We have seen in the preceding chapter that a WCR directly measures the absolute value of the time-averaged Poynting vector (i.e., the intensity) for the superposition of plane or near-plane wavefronts filtered out by the {objective lens, diaphragm} combination, by relaying it onto the sensitive surface of the photodetector. Since all these wavefronts propagate in essentially the same direction, i.e., along the optical axis of the instrument, they can be thought of as forming a parallel beam that can be characterized by all four Stokes parameters rather than only by the first one, i.e., the intensity (Sections 9.2 and 9.3). It turns out that by inserting special optical elements between the relay lens and the detector, it is possible to modify this beam in such a way that the new first Stokes parameter of the beam reaching the photodetector contains information about the second, third, or fourth Stokes parameters of the original beam. This is usually done by using so-called polarizers and retarders, and typically involves a succession of several measurements to fully characterize the four-component Stokes column vector.

The following discussion will be based on the assumption that the WCR faces a monochromatic plane wave. However, the additivity of the Stokes parameters derived in Sections 9.2 and 9.3 allows for a straightforward generalization of the results to polychromatic light assuming that, according to Section 11.5, the range of angular frequencies involved is sufficiently narrow.