Object detection in turbid media by differential backscattering of polarized light

Optical phase modulation with a photoelastic modulator (PEM) and synchronous detection provides a convenient and versatile method of penetrating optically dense scattering media to yield information concerning (a) the chemical structure of the ambient medium, (b) the size and concentration of the suspended scattering particles, and (c) the presence and surface properties of embedded target objects. The salient experimental feature upon which the method is based is that only light which remains polarized contributes to output photocurrents at harmonics of the modulation frequency. Equivalently, polarimetric detection selects those photons which have undergone fewer depolarizing collisions than average. For the PEM configuration employed to delineate embedded objects, signals at (i) the modulation frequency f and (ii) first harmonic 2f respectively give nearly instantaneous measures of the difference in scatterered intensities of (i) left and right circular polarizations (LCP, RCP) and (ii) sigma and pi. linear polarizations. Objects can be discerned, therefore, to the extent that (1) they scatter orthogonally polarized states of light differently and (2) the corresponding scattering from particles suspended in the surrounding medium cancels. Examination of different kinds of targets has shown that polarimetric imaging is sensitive to edges, surface texture, and absorption of the embedded object.