  When use the image mutual information to assess the quality of reconstructed
image in pseudo-thermal light ghost imaging, a negative exponential behavior
with respect to the measurement number is observed. Based on information theory
and a few simple and verifiable assumptions, semi-quantitative model of image
mutual information under varying measurement numbers is established. It is the
Gaussian characteristics of the bucket detector output probability distribution
that leads to this negative exponential behavior. Designed experiments verify
the model.
