The intensity distribution at the exit of a Young-Michelson interferometer
is analysed in Fourier domain. it shows that two numbers are necessary for
describing properly the variation of the visibility of the interferogram fr
inges. One of them is the complex degree of spatial coherence, which descri
bes the correlation between the contributions from the Young's slits. The s
econd number describes the correlation between the Young's interferograms r
eflected by the mirrors of the Michelson interferometer, that is, the corre
lation of optical fields that contain another correlation term.