Partial polarisation, partial coherence and their spectral description for polychromatic light - Part I

The statistical properties of the light disturbance (regarded as a super-position of monochromatic constituents covering a spectrum of frequencies) is described. It is shown that the state of partial polarisation of a poly-chromatic beam can be completely characterised by four Stokes spectral functions which are generalisations of the customary Stokes parameters and which express the state of partial polarisation as a function of frequency; an equivalent method of description is by a spectral coherency matrix. It is pointed out that the customary set of Stokes parameters (or the equivalent coherency matrix) used for characterising the optical state of a quasi-monochromatic beam is really inadequate for the purpose, since beams having the same set of Stokes parameters may not be optically equivalent for a certain class of experiments