The degree of polarization (DOP) of laser-speckle fields, where the speckles were generated by a polarized laser beam incident upon two kinds of samples: ground glass and wax, was investigated within a single coherence area as well as over multiple coherence areas. For the surface-scattering ground glass, the incident polarization state was preserved in the speckle field, and hence the DOP remained at unity regardless of the area of detection. For the volumescattering wax, the polarization

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... s varied with positions in the field, and consequently the DOP depended on the area of detection: the DOP decreased with an increasing area of detection, and only when the area was much smaller than the coherence area would the DOP approach unity. A numerical simulation explained the experimental observation. These results are important for the understanding of polarization phenomena in turbid media such as biological tissue. Fig. 2 Measured DOP, DOLP, and DOCP as functions of the area of detection. A d (ϭd 2 2 /4) is the area of detection, where d 2 is the diameter of the second iris; A s (ϭd s 2 /4) is the average area of the coherence areas, where d s is the average diameter of the coherence areas. (a) Measurements within a single coherence area, where A s ϭ171 mm 2 . (b) Measurements over multiple coherence areas, where A s ϭ0.43 mm 2 . Fig. 3 Normalized probability density functions of Stokes parameters, which were measured in the speckle fields generated by the wax sample: (a) S 0 , (b) S 1 , (c) S 2 , (d) S 3 . The probability density function of the first Stokes parameter S 0 in the speckle field generated by the ground-glass sample is also given in (a) for comparison. ͗S 0 ͘, ͗S 1 ͘, ͗S 2 ͘, and ͗S 3 ͘ are the average values.