Variable contrast in the image of non-uniform objects is obtained by polarizing orthogonally the deviated and undeviated bundles of light due to diffraction by the object. Advantage is taken of this orthogonality of polarization to effect variable phase difference between, and variable amplitude ratio of the deviated and undeviated bundles. These modified, interfering bundles produce the image and permit continuously variable contrast.
Of many possible polanret systems two are described. In the preferred system, the phase difference between the deviated and undeviated bundles is varied throughout one wave-length by rotation of the polarizer, and the amplitude ratio is varied practically in all proportions by rotation of the analyzer. The light is plane polarized either before or after passage through the object, passes through a uniform quarter-wave plate, through the zonal polarizer, and finally through the analyzer. The zonal polarizer consists of two thin, adjoining, plane-polarizing elements oriented at right angles. One element covers the image of the condenser diaphragm as formed by the intervening lenses of the condenser and objective, the other, the remaining area of the clear aperture. The principal axes of the quarter-wave plate bisect the transmission directions of the zonal polarizer.
Wide range in contrast has been obtained with experimental polanret systems with microscope objectives of long focal length.
HAROLD OSTERBERG, "The Polanret Microscope," J. Opt. Soc. Am. 37, 726-730 (1947)