  The calculation of autocorrelation functions represents a routinely used tool
to characterise quantum states of light. In this paper, we evaluate the
$g^{(2)}$ function for detected photons in the case of mesoscopic multi-mode
twin-beam states in order to fully investigate their statistical properties
starting from measurable quantities. Moreover, we show that the second-order
autocorrelation function is also useful to estimate the spurious effects
affecting the employed Silicon-photomultiplier detectors.
