The transfer of the coherence by a multielement optical system is studied, within the framework of the fractional Fourier transform. The method relies on the fundamental fact that the coherence is transformed by a linear filter univocally connected to the filter acting on the electric field. The results show that, the Van Cittert-Zernike theorem can be expressed in terms of the fractional order Fourier transforms of the mutual intensities over sphericals surfaces M1 and M2; which allows, among others, to situate this theorem in a quite general context.
Keywords: Fractional Fourier transform, Van Cittert-Zernike theorem, Optical transformations, wave propagation.