Topics in Particle and Dispersion Science

Contrast improvement with circularly polarized light

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Contrast of an image of an object in turbid media (TM) can be increased by illuminating the object with circularly polarized light and viewing the scene through a circular polarizer of the same handedness as that of the illuminating light. This was demonstrated quite some time ago by Gilbert and Pernicka (1969) for an object with a rough surface submerged in seawater.

These authors postulated that most particles suspended in water have smooth surfaces. Thus, light is reflected once, either at the front or back surface of each such particle. This changes the handedness of the circular polarization of light backscattered by the particles in seawater. In contrast, light is reflected many times by a random relief of the rough surface of the object, with the fraction of even-numbered reflections being roughly equal to that of the odd-numbered reflections. Hence, the object backscatters light of both handedness in roughly equal amounts.

By illuminating an underwater scene with circularly polarized light and viewing it through a circular polarizer, most of light backscattered by suspended particles is attenuated, while only one half of light backscattered by the object is attenuated. This improves the contrast of the object image. Gilbert and Pernicka observed the image contrast improved by a factor as high as 19 in a test tank with unspecified particle properties.

Kartazayeva et al (2005) have recently revisited this technique to examine experimentally the effect of the particle size on the image contrast. They found significant improvement in the image contrast when light with circular polarization was used to view a reflective object in a medium containg large spherical polystyrene particles (~10 µm) as opposed to no measurable improvement for small (~0.2 µm) particles. See also Polarization memory.

CITATION: Jonasz M. 2006. Contrast improvement with circularly polarized light (www.tpdsci.com/Tpc/ImgTMPolC.php). In: Top. Part. Disp. Sci. (www.tpdsci.com).

HISTORY: Published: 18-Jan-2006 Modified: 03-Jul-2006 Peer-reviewed: PENDING

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