Radial profiles of the N_H,mod/A_K ratio (upper panel) and the N_H ratio between the modelled column density and the one derived from the NIR extinction for three values of R_V (lower panel). The N_H,mod/A_K ratio attains a mean value of 1.3 × 10²² cm^-2 mag^-1 with a scatter of 12%. This corroborates the estimate of Vuong et al. (2003) using the standard metallicity of the ISM. A good agreement between the modelled column density and the one based on the extinction is achieved for a slightly increased R_V value of about 4.