Polarized light imaging is a potential tool to obtain an adequate description of the properties of depolarizing media such as biological tissues. In many biomedical applications, for instance, dermatology, ophthalmology, or urology, imaging polarimetry provides a noninvasive diagnosis of a wide range of disease states, and, likewise, it could be applied to the study of internal tissues though the use of endoscopes that use optical fibers. We introduce an algebraic method, based on the Mueller-coherence matrix, for a clearer analysis of the polarization characteristics of depolarizing media via the entropy factor. First-order errors introduced by the measurement system are corrected. Entropy defines three kinds of media according to their depolarizing behavior, and several examples corresponding to each region are shown. The calculation of this factor provides clearer information than that provided by the traditional Mueller matrix in the analysis of biological tissue properties by polarization measurement techniques.

A very practical method for the analysis of an optical device is to employ its Mueller matrix [l]. This matrix characterizes any device in terms of its optical properties. To obtain the matrix elements a series of intensity measurements is performed, by the use of polarizing devices (polarizers and retarders) to modify the incident polarization state @'S) and analyse the emerging PS. Since the resulting Mueller matrix is be point of departure for any further analysis of the optical device, exactness is of main importance. Therefore, the small changes introduced in its elements taking into account the fmt-order errors of the polarizing devices should be adequately corrected In this work, the variation of the Mueller matrix of a device under test because of these errors is analysed, in order to obtain its correct values from the measured elements, by means of the Mueller-Coherence matrix. 2.-First-order errors in polarization devices using the Jones matrixThe following defects in polarizing devices lead to fust-order errors. Second-order errors are those caused by effects that mutually influence each other, and will not be taken into account in this work a) Optical activiw (rpl Certain devices act as ~t u r a l l y polarization rotators, a property known as optical activity. In these materials waves with right-and left-circular polarizations travel at different phase velocities. The optical activity factor expresses the r o w power (rad/"). b) Dichroism (4,The material property to attenuate one defined orthogonal component more than the other is known as dichroism. The dichroism factor expresses this difference in attenuation (mm-'). c) Leakage (9When depolarized light crosses through a h e a r polarizer, the undesired orthogonal component will not be perfectly zero on emission. This leaking effect is expressed by the extinction ratio. d) Strain (4When an optical device experiences deformation because of unequal pressure on different places for example, slightly wrong data are extracted out of measurement of its optical properties. A different retardance is introduced for the two orthogonal components ( r d m m ) . The non-desired effects that can be supposed in the linear polarizer and the linear retarder are:These errors are now introduced into the Jones calculus [2], modifying the ideal expressions of the matrices of these devices according to values of the non-desired parameters [3, 41. The next step will be to transform these modified matrices considering the first-order errors into the corresponding Mueller matrices. 3.-Correction of the measured Mueller matrixThese expressions in the Jones matrices will now be translated to the Mueller formalism. To actuate this conversion, the SW(4)-0'(6) homomorphism [5] will be used. The Jones matrices considering the fmt-order errors will be converted into the corresponding MuelIer matrix by means of the Muellercoherence matrix. The expressions of the Mueller matrix elements considering the first-order errors become very complicated. As an example, the first row of the M...