MIR observations of the OMC-3 field. Frame a shows the observed 8 μm surface brightness. Frame b shows the masks used, where the value 1 corresponds to the chosen filament region (shown with dashed contours in the other frames) and the value -1 corresponds to masked stars. Frame c shows the estimated extended emission, I^ext, and frame d the resulting 8 μπι optical depth. Frame a also indicates the four filament fragments that were chosen for further analysis (solid white lines with labels A–D).