The concept of characterizing turbid media using backscattering of polarized light and Mueller matrices was introduced in 1997 by Andreas Hielscher et. al. The goal of my project is to create a similar polarization-imaging apparatus and apply it to various biologically interesting liquids. 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. A Mueller matrix is a four by four matrix that describes the optical properties of a medium or device with respect to different types (linear, circular) and orientations of polarized light. Thus Mueller matrices can provide a very complete description of the way a particular medium affects polarized light.
In this project, red light from a polarized HeNe laser is scattered from the medium, and the scattered light is focused into a cooled CCD camera. The polarization of the incoming light and the orientation of the analyzer directly in front of the camera are adjusted to create 49 individual images which, following the method outlined by Hielscher et. al., can be manipulated to form the 16 elements of a Mueller matrix. 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. A setup has been created to redirect laser light vertically down through a small (2 mm) hole in a slanted mirror on to a sample held in an open container; the same mirror redirects most of the the backwards-scattered light towards a polarization imaging system. This arrangement allows the scattered light to be studied without interference from any container wall. In addition, the polarization properties of several candidate HeNe lasers have been measured and a web site with many references about this topic has been constructed. I am also helping to specify the CCD camera that will be purchased for the experiment.
My initial measurements will be done with milk as the scattering medium. The intention is that the Mueller matrix image might reflect on the changing optical properties of the fat and protein constituents as a function of some variable such as pH or temperature.
This research was supported by the Simons Foundation.