Comparisons of different “worst” cases of , for face-on spirals and discs, as listed in Table 6. Top panel: dust SEDs corresponding to the spherical dust mass of 1 × 10⁻⁵ M_⊙ (black dashed line), i.e. face-on 100% spiral of mass 2 × 10⁻⁷ M_⊙ and a few additional SEDs down to 1 × 10⁻⁸ M_⊙ (red lines), and a face-on 5% spiral with the dust mass 8 × 10⁻⁷ M_⊙ (cyan line). Bottom panel: dust SEDs corresponding to the spherical dust mass of 2 × 10⁻⁵ M_⊙ (black dashed line), i.e. a face-on disc with the dust mass 8 × 10⁻⁷ M_⊙ (green line), and a face-on 0.5% spiral with the dust mass 1 × 10⁻⁵ M_⊙ (magenta line). The vertical yellow dotted line marks the wavelength limit of 2 μm.