cloud. We can thus express the mid-IR intensity along the line of sight as: $$\begin{aligned}
        I(\bar{r}) & = & [<I_{back}>  + \delta I_{back}(\bar{r})] \cdot 
          e^{-\tau_{\lambda}(\bar{r})}\nonumber\\ 
&  & + [<I_{fore}> + \delta I_{fore}(\bar{r})],
\label{itau}\end{aligned}$$ where $I_{back}$ and $I_{fore}$ are the background and foreground intensities respectively, and $\tau_{\lambda}(\bar{r})$ is the dust opacity at a projected radius $\bar{r}$ from core center and at observed wavelength $\lambda$. The foreground emission, $I_{fore}(\bar{r})$, includes the contribution from the zodiacal light emission, $I_{zodi}$. The mid-infrared background emission, $I_{back}(\bar{r})$, is thought to arise from very small grains presenting aromatic emission features (Léger & Puget 1984, Allamandola, Tielens & Barker 1989, Boulanger et al. 1996). These grains/molecules are excited by the interstellar far-UV (FUV) radiation field and re-emit this energy as a group of spectral lines/bands in the mid-infrared, the Unidentified Infrared Bands (UIBs). The ISOCAM LW2=\[5.0, 8.5 $\mu$m\] and LW6=\[7.0, 8.5 $\mu$m\] filters that we used for our observations include the 6.2 $\mu$m and 7.7 $\mu$m emission features of these UIBs.

According to recent studies, the small particles responsible for the UIBs are distributed throughout molecular clouds but because of high interstellar extinction in the FUV band, they are ususally only excited in the outer regions where $A_{v}\simlt 