A waveplate is an essential optical component used to control and measure the polarization state of light. Waveplates modify polarization through a parameter called retardance, without affecting the degree of polarization. Accurate measurement of retardance is critical for maintaining precise polarization control and verifying waveplate performance.

The most commonly required retardances are half-wave and quarter-wave. Even small deviations from the intended retardance can affect the purity of the output polarization state, making accurate characterization important for both waveplate manufacturers and users.