SKIRT results for a smooth spiral galaxy model with varying optical medium properties, viewed at inclinations of 20° (top row) and 80° (bottom row). The left column shows the surface brightness assuming a dust medium at V-band wavelengths, using an arbitrary colour scale. The middle column shows the average surface brightness along a central horizontal slice, assuming the model is observed at a distance of 10 Mpc. Results are shown for three types of media replacing the dust (green, orange and blue colours) and calculated without (solid curves) or with (dashed curves) explicit absorption. See Sect. 5.3 for details. The right column shows the mean intensity of the radiation field inside each of these model variations, averaged along the line of sight (weighted by medium mass) and averaged along a horizontal slice of the view.