Observations of the column densities of CO and H₂ (open circles, Sheffer et al. 2008) compared to the predictions of the TDR model. The model (filled symbols) are computed for several densities, n_H = 20 (crosses), 50 (squares), and 300 cm^-3 (upside-down triangles) and total column densities, N_H = 1.1 (red), 2.2 (orange), 4.5 (magenta), 9.0 (cyan), and 18 (blue) × 10²⁰ cm^-2. Top panel: the lines of sight are reconstructed with a homogeneous shielding. The value adopted for A_V varies along each curve from left to right from 0.005 mag to A_V,max, where A_V,max is the shielding equivalent to a column density of matter of N_H/ 2. Bottom panel: the lines of sight are reconstructed with a PDR-like shielding function of N_H (Sect. 3.3), hence A_V is no longer a free parameter.