1$ (Hollenbach & Tielens 1997, Bernard et al. 1993). For this reason, the interiors of (quiescent) clouds should be relatively free of UIB emission, and only their outer envelopes should significantly contribute to the mid-IR background and foreground (e.g. Bernard et al. 1993, Boulanger et al. 1998).

As a first step, we neglected the spatial fluctuations of the background and foreground, $\delta I_{back}(\bar{r})$ and $\delta I_{fore}(\bar{r})$, and thus assumed $I_{back}$ and $I_{fore}$ to be equal to their mean values across the field, $<I_{back}>$ and $<I_{fore}>$ (see \[nonuni\] below for a critical discussion of this assumption).

For a given gas-to-dust ratio, the H$_{2}$ column density $N_{H_{2}}$ is related to the dust opacity by $\tau_{\lambda}(\bar{r})=\sigma_{\lambda} \cdot N_{H_{2}}(\bar{r})$, where $\sigma_{\lambda}$ is the dust extinction cross section[^7] at the wavelength of observation $\lambda$. We assume $\sigma_{\lambda}$ to follow the Draine & Lee (1984) dust model in the mid-IR as a first approximation, but the value is rather uncertain (see section \[uncert\]). We adopted $\sigma_{6.75\mu m}=1.2\times 10^{-23}$ cm$^{2}$ for the LW2 filter, $\sigma_{7.75\mu m}=1.35\times 10^{-23}$ cm$^{2}$ for the LW6 filter, and $\sigma_{15\mu m}=1.6\times 10^{-23}$ cm$^{2}$ for the LW9 filter, i.e., we calculated $\sigma_{\lambda}$ at the central wavelength of each filter. A more accurate value could 