ABSTRACT
Unpolarized light and partially polarized light are the dominant forms of light in the universe. Unpolarized light has a randomly varying polarization state with no preference for any particular state. Unpolarized light is incoherent; it does not form stable interference patterns. Partially polarized light has a randomly changing polarization state with a tendency toward a particular polarization state. Polychromatic light, light of multiple wavelengths, has an electric field amplitude with a random appearance with irregular periods and peaks of different heights. The spectral bandwidth determines how rapidly the polarization ellipse can evolve in partially polarized light. The four Stokes parameters are defined in terms of six polarized flux measurements performed with ideal polarizers. Stokes parameters are suitable for characterizing incoherent or coherent light beams. The Stokes parameter’s non-orthogonal coordinate system is extremely clever; this unusual coordinate system correctly handles the superposition and interference of multiple incoherent beams of light. The Stokes parameters do not differentiate between light polarized at 0 and 180 because for incoherent light there is no effective difference. The plus and minus signs of the Stokes parameter elements represent orthogonal polarizations. Adding orthogonally polarized incoherent beams reduces the degree of polarization of the light. The degree of polarization (DoP) characterizes the randomness of a polarization state. The polarized flux is the amount of flux which is polarized. Orthogonal polarization states are not defined for partially polarized states; for example unpolarized light does not have an orthogonal polarization state. The Poincaré sphere is a geometric representation for polarization states which maps all fully polarized states onto the surface of a sphere. The origin (0, 0, 0) of the Poincaré sphere represents unpolarized light. The interior of the Poincaré sphere represents partially polarized light with the distance from the origin indicating the DoP. The Poincaré sphere simplifies the analysis of many polarization problems, particularly problems involving retarders.
