Line luminosity of C¹⁸O vs. ¹³CO (J = 2−1) obtained implementing isotope-selective processes. Different colors represent models with different disk gas masses. The scatter of the points is due to the exploration of various disk parameters (γ,R_c,ψ,h_c, andi). Models shown in the left panel represent T Tauri disks, while those in the right panel represent Herbig disks, all located at a distance of 100 pc.