Circular polarizer
= Circular polarizer
= Circularly polarizing filter
{synonym}
A reciprocal optical <circular polarizer> can be assembled from a linear <polarizer> followed by a <quarter-wave plate> whose axes make $45^\circ$ to the transmitted linear state. Forward passage converts <unpolarized light> into circular <light> of half the ideal incident <intensity>. Reverse passage encounters the retarder first; it leaves <unpolarized light> unpolarized until the linear <polarizer> transmits half as linear <light>. Thus the one-pass <intensity> can be equal in both directions even though the outgoing polarization differs. Use a <coherency matrix>, not a single <Jones vector>, for unpolarized input.