Illustrations of the two scenarios for the flux-extinction correlation. In both cases, the H₂ emission arises from the thin external shell of clumps. In scenario A, these clumps are optically thick to H₂ emission and the emitting shells have variable optical depth (τ₁≠τ₂≠τ₃≠τ₄≠τ₅). In this case, only the most external clumps are visible (1, 2, 4, 5, but not 3). In scenario B, the clumps are optically thin and are all identical, with optical depth of the emitting shell τ = τ_C ∕100. In this case, the emission coming from a background clump is still visible, provided that there are not too many clumps on the line of sight.