Circular polarizers block all photons rotating in one direction while allowing those rotating in the opposite direction to pass. These optical polarizers are assemblies that consist of a linear polarizer and a wave retarder, usually a quarter waveplate or half waveplate. Linear polarizers are optical components that transmit light waves along one axis and absorb them along the other. The transmitting and absorbing axes of linear polarization are oriented at 90 degrees to each other. Waveplates and retardation plates are optical elements with two principal axes, one slow and one fast, which resolve an incident polarized beam into two mutually perpendicular polarized beams. Specialty circular polarizers are also available.
Selecting circular polarizers requires an understanding of half-wave and quarter-wave retardations. Half-wave retardation plates are made of a double-refracting crystal having such a density that a phase difference of one-half cycle is formed between the ordinary and extraordinary elements of light passing through. Quarter waveplates are made of a double-refracting crystal having such a density that a phase difference of one-quarter cycle is formed between the ordinary and extraordinary elements of light passing through. As components of circular polarizers, they are used for conversion of linearly polarized radiation into circularly polarized radiation when the input polarization is oriented at an angle of 45 degrees to the waveplate optical axis.
There are two basic types of circular polarizers: circular dichroic polarizers and circular beam separators. The term "dichroic" refers to how light rays with different polarizations are absorbed in different amounts. Circular dichroic polarizers consist of a dichroic linear polarizer and a quarter-wave retarder. Circular beam separators also consist of a quarter-wave retarder, but feature an assembly that is oriented with the fast axis at 45 degrees to the transmission axis of a laser line beam-spitting cube. With these circular polarizers, the transmitted beam is circularly-polarized regardless of the input beam’s polarization state.
Normally, circularly polarized light is created when one linear electric field component is phase-shifted in relation to the orthogonal component. Circular polarizers may be also be used as optical isolators, transmitting left-circular light or right-circular polarized light for an input beam of any polarization state. By aligning the linear polarizer component with the quarter-wave retarder component, circular polarizers can turn linearly-polarized light into circularly-polarized light. Since specular reflection causes circularly polarized light to exhibit shift in orientation or handedness, the reflected circularly-polarized light becomes linearly-polarized and rotates 90 degrees from the light leaving the isolator. As a rule, horizontally-polarized light that falls upon a vertically-aligned polarizer is rejected.
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