UVCS Tutorial: The WLC Polarizer Assembly

The visible image of the corona that passes through the WLC pinhole is projected onto the WLC polarizer assembly, which consists of four closely-spaced optical components:

• A half-wave retarder that alters the optical path length for one direction of polarization while leaving the other direction unchanged. This results in a net rotation of the plane of polarization of linearly polarized light. Measurements with the WLC are typically made with the half-wave retarder rotated into one of three possible positions (with angular separations of 30 degrees), but any rotation angle is possible.

• A fixed linear polarizer, made of Polaroid HN38S material, which transmits light with a specific plane of polarization.

• A lens that focuses the light from the pinhole onto a plane at the location of the detector.

• A bandpass filter that transmits light only in the range of 4500 to 6000 Angstroms, both to ensure the proper operation of the other components and to make the analysis of the resulting light more straightforward.

The intensities measured at the three rotational positions of the half-wave retarder can be combined mathematically to compute the degree of linear polarization (which we typically call the "polarization brightness," or pB). Measurements of pB allow the electron density in the corona to be deduced. More information about how optical systems like this work can be found in this polarization tutorial.

Go back to the Top-level tutorial page.