Polarization of light is a state in which rays of light exhibit different properties in different directions. When the electric field vector of the EMR oscillates in a single, fixed plane all along the beam, the light is said to be linearly (plane) polarized; when the plane of the electric field rotates, the light is said to be elliptically polarized because the electric field vector traces out an ellipse at a fixed point in space as a function of time; and when the ellipse happens to be a circle, the light is said to be circularly polarized.
The degree of polarization is the quantity that characterizes the ratio of the intensity of polarized light to the total intensity of light, PL = (7ц – /±)/(7ц + IJ, where Тц is the intensity of light polarized in parallel and I± perpendicular to polarization plane. When polarized light traces a tissue, its depolarization (destruction of light polarization) happens because of the complex character of light’s interaction with the inhomogeneous (scattering) medium (i. e., tissue). As a characteristic of such an interaction, the depolarization length is introduced. The depolarization length is the length of light beam transport in a scattering-depolarizing medium in which the polarization degree decays to a definite level compared to the totally polarized incident light.
Many tissues, including skin, feature polarization anisotropy which is an inequality of polarization properties along different axes. Polarization properties of light propagating within a tissue are sensitive to changes in tissue morphology, for instance, due to skin collagen aging. On this basis, a number of polarization-gating techniques were designed. These techniques provide a selection of diffuse photon groups with different path lengths, in particular ballistic or least-scattering photons that carry information about tissue structure. Skin polarization optical imaging and spectroscopy techniques were recently suggested.
A polarizer is a device, often a crystal or prism, which produces polarized light from unpolarized light of a conventional light source. Laser light is principally polarized, and depending on laser construction it can provide a high degree of linear or circular polarization.