Optical depth maps of the sources obtained using Eq. (2) for the four different wavelengths observed. Contours are in intervals of 10% of the intensity peak for IRAS 17009−4154, IRAS 18229−1127, and IRAS 18454+0001. For IRAS 15534−5422 the contours are in intervals at 10% for [Ne ii] and SiC, intervals at 20% for Q1, and for the PAH1 map the contours start at 70% of the intensity in intervals of 2.5% of the intensity peak.