Starting from the well-known description of a first-order optical system by means of its ABCD matrix, a propagation law for the second-order moments of the Wigner distribution function of partially coherent light in such a system is derived. It is shown that these second-order moments can be described by two parameters: a real parameter, which measures the overall coherence of the light and which remains invariant on propagation, and a complex parameter, whose propagation law takes the bilinear form that also describes the propagation of the complex beam parameter of a complex coherent Gaussian beam.

To: Papers by Martin J. Bastiaans