Left: integrated intensity ratio map for filament 14 in IC 5146. Only those pixels where the column density derived from Herschel is higher than10²¹ H₂ cm^-2 have been considered (black contour in Fig. 3). Middle: map of the mean optical depth of the ¹³CO(2−1) transition. The contours in green correspond to a value of 1. Right: map of the mean optical depth of the C¹⁸O(2−1) transition. The mean optical depth of the C¹⁸O transition is τ(C¹⁸O)  <  1 over the entire filament.