Comparison between calculations performed using our non-local treatment of radiative transfer (squares) and the LVG approximation for a cloud model similar to that used previously (from Bernes 1979) for different values of the relative CO abundance (which led to an optically thin case for X(CO) = 5 × 10^-9, lower panel). The green dotted lines represent a modification of the LVG approach trying to simulate boundary effects.