For a distant spatially incoherent quasimonochromatic source in the small-angle regime, the equal-delay mutual coherence across projected baseline is the Fourier transform of its angular intensity. With matched collector response, the normalized degree is . Equal collector intensities and matched polarizations make fringe visibility . Finite bandwidth and unmatched optical paths introduce additional temporal-coherence averaging.
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The Van Cittert–Zernike theorem is a fundamental result in the field of imaging and optics, particularly relevant to the theory of image formation in astronomy and other fields where diffraction-limited imaging is important. The theorem provides a mathematical framework for understanding how the intensity distribution of a diffraction-limited image can be reconstructed from the visibility of spatial frequencies in an observed object.