early type galaxies [@Madden1997].

Analysis of a wealth of ISOCAM data has shown that the flux ratio of the broad band filters centered at 15$\mu$m and 6.75$\mu$m (LW3/LW2 or f$_{15\mu m}$/f$_{6.7\mu m}$) provides a diagnostic of the dominant global MIR emission characteristic of regions, the diffuse interstellar medium or photo-dissociation regions [@Verstraete1996; @CesarskyD1996b; @Dale2001; @Roussel2001]. It has been shown that while quiescent star forming regions typically have f$_{15\mu m}$/f$_{6.7\mu m}$$\sim$1, in active sites of massive star formation this ratio increases due to the increasing contribution of the continuum emission in the 15$\mu$m bandpass [@Sauvage1996; @Mirabel1998; @Vigroux1999; @Dale2001]. However, one should note that the use of this indicator alone is not sufficient to distinguish between the MIR spectrum due to star formation or an AGN, since in AGNs the hot dust continuum arising from the torus also has f$_{15\mu
m}$/f$_{6.7\mu m}$ $>$1. Such a degeneracy may be resolved using the flux ratio of the 6.75$\mu$m LW2 filter (sampling the 6.2 and 7.7$\mu$m UIBs) to the narrower LW4 filter which is centered at 6.0$\mu$m only contains the 6.2$\mu$m UIB. As the continuum variation between these two filters is negligible, the f$_{6.7\mu
m}$/f$_{6\mu m}$ (LW2/LW4) ratio estimates the intensity of UIBs relative to 