two ghost imagers, $$\begin{aligned}
\label{eq:CContrast}
&\mathcal{C}_C = \frac{\pi \rho_L^2}{A_T},\end{aligned}$$ and $$\begin{aligned}
\label{eq:QContrast}
&\mathcal{C}_Q = \frac{\pi \rho_L^2}{A_T}\frac{\Omega_BT_0}{4}\left(1+\frac{1}{4\sqrt{\pi}\mathcal{I}} \right).\end{aligned}$$

If $|{\mathcal{T}}({\bm\rho})| \sim 1$ where it is nonzero, then $A_T$ is the area of the target. Moreover, $\rho_L^2$ is the spatial-resolution area in the absence of turbulence. It follows that the pseudothermal ghost-image contrast is approximately $1/N$, where $N$ is number of no-turbulence resolution cells in the image. Note that our analysis to this point has assumed a [dc]{}-coupled correlator in Fig. \[fig:setup\]. The contrast of a pseudothermal ghost image can be improved by suppressing the background term through [ac]{}-coupling, i.e., measuring the cross covariance, rather than the cross correlation, between the bucket and high spatial-resolution photodetector’s outputs, as was done in the pseudothermal experiments of Scarcelli *et al. [@Scarcelli2006].*

The pseudothermal imager’s contrast does not depend on its source brightness, $\mathcal{I}$, but that for the SPDC does. At high source brightness, $\mathcal{I} \gg 1$, the SPDC image contrast reduces to the result we found for the pseudothermal case. The normal operating regime for a cw SPDC, however, is one of low brightness, $\mathcal{I} \ll 1$, for which Eq.  reduces to $$\mathcal{C}_Q \approx \frac{\sqrt{\pi}}{16}\,\frac{\Omega_B}{P}\frac{a_L^2}{A_T}.$$ The number of photon pairs emitted by an SPDC 