  The {\em Van Cittert Zernike} theorem consists of a simple relation between
the measured coherence of the radiation and the characteristics of the emitting
stochastic source. In the present work, an extension of the theorem to
partially correlated sources in curved spacetime is derived. The generalized
theorem is used to evaluate the changes in the spectrum of light travelling
through a gravitational lens. Some possible astrophysical applications are
discussed, accompanied by a numerical estimation of the spectrum modification
in the process of propagation through a gravitational lens.
