Abstract
The analogy between the van Cittert-Zernike theorem and diffraction in focal regions is discussed. For wavefields that can be approximated by a generalized quasihomogeneous wave model, the spatial coherence is described by a generalized van Cittert-Zernike theorem even close to the source. The main difference between the classical and the generalized theorem is that the source intensity in the former is replaced by the source radiance in the direction of the observation point in the latter. The spatial coherence is related to the source radiance by a modified Debye integral, which implies that the wave energy is propagated according to the laws of classical radiometry. No assumption of source incoherence is involved, and the results apply to radiation from both primary sources and secondary sources like rough scattering objects or illuminated apertures in optical systems.