Comparison of spectra calculated by the T-matrix formalism for oblate spheroidal particles of equivalent sphere radius 0.1 μmm (except blue line: r = 1μm) with the mean of 23 ISO-SWS spectra (grey line). Purple line: weighted sum of orientation-averaged spectra of spheroids of hypothetic isotropic materials, calculated for a limited distribution (LDE1) of axis ratios (1:1.25–1:1.58). Black dotted line: same, but for a sole axis ratio 1:1.4. Red line: average of the three spectra (two of them identical) with the c-axis parallel to the particle symmetry axis for the LDE1 axis ratio distribution. Yellow line: same for the sole axis ratio 1:1.4. Blue line: single FDTD spectrum of an oblate corundum particle with r = 1μm and ratio of 1:1.4.