Measured and calculated SPDC photon flux generated in a 1-cm long x-cut KTP crystal as a function of the incident pump intensity. $I_p^{\lim }$is the pump intensity delimiting the low and high pump intensity regimes of SPDC. SCM stands for semi-classical model (present work), NMM for nonlinear momentum model [10, 11] and NHM for nonlinear Hamiltonian model [12]. The interpolation plots for NMM and NHM models are obtained by multiplying the second-order nonlinearity ${\chi }^{\left(2\right)}$ by a factor of 3, while the SCM it is obtained by multiplying the vacuum fluctuations amplitude $\Delta E_{s,i}^{\mathrm{vacuum}}$ by a factor of 2.3.