Opacities of the emission lines in shocks as functions of the pre-shock density and the shock velocity (top row) and in PDRs as a function of the gas density and the illumination factor (bottom row). All other parameters are set to their standard values given in Table 1. The left column is associated with the H₂ 1-0 S(1) line, the middle column with the [CII] 158 μm line, and the right column with [OI] 63 μm line. The color-coding indicates the central opacity of the lines τ_S(ν₀) and τ_P(ν₀), with a different color scale for shocks and PDRs.