By cementing a half-wave plate of Cellophane over a portion of a sheet of Polaroid, two fields, separated by a photometric line of demarcation are produced. Since the directions of vibration of the polarized light in the two fields are mutually perpendicular, a sensitive device for the detection of polarized light is realized.
A. H. PFUND, "A Polaroid Half-Shade Analyzer," J. Opt. Soc. Am. 26, 453-453 (1936)