Example of the comparison of two sources spectra with spherical clouds models. Upper panels: CO₂ anti-symmetric stretching mode observed fluxes with the ISO for Elias 29 and NGC7538 IRS9. A straight continuum baseline passing through points located at several times the band-FWHM from the band centre (magenta) is over-plotted, as well as spherical cloud radiative transfer models outputs for M15 mixture presented in Fig. 7 with different distribution upper size limits (MRN (green), τ₁ (red), τ₃ (blue), τ₅ (brown)) scaled by an arbitrary (but common) factor for each observation. For Elias 29, the model spectra are for total visual extinction of 30. For NGC7538 IRS9, the model spectra are for total visual extinction of 60. Lower panels: same comparison but in transmittance, after baseline correction applied to each spectrum, which minimises baseline effects in the comparison.