Photon bunching in chaotic fields with uni-dimensional probability density

In a recent experiment the intensity correlation function of twin beams generated by spontaneous parametric down-conversion was found to present a ratio B between the zero-delay value and the infinite-delay value equal to 3, at variance with the bunching ratio B=2 of standard chaotic fields characterized by a two-dimensional Gaussian probability density. In this letter it is pointed out that, for the class of chaotic fields characterized by a fixed phase and a zero-average fluctuating amplitude with a uni-dimensional Gaussian probability density, B is equal to 3, and it is argued that the spontaneous parametric down-conversion field belongs to this latter class, thus giving a simple quantitative explanation to the reported experimental results.