Spheroid and coated sphere Q_abs and Q_sca factors, the latter multiplied by 3 × 10⁴ for clarity, for the parallel (solid lines) and perpendicular (dashed lines) cases, normalised to the same core and mantle volumes. The data shown are for amorphous silicate (a-Sil) cores (effective radius 80 nm) with 20 nm deep aromatic-rich carbon (a-C) mantles (assumed radius of 10 nm; see Sect. 4), as a function of wavelength and e = (a_c/b_c) = 0.5, 0.667, 1, 1.5, and 2 for co-axial (black), constant-depth (blue), and confocal (red) mantles. Also shown for comparison are the data for mantled spheres (cobalt) of outer effective radius a_eff = 80 + 20 = 100 nm.