Parametric down-conversion experiments with stationary fields

Returning from the Wigner to the Hilbert-space formalism, we have augmented the detailed analysis of the four experiments by Casado et al. [Phys. Rev. A55, 3879 (1997)]. Starting from realistic, though arbitrary, parameters we have made numerical calculations of the visibility of fourth-order interference. We have found that two of these experiments are rather proposals than reported ones and we have formulated an alternative to the description of one of them.