure 3 below, 4 retardation values are shown relative to an original, sine wave. The orange wave is retarded by a quarter of a wave, the yellow by a half, the green by three-quarters, and lastly, the blue by a full wave. The retardation by a quarter wave changes the sine wave to a cosine wave and the full wave retardation lets the wave elapse itself. The most popular waveplates are quarter and half waveplates since they can be stacked to achieve additional retardation values.