Gabor's signal expansion is reviewed by applying it first to a coherent optical (or deterministic) signal; it is shown that such an optical signal can be expressed as a superposition of optical rays that appear at discrete directions. The expansion is then applied to partially coherent light, yielding expansion coefficients that express the correlations that exist between the different optical rays. It is shown how these expansion coefficients are related to the mutual power spectrum of the partially coherent light and how they are propagated through linear systems. The special case of incoherent light is considered in more detail.
doi:10.1016/0030-4018(91)90235-6