= Two-element polarization identification
Suppose the unknown ideal elements are a linear <polarizer> and a <quarter-wave plate>, and the two unknown sources are unpolarized and circularly polarized. Test both element orders while rotating their relative axes and monitoring output <intensity>. Only circular input followed by the <quarter-wave plate> and then the <polarizer> gives complete modulation: the plate makes linear <light> and the analyzer obeys <Malus's law>. Polarizer-first output is constant half-intensity because the following lossless plate changes polarization but not <intensity>. Unpolarized input is constant in either order. This identifies both elements and both sources without comparing source brightnesses.
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