1995A&A...293L..65X

We compare the ultra-violet, optical, and far-infrared emission for a sample of 135 spiral galaxies in order to address the widely debated problem concerning the opacity of spiral disks. We find that the re-radiation of the dust, estimated from the far-infrared emission, is on average only 31±1 percent of the bolometric luminosity of a spiral galaxy, indicating that less than one third of the stellar radiation is absorbed and then re-radiated by dust in a spiral disk. Applying a radiation transfer model which assumes a "Sandwich" configuration for the spiral disk, and fully takes into account the effect of scattering, we find for our sample a median of the face-on blue (4400A) optical depth τ_B=0.49 and the mean <τ_B≥0.60±0.04, indicating that most spiral galaxies in our sample are not opaque for blue light (τ_B<1).