Abstract


The concept of characterizing turbid media using backscattering of polarized light and Mueller matrices was introduced in 1997 by Andreas Hielscher [1], This research creates a similar polarization-imaging apparatus and applies it to varying concentrations of small latex spheres in deionized water, focusing on the changes in the matrix element intensity patterns.

When light scatters from a turbid medium (a liquid with many fine suspended particles) it gradually loses its polarization at a rate which depends on the number of scatterings, the details of the medium and the type of polarization. Because almost all particles are unable to re-radiate photons in the same plane the incident light was polarized in, the scattered light forms interesting dipole-shaped patterns.

In the apparatus that was created, HeNe laser light is scattered by the medium, and the scattered light is focused into a CCD camera. The polarization of the incoming light and the orientation of an analyzer directly in front of the camera are adjusted to create several individual images which can be manipulated to form the 16 elements of a Mueller matrix. A Mueller matrix is a 4x4 matrix that describes the optical properties of a medium or device with respect to not only linear polarized light, but to circular or elliptical polarized light as well. Using images as the matrix elements instead of single values allows every point in the medium to be represented. With the help of false coloring, the 16 distinct Mueller matrix images look strikingly different and can be used as a ``fingerprint'' of the scattering medium for identification and comparison. Creating the setup involved in part carefully characterizing the polarization optics, and selecting and testing a suitable CCD camera.

Mueller matrix images obtained from a dilute sample of 0.2 um diameter latex spheres show the characteristic multi-lobed dipole patterns that laws of scattering predict. Repeating the procedure using a sample of one half the concentration of the initial mixture produced a decidedly different set of 16 matrix elements. Graphed comparisons of intensity values in a selected element clearly indicate how the Mueller matrix for a latex sphere solution is altered with respect to concentration.












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